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Survival of bacteria on wood and plastic particles: Dependence on wood species and environmental conditions

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Abstract

The survival of two hygienically relevant bacteria, Escherichia coli pIE639 and Enterococcus faecium , was followed on wooden sawdust of seven different European woods (pine, spruce, larch, beech, maple, poplar, and oak) versus polyethylene chips by using cultivation-dependent and molecular-based methods in parallel. The survival of the bacteria on wood was dependent on various factors such as the wood species, the type of the inoculated bacterium, the ambient temperature, and humidity. The bacterial titre decreased fastest on pine followed by oak compared to the other woods and plastic. Cultivation-independent analysis employing DNA extraction, Southern blot hybridisation, and PCR-based detection of marker genes of the test bacteria confirmed this result. The decline in bacterial numbers correlated with the decrease of bacterial DNA in the samples. Amounts of DNA of E. coli and E. faecium recovered from pine and oak-wood sawdust were generally lower compared to the other woods and plastic. The presented study shows that pine and oak exhibit substantially better hygienic performance than plastic and indicates an antibacterial effect caused by a combination of the hygroscopic properties of wood and the effect of wood extractives.
Holzforschung, Vol. 59, pp. 72–81, 2005 Copyright by Walter de Gruyter Berlin New York. DOI 10.1515/HF.2005.012
Survival of bacteria on wood and plastic particles:
Dependence on wood species and environmental conditions
Annett Milling
1
, Rolf Kehr
2
, Alfred Wulf
2
and
Kornelia Smalla
1,
*
1
Institute for Plant Virology, Microbiology and Biosafety,
Federal Biological Research Centre for Agriculture and
Forestry (BBA), Braunschweig, Germany
2
Institute for Plant Protection in Forests, Federal
Biological Research Centre for Agriculture and
Forestry (BBA), Braunschweig, Germany
*Corresponding author.
Institute for Plant Virology, Microbiology and Biosafety, Federal
Biological Research Centre for Agriculture and Forestry (BBA),
Messeweg 11/12, 38104 Braunschweig, Germany
Tel.: q49 531 299 3814, Fax: q49 531 299 3013
E-mail: k.smalla@bba.de
Abstract
The survival of two hygienically relevant bacteria, Esche-
richia coli pIE639 and Enterococcus faecium, was fol-
lowed on wooden sawdust of seven different European
woods (pine, spruce, larch, beech, maple, poplar, and
oak) versus polyethylene chips by using cultivation-
dependent and molecular-based methods in parallel. The
survival of the bacteria on wood was dependent on var-
ious factors such as the wood species, the type of the
inoculated bacterium, the ambient temperature, and
humidity. The bacterial titre decreased fastest on pine fol-
lowed by oak compared to the other woods and plastic.
Cultivation-independent analysis employing DNA extrac-
tion, Southern blot hybridisation, and PCR-based detec-
tion of marker genes of the test bacteria confirmed this
result. The decline in bacterial numbers correlated with
the decrease of bacterial DNA in the samples. Amounts
of DNA of E. coli and E. faecium recovered from pine and
oak-wood sawdust were generally lower compared to the
other woods and plastic.
The presented study shows that pine and oak exhibit
substantially better hygienic performance than plastic
and indicates an antibacterial effect caused by a com-
bination of the hygroscopic properties of wood and the
effect of wood extractives.
Keywords: bacteria; DNA extraction; hygiene; plastic;
wood.
Introduction
Wood has a long tradition as a natural material used by
humans for centuries. Since the 1960s, there has been a
continuous debate about the hygienic properties of wood
and wooden products, and as a result the use of wood
is seen critically in many sectors. Nowadays, it seems to
be a general perception that products made of synthetic
materials such as stainless steel and plastics are more
hygienic and cleaner than those made of natural mate-
rials such as wood. In contrast, wood is well-known as
a porous material that can absorb and retain bacteria,
and thus it is regarded as impossible to be kept com-
pletely clean and decontaminated.
Numerous scientific studies have evaluated the hygi-
enic potential of wood compared to plastics and stain-
less steel and resulted in completely different
observations. On the one hand, contamination experi-
ments showed that plate counts from wood were greater
than those from all the boards made of plastics or metal
tested (Kelch and Palm 1958; Ro¨ del et al. 1994). Fur-
thermore, also after different cleaning procedures, high
rates of bacteria were recovered from the wooden sur-
faces, indicating that those surfaces could not be decon-
taminated efficiently (Gilbert and Watson 1971;
Kampelmacher et al. 1971; Borneff et al. 1988a,b; Abris-
hami et al. 1994; Ro¨ del et al. 1994). On the other hand,
the results published by Ak et al. (1994a,b) indicated that
wood is safer in contact with foodstuffs than plastics.
After contamination of cutting boards made of nine dif-
ferent hardwoods and plastics with several hygienically
relevant gram-negative bacteria, significantly fewer viable
bacteria were detected on the wooden boards than on
the plastic boards, regardless of new or used status of
the boards. In the frame of the Nordic Wood Project
‘Wood in Food’’, the hygienic properties of wood, plastic,
and steel were compared (Koch et al. 2002). The survival
of Bacillus subtilis and Pseudomonas fluorescens, found
in the meat industry, was followed in two sets of exper-
iments: (I) on boards made of beech, oak, and ash, rep-
resenting commonly used materials for tabletops or (II)
on spruce and pine, simulating the usage of pallets. A
remarkably great difference in the survival of the bacteria
on the surface of the samples was observed between
wooden samples and plastic and steel. Oak showed the
highest decrease rate in bacterial titre, followed by beech
and ash. Bacteria survived longest on plastic followed by
stainless steel. In the experiments with pine and spruce,
pine performed better than spruce both at low and at
high moisture content. A recent study performed by our
group (Scho¨ nwa¨ lder et al. 2000, 2002) showed that the
survival of the test bacteria Escherichia coli pIE639 and
Enterococcus faecium depended on different factors
such as tree species, the initial inoculum size, and the
characteristics of the inoculated bacterium. Pine-wood
boards exhibited better hygienic performance than other
wooden boards made of spruce, beech, and poplar or
plastic boards. The study indicated an antibacterial effect
of wood, especially pine, presumably caused by a com-
bination of the hygroscopic properties of wood and wood
extractives.
Nevertheless, it is difficult to compare the different
studies because the very wide disparities between the
Hygienic properties of wood and plastic 73
experimental conditions such as type of wood, surface
state of wood, orientation of wood fibres in cutting
boards, humidity level, and fouling of wood prior to con-
tamination. Also, the origin and type of micro-organisms
used to contaminate surfaces, the method of surface
contamination, and sampling methods have a strong
impact on the obtained results (Carpentier 1997).
A lack of information regarding the optimum conditions
for the use of wood has been ascertained (Carpentier
1997), and more information is needed to understand the
activities of bacteria in wood in general. The aim of the
experiments presented in this study was to evaluate the
recent findings and to better define factors that could
influence the survival of bacteria on wood. Based on pre-
liminary results obtained by our group (Scho¨ nwa¨ lder
1999; Scho¨ nwa¨ lder et al. 2000), the survival of E. coli and
E. faecium on wood and plastic in dependence of wood
species, type of the inoculated bacterium, and environ-
mental conditions was followed. For the first time, a com-
bination of cultivation-dependent and DNA-based
methods was used for a hygiene study. In contrast to all
other hygienic investigations, in the current work small
wooden particles (sawdust) of seven different European
wood species were investigated to minimise influences
caused by compact wooden boards such as capillary
properties, orientation of wood fibres, or the water-reten-
tion capacity and to achieve results that are more inde-
pendent of the physical properties of wood according to
Scho¨ nwa¨ lder (1999).
Material and methods
Bacteria and growth conditions
Escherichia coli pIE639 (Tietze et al. 1989) and Enterococcus
faecium (Klare et al. 1995) were used in the experiments as test
bacteria and model organisms for hygienically relevant gram-
negative and gram-positive bacteria. Both bacterial strains carry
antibiotic resistance genes, which were used as selection and
as molecular markers. By addition of antibiotics to the medium,
it was possible to follow precisely the behaviour of the test bac-
teria on the different materials also under semi-sterile conditions.
E. coli was grown overnight in Luria-Bertani broth (LB) contain-
ing 100
mgml
y
1
cycloheximide, 50 mgml
y
1
l rifampicin, and
50
mgml
y
1
streptomycin at 378C for 1624 h, E. faecium in LB
broth containing 25
mgml
y
1
vancomycin at 378C for 2436 h.
For molecular detection, the streptothricin acetyltransferase
gene (sat3) of E. coli, coding for nourseothricin resistance and
localised on the InQ plasmid pIE639, and the chromosomally
localised vancomycin resistance gene (vanA) of E. faecium were
used.
Material
The tested wood included Scots pine (Pinus silvestris L.), Nor-
way spruce (Picea abies Karst.), European larch (Larix decidua
Mill.), black poplar (Populus nigra L.), sycamore maple (Acer
pseudoplatanus L.), beech (Fagus silvatica L.), and pedunculate
oak (Quercus robur L.). Trees of a diameter of 15 to 25 cm at
breast height (dbh) were taken from forests in the immediate
vicinity of Braunschweig in Northern Germany. The trees were
cut into 2-cm thick boards longitudinally, air-dried at ambient
temperatures for 612 months, and converted to sawdust using
a circular saw. Experiments were done with mixed samples of
heartwood and sapwood in heartwood-forming species (oak,
pine, and larch) and with mixed sapwood samples taken across
the wood diameter in the other species. The proportion of heart-
wood averaged 60–70% in pine, larch, and oak samples. As
reference material, polyethylene chips (5=10 mm, 0.5 mm thick)
were used.
Determination of dampness
A characteristic factor of wood is its water content. To determine
the moisture content (or dampness), two replicates of sawdust
(1 g) of each wood type were kiln-dried in an oven at 1038C for
at least 12 h until a constant mass was reached and cooled
down afterwards in the desiccator. Finally, the dry weight was
determined, and the moisture content based on the dry weight
was calculated from the difference of the measured values. The
actual moisture content of the wooden and plastic samples was
checked during the experiments according to the samplings
done.
Inoculation
Before each experiment, the sawdust was dried and disinfected
for at least 12 h at 1038C; polyethylene chips were treated under
UV light for 2 h. After drying, the moisture content was stan-
dardised to 2% of dry weight. In order to characterise the inter-
actions between wood particles and bacteria, 25 g or 50 g
sawdust of the appropriate woods and polyethylene chips were
treated by spraying with a defined volume of the test bacteria
to achieve the desired initial inoculum size. Before inoculation,
the cell density of the inoculum was adjusted. The overnight
culture of the bacteria was centrifuged for 10 min at 2000 g and
the resulting pellet was resuspended in the required volume of
sterile 0.85% (w/v) saline. All experiments were performed with
two replicates per treatment. In the following, the inoculation
performed in the different experiments is described.
Survival of E. coli and E. faecium on wooden
particles of different wood species
The dried sawdust of pine, larch, spruce, maple, beech, poplar,
and oak and polyethylene chips as reference material were treat-
ed with 0.5 ml g
y
1
of an appropriately diluted cell suspension of
the test bacteria to achieve an initial inoculum level of
5=10
8
CFU g
y
1
of E. coli pIE639 or 1=10
6
CFU g
y
1
of E. fae-
cium, respectively. We decided to work with different initial cell
densities of E. coli compared to E. faecium because in prelimi-
nary tests it turned out that approximately this amount of the
test bacteria can be killed on pine wood within 24 h (Scho¨n-
wa¨ lder 1999). Since pine wood so far has shown the best anti-
bacterial characteristics (Scho¨ nwa¨ lder et al. 2002), we used the
possible decreasing rates of pine wood as reference value. The
initial moisture content of the wooden material was 50% after
inoculation. The samples were incubated at 218C. In order to
compare the survival on both test strains directly, an additional
experiment was performed. Pine-wood sawdust and plastic
splinters from polyethylene were treated with 0.5 ml g
y
1
of a
higher concentrated suspension of E. coli and E. faecium to
achieve an initial cell density of 1=10
9
CFU per gram of wood
or plastic and were incubated at 218C.
Survival of E. coli on pine wood in dependence of
the temperature
To examine whether survival of the test bacteria on pine-wood
particles and plastic chips is dependent on the ambient tem-
perature, samples of both materials were inoculated with 0.5 ml
of E. coli to achieve an initial germ load of 5=10
8
CFU g
y
1
and
74 A. Milling et al.
an initial moisture content of 50% of the wooden material. The
samples were incubated at 48C (relative humidity (RH) 100%),
218C (RH 60%), and 378C (RH 23%).
Extraction of bacterial cells and bacterial counts
At fixed time intervals, wood particles and plastic chips (3 g)
were transferred to sterile plastic bags and an extraction buffer
(0.85% (w/v) NaCl, 0.1% (w/v) Bacto-Tryptone, 0.1% (v/v) Tween
20 obtained from Merck, Darmstadt, Germany) was added in a
1:10 relation. The samples were mechanically treated in a Stom-
acher lab blender (Seward Medical, London, UK) for 3 min at
260 rpm to dislodge the adhering bacteria. Serial dilutions were
plated onto appropriate solid growth media. Escherichia coli was
cultivated on Plate Count Agar (Merck) containing 100
mgml
y
1
cycloheximide, 50 mgml
y
1
rifampicin, and 50 mgml
y
1
strep-
tomycin; Enterococcus faecium on Plate Count Agar containing
25
mgml
y
1
vancomycin. After an incubation period of 2472 h
at 378C, the grown colonies were counted to determine the titre
of culturable bacteria per gram of sample (CFU g
y
1
).
DNA extraction
The cell suspension obtained after treatment in the Stomacher
blender was centrifuged at 10 000 g for 30 min. Total community
DNA was extracted from the bacterial pellet according to the
protocol of van Elsas and Smalla (1995) originally developed for
DNA extraction from soil. Briefly, the DNA was extracted by lyso-
zyme, bead beating, and alkaline sodium dodecyl sulphate treat-
ments followed by phenol and chloroform extractions. The crude
DNA was purified by glass milk (GENECLEAN
SPIN kit, BIO101,
Vista, CA, USA) according to the manufacturers’ instructions.
PCR amplification
PCR amplifications were carried out with 1 ml of purified DNA
extracts obtained as described above. PCR products of the sat3
gene of E. coli were generated as described by Pukall et al.
(1996). The vanA gene of E. faecium was amplified according to
Klare et al. (1995).
Southern blotting und hybridisation
The quality and quantity of crude DNA received after the extrac-
tion was analysed in an agarose gel (0.8% w/v). Subsequently,
the DNA was blotted from the agarose gel matrix onto an
uncharged nylon membrane (Amersham, Freiburg, Germany) by
capillary transfer (Sambrook et al. 1989). For molecular detec-
tion, Digoxigenin-labelled probes specific for the streptothricin
acetyltransferase gene (sat3) in E. coli pIE639 and the vanco-
mycin resistance gene (vanA) in E. faecium were used. In order
to prepare the probes, PCR products of the sat3 and the vanA
gene, respectively, were generated as described above, excised
from an agarose gel (1.0% w/v), eluated and purified by using
the Qiaex II gel extraction kit (Qiagen, Hilden, Germany), and
labelled by a standard random priming according to the instruc-
tions of the DIG DNA labelling kit (Roche, Mannheim, Germany).
Hybridisation at 428C, washing and detection of DNA hybrids
was carried out as recommended by Roche.
Inhibitory potential of aqueous wood extracts
Wood extracts were prepared by adding 20 g of wood sawdust
to 180 ml sterile water, incubated at 808C for 2 h and sterile
filtered through a 0.2
mm cellulose acetate filter (Sartorius, Go¨t-
tingen, Germany). The woods tested were pine, spruce, larch,
beech, poplar (as described before), and, in addition, pure pine
heartwood. One hundred microlitres of each wood extract were
transferred in a 96-well microtitre plate and supplemented with
an overnight grown culture of the test bacteria. The final cell
density in the wells was 1=10
9
CFU ml
y
1
E. coli and
1=10
6
CFU ml
y
1
E. faecium, respectively, according to the inoc-
ulum levels used to contaminate wood dust. Pure water and
tannic acid (5% w/v, 10% w/v; Sigma, Steinheim, Germany)
were used as controls. Each approach was repeated four times.
Plates were incubated overnight at 378C. A tetrazolium salt as
indicator of viability (Rodriguez et al. 1992), 2,3,5-Triphenyltetra-
zolium chloride (0.1 mg ml
y
1
; Merck) was added to the wells.
After a further incubation of 30 min at 378C in the dark to allow
colour development, bacterial growth was determined by meas-
uring the optical density at 595 nm with a microtitre plate reader
(model V
max
; Molecular Devices Corp., Menlo Park, CA, USA).
Results
Survival of E. coli pIE639 and E. faecium on wooden
particles of different wood species
The survival of E. coli and E. faecium was followed on
sawdust of several wood species using cultivation-based
and cultivation-independent methods. First, wooden and
plastic samples were inoculated with initial cell densities
of 5=10
8
CFU g
y
1
E. coli or 1=10
6
CFU g
y
1
E. faecium,
respectively. Samples of the inoculated woods and plas-
tic chips were taken at fixed time intervals and plate
counts were done. A very different survival behaviour of
the test bacteria on the different wood species and plas-
tic was found (Figures 1a,b). A dramatic reduction in bac-
terial numbers of E. coli was detected on pine and oak
wood. Already after 24 h, no culturable bacteria could be
detected on pine, and on oak after 48 h. These results
correspond to a log 9 reduction in bacterial numbers of
E. coli within 24 h on pine and within 48 h on oak,
respectively. However, a decline in E. coli numbers, cor-
responding to a log 2 germ reduction on spruce, larch,
maple, poplar, and beech, became apparent only during
the first 24 h. Afterwards, the bacterial titre remained
constant over several days at high levels of
10
6
–10
5
CFU g
y
1
. The bacterial titre on the plastic chips
developed in a similar way compared to spruce, larch,
maple, poplar, and beech and also remained at levels of
10
6
–10
4
CFU g
y
1
during the whole sampling time of
9 days. However, 12 orders of magnitude fewer cultur-
able E. coli could be recovered from the plastic chips
compared to the sawdust of spruce, larch, maple, beech,
and poplar after 9 days. A drastic decrease of the E. fae-
cium titre was observed on pine, oak, and larch. After
24 h, a log 6 reduction in CFU numbers was obtained,
and no culturable bacteria could be recovered from these
woods anymore. In the other woods, the germ reduction
was fastest in the first 24 h. The bacterial numbers con-
tinued to decline and resulted in a complete germ reduc-
tion of E. faecium on spruce and maple after 7 days. E.
faecium survived longer on beech and poplar, where
10
1
–10
2
CFU per gram wood dust were recovered even
after 7 days. However, E. faecium survived longest and
in larger quantities on the plastic chips compared to all
woods tested. Within the first days a very stable bacterial
titre was observed on plastic. The first reduction in bac-
terial numbers began 4 days after inoculation and result-
ed only in a log 2 reduction after 7 days. All samples
dried equally fast. The moisture content of all different
Hygienic properties of wood and plastic 75
Figure 1 Survival of E. coli pIE639 (a) and E. faecium (b) on wooden sawdust and plastic chips. Initial inoculum size: E. coli,
5=10
8
CFU g
y
1
;RT:218C; relative humidity (RH) 55%; E. faecium,1=10
6
CFU g
y
1
;RT:218C; RH 55%.
woods tested was around 58% after 12 h. The plastic
chips were completely dried after this time (data not
shown).
In addition, the survival of the test bacteria was fol-
lowed on pine-wood sawdust and plastic splinters from
polyethylene after both samples were inoculated with the
same cell density of 1=10
9
CFU g
y
1
(data not shown).
Again, the bacterial titre of both strains decreased con-
siderably faster on pine wood compared to plastic. How-
ever, the gram-positive E. faecium always survived longer
than the gram-negative E. coli on both kinds of material.
Immediately after inoculation, a drastic germ reduction
started on pine-wood. After 24 h and 48 h, respectively,
no culturable E. coli or E. faecium were recovered from
pine-wood sawdust. In contrast, the decline in CFU on
plastic resulted only in a log 3 reduction of bacterial num-
bers of E. coli within 7 days. Almost no decrease in bac-
terial numbers of E. faecium was observed on plastic;
rather, the gram-positive bacteria could be recovered at
very high levels of more than 10
4
CFU per gram of pol-
yethylene chips also after 7 days.
In parallel, a cultivation-independent detection of the
inoculated bacteria was performed. Results concerning
the persistence of E. coli on the different materials are
presented in Figure 2. The molecular detection showed
that the decrease of CFU numbers on pine correlated
with the decrease of E. coli DNA on wood. It was shown
that the DNA contents of E. coli on pine wood decreased
rapidly, and that no DNA of the test organism could be
detected after 24 h. In contrast, only a slight reduction
of the DNA contents of E. coli was observed on spruce,
larch, maple, beech, poplar, and plastics according to the
bacterial numbers determined. The molecular detection
of E. coli DNA on oak was not successful. Although
numerous culturable bacteria were recovered from oak
wood after the first 24 h, no corresponding DNA signal
became apparent after hybridisation (Figure 2a). The ini-
tial DNA contents on maple and beech detected after
hybridisation did not correlate with the amount of extract-
ed DNA in the agarose gel (data not shown). The intensity
of corresponding PCR signals obtained after 48 h was
lower for pine and oak wood samples compared to the
other wood species and plastic when fewer amplification
cycles were used (Figure 2b). PCR products of the sat3
gene of E. coli were obtained for all wood species tested
at all samplings with increasing PCR cycles up to 35 only.
The DNA tended to decline independently of the type of
bacteria. Also, amounts of DNA of E. faecium recovered
from pine and oak-wood sawdust were generally lower
compared to the other woods and plastic (data not
shown).
76 A. Milling et al.
Figure 2 Detection of E. coli pIE639 DNA on wooden sawdust and plastic chips. Initial inoculum size: 5=10
8
CFU g
y
1
;RT:218C;
RH 55%. (a) Southern blot of the extracted crude DNA after hybridisation with sat3-probe. (b) Southern blot of the PCR-amplified
DNA (852 bp) after 23 amplification cycles and hybridisation with sat3 probe. M, DNA molecular weight marker III, DIG-labelled
(Roche, Mannheim, Germany); A, pine; B, spruce; C, larch; D, maple; E, beech; F, oak; G, poplar; H, polyethylene; samples were
analysed after 1, 1 h; 2, 24 h; 3, 48 h; 4, 96 h; 5, 216 h.
Further experiments to characterise the interactions
between wood and bacteria in more detail were per-
formed with pine wood only, because this wood showed
the best antibacterial properties.
Survival of E. coli on pine wood in dependence of
the ambient temperature and humidity
The experiment aimed to test whether survival of the test
bacteria on pine wood particles and plastic chips is
dependent on the ambient temperature. Figure 3a shows
that the number of culturable bacteria decreased faster
on both materials at higher temperatures and corre-
sponding lower humidities. The recovery of E. coli from
pine sawdust was generally lower compared to the
recovery of E. coli from plastic chips at all temperatures.
After 24 h, no culturable bacteria could be detected on
pine wood at 378C, while on plastic chips almost
10
6
CFU g
y
1
were determined. In contrast, if the pine
wood samples were incubated at 48C, a complete
decrease of bacteria required more than 6 days. The
germ reduction clearly slows down with decreasing tem-
peratures, yet a clear decrease of the germs on pine took
place also at 48C. A decline in bacterial numbers on plas-
tic chips was only observed at 218C and 378C. In con-
trast, the bacterial titre on plastic remained almost
unaltered at high levels of 10
7
CFU g
y
1
over 6 days at
48C.
Hygienic properties of wood and plastic 77
Figure 3 Survival of E. coli pIE639 on pine-wood sawdust and plastic chips (a) and drying process of pine-wood sawdust and
plastic chips (b) depending on the ambient temperature. (a) Initial inoculum size: 5=10
8
CFU g
y
1
; incubation at 48C (RH 100%), 218C
(RH 60%), and 378C (RH 23%). (b) Initial moisture content: 50%; incubation at 48C (RH 100%), 218C (RH 60%), and 378C (RH 23%).
Although wood and plastic dried equally well at ambi-
ent temperatures of 378C (RH 23%) and 218C (RH 60%)
within 24 h, bacterial numbers decreased conspicuously
faster on pine wood than on plastic. The drying process
substantially slowed down at a temperature of 48C and
a corresponding humidity of almost 100%, and the pine-
wood sawdust dried more slowly than the plastic chips
(Figure 3b). Nevertheless, a clear germ reduction on pine
started already after 4 h at 48C. In contrast, almost no
decline in CFU was observed on plastic chips during the
first 48 h. Even 6 days after inoculation, the analysis of
the plastic chips resulted only in a log 1 reduction in bac-
terial numbers (Figure 3a).
Testing for inhibitory potential of aqueous wood
extracts
In order to clarify the mechanism of the detected anti-
bacterial effect, the survival of the test bacteria in the
aqueous wood extract was analysed. An inhibition of
bacterial growth of E. coli pIE639 became apparent only
in the aqueous extracts of pine according to the inhibition
capacity of a 5% (w/v) solution of tannic acid (Figure 4a).
However, clear differences between the inhibitions of E.
coli in extracts made of pure pine heartwood and
extracts made of pine with a heartwood content of only
about 60% were not detected. Aqueous extracts of
spruce, larch, beech, and poplar supported a level of
bacterial growth comparable with that in water. Growth
of E. faecium was completely suppressed in the extracts
from pure pine heartwood, larch, and in the tannic acid
suspensions. The lower heartwood content in the mixed
pine wood sample caused a slight reduction of the inhi-
bition of E. faecium. Some inhibition was also observed
in the extract of spruce, while the growth of E. faecium
in the extract made of poplar and beech was not sup-
pressed (Figure 4b). The intensity of the inhibitory activity
of the aqueous wood extract correlated with the results
of plate counts and DNA analysis.
Discussion
The most important finding of the study was that the
hygienic potential of wood cannot be generalised,
because the survival of bacteria was completely different
on the different wood species tested. In the experiments
presented, the survival of Escherichia coli pIE639 and
Enterococcus faecium on wood depended on various
factors such as the wood species, the type of the inoc-
ulated bacterium, the ambient temperature, and humidity.
78 A. Milling et al.
Figure 4 Growth of E. coli pIE639 (a) and E. faecium (b) in aqueous wood extracts, tannic acid, and water after incubation overnight
at 378C. Initial cell densities: E. coli,1=10
9
CFU ml
y
1
; E. faecium,1=10
6
CFU ml
y
1
.
First of all, plate counts showed that the test bacteria
had remarkably different abilities to survive on the differ-
ent wood species. The rate of germ reduction was
always fastest on pine-wood sawdust followed by oak
compared to spruce, maple, beech, poplar, and poly-
ethylene. In previous studies performed by our group,
the survival of E. coli and E. faecium was followed on
wooden sawdust (Scho¨ nwa¨ lder 1999) and on compact
wooden boards (Scho¨ nwa¨ lder et al. 2002). In those stud-
ies, pine also exhibited strong antibacterial characteris-
tics, comparable to the currently presented results. In the
current study, E. coli survived longer on spruce, maple,
beech, and poplar than on plastic, while in contrast E.
faecium survived longer on the plastic chips compared
to the wooden samples. Furthermore, on larch wood a
drastic decrease in bacterial numbers of E. faecium com-
parable with that on pine and oak became apparent,
whereas the survival of E. coli was not suppressed on
larch dust. Contrary to that finding, we demonstrated in
the earlier study that the germ-reducing properties of
larch wooden boards against both E. coli and E. faecium
were almost as good as for pine wood, but a high vari-
ation concerning the antibacterial effect was observed
(Scho¨ nwa¨ lder et al. 2002). The antibacterial effect of pine
wood was reproducible when the survival of the test bac-
teria was followed by plate counts as described before
on eight independent pine wood samples (ratio of heart-
wood to sapwood: 70:30) originating from different areas
of Germany and Russia. The decrease in bacterial num-
bers of applied bacteria on pine-wood sawdust resulted
always in at least 5=10
8
CFU g
y
1
of E. coli and
5=10
6
CFU g
y
1
of E. faecium, respectively, within 24 h
for all pine wood samples tested (data not shown).
Differentially, antibacterial effects were observed by Ak
et al. (1994a) only after a contamination of wooden
boards with high levels (10
8
CFU 25 cm
y
2
)ofE. coli on
three successive days without intermediate cleaning.
Bacterial recoveries from cherry and maple were signifi-
cantly fewer than from basswood, birch, and butternut.
But, in general, significantly fewer viable bacteria could
be recovered from the wooden surfaces compared to
plastic regardless of the wood species and the type of
the inoculated strain (Ak et al. 1994a,b). Gehrig et al.
(2000) found no differences in bacterial survival rates on
boards of maple and beech. The study by Koch et al.
(2002) revealed that oak showed the best result in elim-
ination of Pseudomonas fluorescens and Bacillus subtilis
on the surface of boards compared to beech and ash.
Again, the survival rates of bacteria on plastic and stain-
less steel were always higher than on wooden boards.
Furthermore, it was concluded that pine revealed a better
hygienic performance than spruce after testing both
wood species in a second set of experiments (Koch et
al. 2002). Earlier research repeatedly demonstrated that
more micro-organisms were present on wooden boards
compared to metal or plastic surfaces after contact with
food products (Kelch and Palm 1958; Gilbert and Watson
1971; Kampelmacher et al. 1971; Borneff et al. 1988a,b).
Hygienic properties of wood and plastic 79
However, those studies did not provide sufficient results
regarding important details of the experimental design,
because the wood species used in the experiments was
not mentioned.
The gram-positive E. faecium survived noticeably long-
er on both pine wood dust and plastic chips compared
to the gram-negative E. coli, confirming the findings of
Scho¨ nwa¨ lder et al. (2002). Differences in the cell wall
structure of gram-positive bacteria compared to gram-
negative ones might explain this result. Quite possibly,
the higher number of layers in the cell wall of gram-pos-
itive bacteria compared to that of gram-negative bacteria
could protect the former against desiccation and wood
components with antibacterial potential for a longer time.
In contrast, experiments performed by Koch et al. (2002)
indicated the slightly higher survival rates of the gram-
negative P. fluorescens compared to the gram-positive
B. subtilis on boards made of ash, beech, oak, plastic,
and stainless steel. Ak et al. (1994a) demonstrated
almost similar survival behaviour of three different gram-
negative strains (E. coli O157:H7, Listeria monocyto-
genes, Salmonella typhimurium) on hard maple boards.
In addition, earlier research conducted by our group with
wooden boards (Scho¨ nwa¨ lder et al. 2002) and with pine
dust (Scho¨ nwa¨ lder 1999) revealed that the rate of germ
reduction was strongly influenced by the initial inoculum
size. The more bacteria were applied, the longer cultur-
able bacteria could be recovered.
Furthermore, it was shown that moisture content of the
test material and humidity are key factors for germ devel-
opment on different materials, and an obvious delay in
germ reduction was observed with increasing humidity
and moisture content, respectively. Nevertheless, the
decline in bacterial numbers on pine wood was faster
than on plastic in a humid environment, too. In addition,
higher temperatures accelerated the drying process and
thus also the decrease of culturable bacteria. Already,
results obtained in preliminary experiments indicated that
the survival of bacteria on pine sawdust was strongly
dependent on the moisture content of wood. The higher
the initial moisture, the longer the drying process and the
longer bacteria could survive (Scho¨ nwa¨ lder 1999). Gehrig
et al. (2000) performed a contamination experiment of
wooden cutting boards made of maple and beech versus
polyethylene boards. Results showed very high numbers
of E. coli on both wood and plastic in a humid environ-
ment. In a drier environment, noticeably fewer bacteria
were recovered from wood samples than from plastic.
Comparison of wet and dry boards of pine and spruce
by Koch et al. (2002) revealed a higher survival of bac-
teria on the wet surface. But, in both cases, the amount
of bacteria decreased with time. The effect of tempera-
ture(RT,48C) and humidification on the survival of E. coli
K12 Hfr on wooden and plastic boards was also tested
by Ak et al. (1994b). In contrast to the other studies men-
tioned, multifactorial analysis of variance showed no sig-
nificant differences in the survival rates of E. coli on wood
among wood types, between temperatures, or between
humidification conditions, but clear differences between
recoveries of bacteria from wooden and plastic surfaces
were demonstrated here also. Humidification was only
critical on plastic at room temperature and led to
increased recovery of bacteria.
Most investigations into the hygienic properties of
wood employed cultivation-dependent techniques such
as agar contact plates, swabbing, rinsing, soaking, and
destructive methods (Kampelmacher et al. 1971; Abris-
hami et al. 1994; Ak et al. 1994a,b; Miller et al. 1996;
Prechter et al. 2002; Scho¨ nwa¨ lder et al. 2002). It has
been proposed that bacteria can survive stressful con-
ditions such as limited nutrient availability, osmotic
stress, oxygen limitation, and large variations in temper-
ature and pH by entering a viable but nonculturable
(VBNC) state. These cells are metabolically active but
cannot be cultured on laboratory media by standard bac-
teriological methods (Roszak and Colwell 1986; Winfield
and Groisman 2003). Even by desiccation of the wood,
the test organisms are exposed to heavy stress. During
drying, some bacteria die while others are only impaired
in their life activity, without being killed. However, these
bacteria can be vitalised if they are returned into a favour-
able environment (Kampelmacher et al. 1971). Further-
more, it was shown that bacteria such as E. coli form
dormant cells in order to survive also under unfavourable
living conditions with limited nutrient offer (Perez-Rosas,
1988; Barcina et al. 1989). Del Mar Lleo` et al. (2003)
reported the survival of stressed vancomycin-resistant
enterococci in the environment by entering the VBNC
state. In order to exclude that the detected reduction in
bacterial numbers, particularly on pine and oak wood, is
mainly caused by viable but nonculturable cells caused
by desiccation stress, it was necessary to use molecular
methods capable of detecting nonculturable bacteria.
After recently published studies had demonstrated the
eligibility of DNA-based methods for direct analysis of
wood-inhabiting fungi (Jasalavich et al. 2000; Vainio and
Hantula 2000) and reported the successful isolation of
functional RNA from woody branches (Lewisohn et al.
1994; Melichar et al. 2000), we decided to use DNA
extraction and PCR for the first time in a hygiene study
to facilitate a reliable and sensitive cultivation-independ-
ent analysis of selected bacteria in wood.
The cultivation-independent analysis showed that the
decrease in CFU numbers correlated with the decrease
of DNA on wood. Especially on pine wood, a rapid
decline of extracted DNA became apparent. These
results indicate that the decrease in bacterial numbers
on wood was not due to the ability of the test bacteria
to enter the VBNC state or due to the transfer of the
bacteria into the wood and close adsorption of the bac-
teria to the wood structure as argued by several authors
(Kampelmacher et al. 1971; Ruosch 1981; Abrishami et
al. 1994; Ro¨ del et al. 1994; Lorentzen et al. 2000). Rather,
the applied bacteria were killed completely due to inter-
action with wood.
Molecular detection was dependent on the wood spe-
cies and could not be successfully accomplished for
beech, maple, and oak. Results indicate some sort of
inhibition during DNA extraction of oak and Southern blot
hybridisation of beech, maple, and oak. It is assumed
that molecular detection was probably disturbed by co-
extracted polyphenolic wood components, because even
humic acids were shown to interfere with DNA hybridi-
80 A. Milling et al.
sation, restriction enzyme digestion, and PCR amplifi-
cation (Smalla et al. 1993; Tebbe et al. 1993). Inhibiting
wood components might be co-extracted directly from
remaining wooden particles in the cell pellet, washed-out
during mechanical treatment of the samples in the Stom-
acher blender, or might have been incorporated into the
cell walls of the bacteria during incubation. DNA and
enzymes become affected by polyphenolic components
(Scalbert 1991; Field and Lettinga 1992), so that the DNA
is no longer available for molecular detection, and
enzymes used for DNA extraction and hybridisation are
inhibited. In preliminary tests, direct extraction of bacte-
rial DNA from wood and DNA extraction after prior wash-
ing and preparing a cell pellet as introduced in this study
was tested. Both methods resulted in comparable find-
ings. The amount of extracted DNA correlated with the
bacterial numbers recovered from the different woods.
However, a higher quality and less inhibited DNA was
obtained after DNA extraction from the cell pellet com-
pared to the direct DNA extraction from wood. Southern
blot hybridisation of DNA extracts was improved by prior
purification of the crude extracts with glass milk as per-
formed before PCR amplification (data not shown).
Results obtained in this study indicate that the detect-
ed germ reduction on wood is caused by an antibacterial
effect of wood based on several physical and chemical
properties of wood. The porous structure and hygro-
scopic characteristic of wood leads to desiccation of
bacteria. Most bacteria are desiccation-sensitive and
require a water potential of y2.8 MPa or less for growth
in wood. This is significantly above the moisture content
of air-dried wood stored indoors, so that properly dried
wood does not offer bacteria enough water for growth
and multiplication (Bavendamm 1974; Schmidt 1994).
However, the present study revealed that desiccation of
wooden material cannot be the only reason for the
effects observed. During the experiments, all woods and
plastic dried rapidly within 24 h, but on pine and oak a
much higher reduction of culturable bacteria as well as a
faster decrease of bacterial DNA on pine became
apparent.
In addition, polyphenolic substances present in wood
(e.g., tannins or flavonoids) could be responsible for an
antibacterial effect (Field et al. 1989; Scalbert 1991; Field
and Lettinga 1992; Cowan 1999; Rauha et al. 2000). It
was clearly shown that wood ingredients extracted by
hot water treatment are involved in the decrease of bac-
teria on wood. Escherichia coli and E. faecium were effi-
ciently killed in aqueous extracts from pine. However,
while E. faecium was drastically decreased in the aque-
ous extracts made of larch, no sign of inhibition was
observed for E. coli in larch extracts. Results indicate dif-
ferent susceptibility of different bacteria towards wood
extracts of the same species. Even differentially inhibitory
effects of extractives (flavonoids) towards bacteria were
observed by Rauha et al. (2000); for example, naringenin
belonging to the group of flavonoids showed strong anti-
bacterial activity against Staphylococcus sp., but E. coli
and Pseudomonas aeruginosa were inhibited to a lesser
degree. Moreover, the same study confirmed inhibitory
characteristics of pine, since ether extracts made of the
pine phloem suppressed the growth of E. coli and Sta-
phylococcus aureus (Rauha et al. 2000). Our data con-
tradict the statement of Ak et al. (1994b) that antibacterial
substances in wood are not water-soluble. In addition,
Miller et al. (1996) also demonstrated a considerable level
of inhibitory activity of aqueous extracts made of white
ash against E. coli O157:H7.
Conclusions
The hygienic properties of wooden and plastic particles
in contact with test bacteria were investigated employing
a combination of classic microbiological and DNA-based
methods. The experiments revealed that the survival of
E. coli and E. faecium was dependent on factors such as
humidity and ambient temperature, and on the type of
the test bacteria on both wooden dust and plastic chips.
Furthermore, it must be stressed that different wood spe-
cies displayed completely different hygienic perform-
ance. In the past, wood has obviously been unfairly
classified as unhygienic. Some wood species like pine
and oak showed excellent antibacterial characteristics,
efficiently killed applied bacteria, and had clear hygienic
advantages compared to other woods and plastics. Fur-
ther research has to be performed under conditions clos-
er to practice (e.g. humid air, effect of organic matter, and
naturally occurring microbial communities) to evaluate
whether the hygienic situation in several areas such as
in private households, transportation of foodstuff, or ani-
mal husbandry could be improved by a careful selection
of suitable wood species and appropriate handling based
on the results obtained.
References
Abrishami, S.H., Tall, B.D., Bruursema, T.J., Epstein, P.S., Shah,
D.B. (1994) Bacterial adherence and viability on cutting board
surfaces. J. Food Safety 14:153–172.
Ak, N.O., Cliver, D.O., Kaspar, C.W. (1994a) Cutting boards of
plastic and wood contaminated experimentally with bacteria.
J. Fd. Prot. 57:1622.
Ak, N.O., Cliver, D.O., Kaspar, C.W. (1994b) Decontamination of
plastic and wooden cutting boards for kitchen use. J. Fd.
Prot. 57:2330.
Barcina, I., Gonzalez, J.M., Iriberri, J., Egea, L. (1989) Effect of
visible light on progressive dormancy of Escherichia coli cells
during the survival process in natural fresh water. Appl. Envi-
ron. Microbiol. 55:246251.
Bavendamm, W. Die Holzscha¨ den und ihre Verhu¨ tung. Wissen-
schaftliche Verlagsgesellschaft, Stuttgart, 1974.
Borneff, J., Hassinger, R., Wittig, J., Edenharder, R. (1988a)
Untersuchungen zur Keimverbreitung in Haushaltsku¨ chen I.
Mitteilung: Problemstellung, Versuche, Ergebnisse. Zbl. Bakt.
Hyg. B. 186:129.
Borneff, J., Hassinger, R., Wittig, J., Edenharder, R. (1988b)
Untersuchungen zur Keimverbreitung in Haushaltsku¨ chen II.
Mitteilung: Beurteilung der Resultate and hygienische
Schlussfolgerungen. Zbl. Bakt. Hyg. B. 186:3044.
Carpentier, B. (1997) Sanitary quality of meat chopping board
surfaces: a bibliographical study. Food Microbiol. 14:3137.
Cowan, M.M. (1999) Plant products as antimicrobial agents.
Clin. Microbiol. Rev. 12:564582.
Del Mar Lleo` , M., Bonato, B., Signoretto, C., Canepari, P. (2003)
Vancomycin resistance is maintained in enterococci in the
viable but nonculturable state and after division is resumed.
Antimicrob. Agents Chemother. 47:11541156.
Hygienic properties of wood and plastic 81
Field, J.A., Kortekaas, S., Lettinga, G. (1989) The tannin theory
of methanogenic toxicity. Biol. Wastes 29:241–262.
Field, J.A., Lettinga, G. (1992) Toxicity of tannic compounds to
microorganisms. In: Plant Polyphenols. Eds. Hemingway,
R.W., Laks, P.E. Plenum Press, New York. pp. 673692.
Gehrig, M., Schnell, G., Zu¨ rcher, E., Kucera, L.J. (2000) Hygienic
aspects of wood and polyethylene cutting boards regarding
food contamination. A comparison. Holz Roh- Werkst.
58:265269.
Gilbert, R., Watson, J.H.M. (1971) Some laboratory experiments
on various meat preparation surfaces with regard to surface
contamination and cleaning. J. Fd. Technol. 6:163–170.
Jasalavich, C.A., Ostrofsky, A., Jellison, J. (2000) Detection and
identification of decay fungi in spruce wood by restriction
fragment length polymorphism analysis of amplified genes
encoding rRNA. Appl. Environ. Microbiol. 66: 47254734.
Kampelmacher, E.H., Mossel, D.A.A., van Schothorst, M., van
Noorle Jansen, L.M. (1971) Quantitative Untersuchungen
u¨ ber die Dekontamination von Holzfla¨ chen in der Fleischver-
arbeitung. Alimenta (Sonderausgabe Lebensmittel tierischer
Herkunft) 7076.
Kelch, F., Palm, A. (1958) Vergleichende Untersuchungen u¨ ber
den Oberfla¨ chenkeimgehalt von Holz- und Metalltischfla¨ chen
in Fleisch verarbeitenden Betrieben. Fleischwirtsch.
10:1720.
Klare, I., Heier, H., Claus, H., Reissbrodt, R., Witte, W. (1995)
vanA-mediated high-level glycopeptide resistance in Entero-
coccus faecium from animal husbandry. FEMS Microbiol.
Lett. 125:165172.
Koch, A.P., Kofod, C.J., Konova, D., Kvist, K.E., Lindegaard, B.
(2002) Wood, plastic and steel a comparison of hygienic
properties. Partial report 10 ‘Wood in the Food’’, Danish
Technological Institute.
Lewisohn, E., Steele, C.L., Croreau, R. (1994) Simple isolation of
fungal RNA from woody stems of gymnosperms. Plant Mol.
Biol. Reporter 12:20–25.
Lorentzen, G., Gudbjo¨ rnsdottir, B., Weidner, I. (2000) Wood in
food-measuring methods. Partial report 1 ‘Wood in the
Food’’, Norwegian Institute of Fisheries and Aquaculture Ltd.,
Icelandic Fisheries Laboratories and Norwegian Institute of
Wood Technology.
Melichar, H., Bosch, I., Molnar, G.M., Huang, L., Pardee, A.B.
(2000) Isolation and purification of functional total RNA from
woody branches and needles of sitka and white spruce.
BioTechniques Euro Ed. 28:292296.
Miller, A., Brown, T., Call, G.E. (1996) Comparison of wooden
and polyethylene cutting boards: potential for attachment
and removal of bacteria from ground beef. J. Food Prot.
59:854858.
Perez-Rosas, N., Hazen, T.C. (1988) In-situ survival of Vibrio
cholorae and Escherichia coli in tropical coral reefs. Appl.
Environ. Microbiol. 54:517.
Prechter, S., Betz, M., Cerny, G., Wegener, G., Windeisen, E.
(2002) Hygiene aspects of wooden or plastic cutting boards.
Holz Roh- Werkst. 60:239248.
Pukall, R., Tscha¨ pe, H., Smalla, K. (1996) Monitoring the spread
of broad host and narrow host range plasmids in soil micro-
cosms. FEMS Microbial. Ecol. 20:5366.
Rauha, J.-P., Remes, S., Heinonen, M., Hopia, A., Ka¨ hko¨ nen, M.,
Kujala, T., Pihlaja, K., Vuorela, H., Vuorela, P. (2000) Antimi-
crobial effects of Finnish plant extracts containing flavonoids
and other phenoloic compounds. Intern. J. Food Microbiol.
56:312.
Ro¨ del, W., Hechelmann, H., Dresel, J. (1994) Hygieneaspekte zu
Schneidunterlagen aus Holz und Kunststoff. Fleischwirtsch.
74:814821.
Rodriguez, G.G., Phipps, D., Ishiguru, K., Ridgway, H.F. (1992)
Use of a fluorescent redox probe for direct visualization of
actively respiring bacteria. Appl. Environ. Microbiol.
58:18011808.
Roszak, D.B., Colwell, R.R. (1986) Viable but nonculturable stage
of Campylobacter jejuni and its role in survival in the natural
aquatic environment. Appl. Environ. Microbiol. 52:531–538.
Ruosch, W. (1981) Der quantitative Keimnachweis auf Oberfla¨-
chen von Holz und Kunststoff. Schweiz. Arch. Tierheilk.
123:97103.
Sambrook, J., Fritsch, E.F., Maniatis, T. Molecular Cloning. A
Laboratory Manual. 2
nd
Edn. Cold Spring Harbor Laboratory
Press, New York, 1989.
Scalbert, A. (1991) Antimicrobial properties of tannins. Phyto-
chem. 30:38753883.
Scho¨ nwa¨ lder, A. (1999) Hygienische Aspekte bei Holz und Holz-
produkten. AFZ Der Wald 15:789791.
Scho¨ nwa¨ lder, A., Kehr, R., Wulf, A., Smalla, K. (2000) Antibak-
terielle Eigenschaften von Holz beachtenswert. Holz-Zentral-
blatt 147:20372038.
Scho¨ nwa¨ lder, A., Kehr, R., Wulf, A., Smalla, K. (2002) Wooden
boards affecting the survival of bacteria? Holz Roh- Werkst.
60:249257.
Schmidt, O. Holz- und Baumpilze. Springer-Verlag, Berlin, 1994.
Smalla, K., Cresswell, N., Mendonca-Hagler, L.C., Wolters, A.,
van Elsas, J.D. (1993) Rapid DNA extraction protocol from
soil for polymerase chain reaction-mediated amplification. J.
Appl. Bacteriol. 74:7885.
Tebbe, C.C., Vahjen, W. (1993) Interference of humic acids and
DNA extracted directly from soil in detection and transfor-
mation of recombinant DNA from bacteria and a yeast. Appl.
Environ. Microbiol. 59:26572665.
Tietze, E., Brevet, J., Voigt, W. (1989) Characterization of new
resistance plasmids belonging to incompatibility group IncQ.
J. Basic. Microbiol. 26:695706.
Vainio, E.I., Hantula, J. (2000) Direct analysis of wood-inhabiting
fungi using denaturing gradient gel electrophoresis of ampli-
fied ribosomal DNA. Mycol. Res. 104:927936.
Van Elsas, J.D., Smalla, K. (1995) Extraction of microbial com-
munity DNA from soils. In: Molecular Microbial Ecology Man-
ual. Kluwer Academic Publishers, The Netherlands. pp. 111.
Winfield, M.D., Groisman, E.A. (2003). Role of nonhost environ-
ments in the lifestyle of Salmonella and Escherichia coli. Envi-
ron. Microbiol. 69:36873694.
Received March 19, 2004
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Thesis
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In 2017, the 560-ha area of hybrid poplar plantation in northern Poland showed symptoms of tree decline. Leaves appeared smaller, turned yellow-brown, and were shed prematurely. Twigs and smaller branches died. Bark was sunken and discolored, often loosened and split. Trunks decayed from the base. Phloem and xylem showed brown necrosis. Ten per cent of trees died in 1–2 months. None of these symptoms was typical for known poplar diseases. Bacteria in soil and the necrotic base of poplar trunk were analysed with Illumina sequencing. Soil and wood were colonized by at least 615 and 249 taxa. The majority of bacteria were common to soil and wood. The most common taxa in soil were: Acidobacteria (14.757%), Actinobacteria (14.583%), Proteobacteria (36.872) with Betaproteobacteria (6.516%), Burkholderiales (6.102%), Comamonadaceae (2.786%), and Verrucomicrobia (5.307%).The most common taxa in wood were: Bacteroidetes (22.722%) including Chryseobacterium (5.074%), Flavobacteriales (10.873%), Sphingobacteriales (9.396%) with Pedobacter cryoconitis (7.306%), Proteobacteria (73.785%) with Enterobacteriales (33.247%) including Serratia (15.303%) and Sodalis (6.524%), Pseudomonadales (9.829%) including Pseudomonas (9.017%), Rhizobiales (6.826%), Sphingomonadales (5.646%), and Xanthomonadales (11.194%). Possible pathogens were Pseudomonas, Rhizobium and Xanthomonas. The initially endophytic character of bacteria is emphasized. Soil and possibly planting material might be the sources of pathogen inoculum.
... This ability is influenced by environmental humidity and temperature, and free water, bound water, and fiber saturation point of the wood [94]. Most bacteria and some fungi are sensitive to desiccation and need a water potential of ≤−2.8 MPa for growth, motility, and nutrient uptake [9,58]. However, properly dried wood (12% moisture) has a higher water potential that does not offer enough water for microbial growth and multiplication [9,58]. ...
... Most bacteria and some fungi are sensitive to desiccation and need a water potential of ≤−2.8 MPa for growth, motility, and nutrient uptake [9,58]. However, properly dried wood (12% moisture) has a higher water potential that does not offer enough water for microbial growth and multiplication [9,58]. This is the reason that the hygroscopic nature of wood leads to faster absorption of moisture as compared to other non-porous contact surfaces, consequently, microbes survive least on wood compared to non-absorptive smooth surfaces such as plastics and metals [12,86,[95][96][97]. ...
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Wood materials are being adopted as nature-based architectural themes inside the healthcare buildings. Concern is raised that the organic and porous character of wood might support microbial survival. Therefore, this review discusses the hygienic properties of wood including the antimicrobial potential and its cleanability in comparison to smooth surface materials. In general, wood has antimicrobial properties owing to its chemical composition and physical structure. However, the hygienic potential of wood is influenced by the type of wood, age of wood, the cleaning method, surface treatment, and its moisture content. This information is intended to guide decision-makers regarding the use of wood in hygienically sensitive places and researchers to help them identify the variables for better utilizing the hygienic potential of this material.
... Psychologically, wood is also considered to be warmer, more comforting, relaxing and inviting as compared to wood laminate by both expert and non-expert subjects [1,2]. In contrast to earlier views, newer studies have shown that wood surfaces suppress microbial activity better than plastic surfaces [3] due to their inherent antiviral, antifungal, antibacterial [4,5] and antistatic qualities [6]. Therefore, wood can even be used safely in a hospital environment [4,[7][8][9]. ...
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Cardiorespiratory interactions (CRIs) reflect the mutual tuning of two important organismic oscillators—the heartbeat and respiration. These interactions can be used as a powerful tool to characterize the self-organizational and recreational quality of sleep. In this randomized, blinded and cross-over design study, we investigated CRIs in 15 subjects over a total of 253 nights who slept in beds made from different materials. One type of bed, used as control, was made of melamine faced chipboard with a wood-like appearance, while the other type was made of solid wood from stone pine (Pinus cembra). We observed a significant increase of vagal activity (measured by respiratory sinus arrhythmia), a decrease in the heart rate (as an indicator of energy consumption during sleep) and an improvement in CRIs, especially during the first hours of sleep in the stone pine beds as compared to the chipboard beds. Subjective assessments of study participants’ well-being in the morning and sub-scalar assessments of their intrapsychic stability were significantly better after they slept in the stone pine bed than after they slept in the chipboard bed. Our observations suggest that CRIs are sensitive to detectable differences in indoor settings that are relevant to human health. Our results are in agreement with those of other studies that have reported that exposure to volatile phytochemical ingredients of stone pine (α-pinene, limonene, bornyl acetate) lead to an improvement in vagal activity and studies that show a reduction in stress parameters upon contact with solid wood surfaces.
... This microbe countering behaviour of wood is generally attributed to its chemical composition of extractives, such as flavonoids and phenolics, which have an inhibitory role against microbial growth 11 . In addition, the survival of different bacteria and fungi has also been reported to be lower on wood as compared to other inanimate materials 11,12 . Therefore, the physical structure of wood is also regarded as a component of antimicrobial activity of wood 9,11 . ...
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Methods to test the safety of wood material for hygienically sensitive places are indirect, destructive and limited to incomplete microbial recovery via swabbing, brushing and elution-based techniques. Therefore, we chose mCherry Staphylococcus aureus as a model bacterium for solid and porous surface contamination. Confocal spectral laser microscope (CSLM) was employed to characterize and use the autofluorescence of Sessile oak (Quercus petraea), Douglas fir (Pseudotsuga menziesii) and poplar (Populus euramericana alba L.) wood discs cut into transversal (RT) and tangential (LT) planes. The red fluorescent area occupied by bacteria was differentiated from that of wood, which represented the bacterial quantification, survival and bio-distribution on surfaces from one hour to one week after inoculation. More bacteria were present near the surface on LT face wood as compared to RT and they persisted throughout the study period. Furthermore, this innovative methodology identified that S. aureus formed a dense biofilm on melamine but not on oak wood in similar inoculation and growth conditions. Conclusively, the endogenous fluorescence of materials and the model bacterium permitted direct quantification of surface contamination by using CSLM and it is a promising tool for hygienic safety evaluation.
... Enterobacteriales are facultative anaerobes, common in water or soil, or are parasites of animals and plants. They can live on/in wood of poplar, and even much longer than on wood of other tree species (Schönwälder et al. 2002;Milling et al. 2005). The most abundant representatives of Enterobacteriales were Serratia and Sodalis (15.3% and 6.5% in wood). ...
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In 2017, a 560-ha area of hybrid poplar plantation in northern Poland showed symptoms of tree decline. Leaves appeared smaller, turned yellow–brown, and were shed prematurely. Twigs and smaller branches died. Bark was sunken and discolored, often loosened and split. Trunks decayed from the base. Phloem and xylem showed brown necrosis. Ten per cent of trees died in 1–2 months. None of these symptoms was typical for known poplar diseases. Bacteria in soil and in the necrotic base of poplar trunk were analyzed with Illumina sequencing. Soil and wood were colonized by at least 615 and 249 taxa. The majority of bacteria were common to soil and wood. The most common taxa in soil were: Acidobacteria (14.76%), Actinobacteria (14.58%), Proteobacteria (36.87) with Betaproteobacteria (6.52%), (6.10%), Comamonadaceae (2.79%), and Verrucomicrobia (5.31%).The most common taxa in wood were: Bacteroidetes (22.72%) including Chryseobacterium (5.07%), Flavobacteriales (10.87%), Sphingobacteriales (9.40%) with Pedobacter cryoconitis (7.31%), Proteobacteria (73.79%) with Enterobacteriales (33.25%) including Serratia (15.30%) and Sodalis (6.52%), Pseudomonadales (9.83%) including Pseudomonas (9.02%), Rhizobiales (6.83%), Sphingomonadales (5.65%), and Xanthomonadales (11.19%). Possible pathogens were Pseudomonas, Rhizobium and Xanthomonas. The potential initial, endophytic character of bacteria is discussed. Soil and possibly planting material might be the reservoir of pathogen inoculum.
... For example, one study found that after a 2-h drying period, more than 90% of cells inoculated onto wood surfaces could not be recovered by vigorous rinsing (Abrishami et al., 1994), while another found that bacteria could not be recovered after only 3-10 min (Ak et al., 1994). Numerous other studies provide similar evidence, although the effect may depend on the species of wood and whether it is coated or not Boersig and Cliver, 2010;Boursillon and Riethmüller, 2007;da Costa et al., 2008;Hedge, 2015;Koch et al., 2002;Milling et al., 2005a;Milling et al., 2005b;Moore et al., 2007;Pailhori es et al., 2017;Sch€ onw€ alder et al., 2002;Vainio-Kaila et al., 2011;Vainio-Kaila et al., 2017). ...
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Indoor environmental quality is a paramount concern among architects. Exposure to VOCs and microorganisms impacts occupant health, yet the role of materials on these exposures remains poorly understood. In this study, we placed four material types in individual microcosms to test whether material type influences bacterial community structure and VOC emission. We used culture-independent methods to characterize bacterial communities and TD-GC-MS to measure VOC emission. We found that viable bacterial communities had different patterns of abundance, diversity, and composition, in comparison with total (viable plus dead cells) bacterial communities. Examining viable bacteria only, Earth had the highest abundance and diversity, unique community composition, and overall negative VOC emission. Timber had the lowest bacterial abundance, composition similar to Gypsum and Concrete, and the highest VOC emission rate. Our research provides further evidence that architects’ decisions about building materials can influence chemical and microbial exposures indoors.
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Hygienic aspects of cutting boards made of wood (european maple, beech and oak) and polyethylene (PE) were compared in order to determine the risk of food contamination in household and commercial kitchen. Boards were contaminated with Escherichia coli bacteria, and the colony forming units (cfu) were retrieved by agar contact methods. Bacteria counts after 15 hour storage at room temperature were compared to values obtained after machine and manual washing processes. Results showed that in very humid environment, both wood and PE showed very high numbers of bacteria. Even machine washing of the wet samples hardly reduced the cfu counted. Probably, the high bacteria density observed was due to the high surface moisture of the samples which led to ideal conditions for the microorganisms on the surface from where they are easily retrieved. In drier environment, contact plates removed significantly less bacteria from wood samples than from PE. The reason for this effect was not clearly established, but it was observed that the porous wood surface dried faster than the polyethylene surface. Also, observations of surface samples in a scanning electron microscope proved that after one month of intensive use polyethylene boards obtained a very rough and cavernous surface similar to wood (but with less profound porosity). On wood, these surface cuts open in the drying process and therefore bacteria cannot survive. However, on PE a retention of bacteria enclosed in caverns and the possibility of later release is suspected. On all materials a significant decrease of bacteria count was achieved upon manual washing with detergent and brush followed by rinsing under warm water. After this treatment, bacteria were recovered only sporadically. For wood, an even higher degree of disinfection could possibly be achieved with the microwave method suggested by Park and Cliver (1996). In general, the results of the present experiments show that wood is not, as commonly assumed, less hygienic than polyethylene. The statement that the use of wood in food processing increased the risk of infestation by microorganisms could therefore not be supported.
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