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Photocatalytic disinfection using titanium dioxide: Spectrum and mechanism of antimicrobial activity

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The photocatalytic properties of titanium dioxide are well known and have many applications including the removal of organic contaminants and production of self-cleaning glass. There is an increasing interest in the application of the photocatalytic properties of TiO(2) for disinfection of surfaces, air and water. Reviews of the applications of photocatalysis in disinfection (Gamage and Zhang 2010; Chong et al., Wat Res 44(10):2997-3027, 2010) and of modelling of TiO(2) action have recently been published (Dalrymple et al. , Appl Catal B 98(1-2):27-38, 2010). In this review, we give an overview of the effects of photoactivated TiO(2) on microorganisms. The activity has been shown to be capable of killing a wide range of Gram-negative and Gram-positive bacteria, filamentous and unicellular fungi, algae, protozoa, mammalian viruses and bacteriophage. Resting stages, particularly bacterial endospores, fungal spores and protozoan cysts, are generally more resistant than the vegetative forms, possibly due to the increased cell wall thickness. The killing mechanism involves degradation of the cell wall and cytoplasmic membrane due to the production of reactive oxygen species such as hydroxyl radicals and hydrogen peroxide. This initially leads to leakage of cellular contents then cell lysis and may be followed by complete mineralisation of the organism. Killing is most efficient when there is close contact between the organisms and the TiO(2) catalyst. The killing activity is enhanced by the presence of other antimicrobial agents such as Cu and Ag.
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MINI-REVIEW
Photocatalytic disinfection using titanium dioxide: spectrum
and mechanism of antimicrobial activity
Howard A. Foster &Iram B. Ditta &Sajnu Varghese &
Alex Steele
Received: 11 February 2011 / Accepted: 12 February 2011 /Published online: 27 April 2011
#Springer-Verlag 2011
Abstract The photocatalytic properties of titanium di-
oxide are well known and have many applications
including the removal of organic contaminants and
production of self-cleaning glass. There is an increasing
interest in the application of the photocatalytic proper-
ties of TiO
2
for disinfection of surfaces, air and water.
Reviews of the applications of photocatalysis in disinfec-
tion (Gamage and Zhang 2010; Chong et al., Wat Res 44
(10):29973027, 2010) and of modelling of TiO
2
action
have recently been published (Dalrymple et al. , Appl
Catal B 98(12):2738, 2010). In this review, we give an
overview of the effects of photoactivated TiO
2
on micro-
organisms. The activity has been shown to be capable of
killing a wide range of Gram-negative and Gram-positive
bacteria, filamentous and unicellular fungi, algae, proto-
zoa, mammalian viruses and bacteriophage. Resting
stages, particularly bacterial endospores, fungal spores
and protozoan cysts, are generally more resistant than the
vegetative forms, possibly due to the increased cell wall
thickness. The killing mechanism involves degradation of
the cell wall and cytoplasmic membrane due to the
production of reactive oxygen species such as hydroxyl
radicals and hydrogen peroxide. This initially leads to
leakage of cellular contents then cell lysis and may be
followed by complete mineralisation of the organism.
Killing is most efficient when there is close contact
between the organisms and the TiO
2
catalyst. The killing
activity is enhanced by the presence of other antimicrobial
agents such as Cu and Ag.
Keywords Antimicrobial .Disinfection .Mechanism .
Photocatalysis .ROS .TiO
2
.Titania
Introduction
The ability of titanium dioxide (titania, TiO
2
) to act as a
photocatalyst has been known for 90 years (Renz 1921),
and its role in the chalkingof paint (formation of
powder on the surface) is well known (Jacobsen 1949).
Interest in the application of the photocatalytic properties
of TiO
2
was revived when the photoelectrolysis of water
was reported by Fujishima and Honda (1972), and this
activity was soon exploited both for the ability to catalyse
the oxidation of pollutants (Carey et al. 1976;Frankand
Bard 1977) and the ability to kill microorganisms
(Matusunga 1985; Matsunaga et al. 1985). Photocatalytic
surfaces can be superhydrophilic, which means that water
spreads on the surface, allowing dirt to be washed off, and
commercial uses include self-cleaning windows (e.g. San
Gobain Bioclean, Pilkington Activeand Sunclean;
Chen and Poon 2009) and self-cleaning glass covers for
highway tunnel lamps (Honda et al. 1998). There are
currently over 11,000 publications on photocatalysis.
Although an early study showed no improved antimicro-
bial activity of TiO
2
for disinfection of primary wastewater
effluent (Carey and Oliver 1980), many subsequent studies
have shown the usefulness of photocatalysis on TiO
2
for
disinfection of water (Chong et al. 2010). These include
killing of bacteria (Rincón and Pulgarin 2004a)and
viruses from water supplies (Sjogren and Sierka 1994),
H. A. Foster (*):I. B. Ditta :S. Varghese :A. Steele
Centre for Parasitology and Disease Research,
School of Environment and Life Sciences,
University of Salford, The Crescent,
Salford, Greater Manchester M5 4WT, UK
e-mail: h.a.foster@salford.ac.uk
Appl Microbiol Biotechnol (2011) 90:18471868
DOI 10.1007/s00253-011-3213-7
tertiary treatment of wastewater (Araña et al. 2002),
purifying drinking water (Wei et al. 1994;Makowskiand
War d a s 2001), treatment of wash waters from vegetable
preparation (Selma et al. 2008) and in bioreactor design to
prevent biofilm formation (Shiraishi et al. 1999). TiO
2
-
coated filters have been used for the disinfection of air
(Jacoby et al. 1998; Goswami et al. 1997,1999;Linand
Li 2003a,b;Chanetal.2005). The advantage of using
photocatalysis along with conventional air filtration is that
the filters are also self-cleaning. TiO
2
has also been used
on a variety of other materials and applications (Table 1).
The potential for killing cancer cells has also been
evaluated (reviewed by Blake et al. 1999;Fujishimaet
al. 2000).
In recent years, there has been an almost exponential
increase in the number of publications referring to
photocatalytic disinfection (PCD), and the total number
of publications now exceeds 800 (Fig. 1). Some of the
early work was reviewed by Blake et al. (1999)and
sections on photocatalytic disinfection have been includ-
ed in several reviews (Mills and Le Hunte 1997;
Fujishima et al. 2000,2008;Carp et al. 2004); reviews
of the use in disinfection of water (McCullagh et al.
2007; Chong et al. 2010) and modelling of TiO
2
action
have been published (Dalrymple et al. 2010). In this
review, we explore the effects of photoactivated TiO
2
on
microorganisms.
Photocatalytic mechanism
For a more detailed discussion of the photochemistry, the
reader is directed to the excellent reviews by Mills and Le
Hunte (1997) and Hashimoto et al. (2005). TiO
2
is a
semiconductor. The adsorption of a photon with sufficient
energy by TiO
2
promotes electrons from the valence band
(e
vb
) to the conduction band (e
cb
), leaving a positively
charged hole in the valence band (h
vb+
; Eq. 1). The band
gap energy (energy required to promote an electron) of
anatase is approx. 3.2 eV, which effectively means that
photocatalysis can be activated by photons with a wave-
length of below approximately 385 nm (i.e. UVA). The
electrons are then free to migrate within the conduction
band. The holes may be filled by migration of an electron
from an adjacent molecule, leaving that with a hole, and the
process may be repeated. The electrons are then free to
migrate within the conduction band and the holes may be
filled by an electron from an adjacent molecule. This
process can be repeated. Thus, holes are also mobile.
Electrons and holes may recombine (bulk recombination) a
Uses and applications Publication
Building materials, e.g. concrete Guo et al. (2009)
Chen and Poon (2009)
Catheters to prevent urinary tract infections Ohko et al. (2001)
Yao et al. (2008c)
Coatings for bioactive surfaces Ueda et al. (2010)
Dental implants Suketa et al. (2005)
Mo et al. (2007)
Fabrics Gupta et al. (2008), Kangwansupamonkon
et al. (2009), Wu et al. (2009a,b),
Yuranova et al. (2006)
Food packaging films Chawengkijwanich and Hayata (2008)
Lancets Nakamura et al. (2007)
Metal pins used for skeletal traction Tsuang et al. (2008)
Orthodontic wires Chun et al. (2007)
Paint Allen et al. (2008)
Photocatalytic tiles for operating theatres Fujishima et al. (1997)
Plastics Paschoalino and Jardim (2008)
Cerrada et al. (2008)
Fujishima et al. (1997)
Protection of marble from microbial corrosion Poulios et al. (1999)
Surgical face masks Li et al. (2006)
Tent materials Nimittrakoolchai and Supothina (2008)
TiO
2
-coated wood Chen et al. (2009)
TiO
2
-containing paper Geng et al. (2008)
Tab l e 1 Some antimicrobial
applications of TiO
2
1848 Appl Microbiol Biotechnol (2011) 90:18471868
non-productive reaction, or, when they reach the surface,
react to give reactive oxygen species (ROS) such as O
2
(2) and OH (3). These in solution can react to give H
2
O
2
(4), further hydroxyl (5) and hydroperoxyl (6) radicals.
Reaction of the radicals with organic compounds results in
mineralisation (7). Bulk recombination reduces the effi-
ciency of the process, and indeed some workers have
applied an electric field to enhance charge separation,
properly termed photoelectrocatalysis (Harper et al. 2000).
TiO2þhn!ecbþhvb þð1Þ
O2þecb!O2ð2Þ
hvbþþH2O!OH þHþaq ð3Þ
OH þOH !H2O2ð4Þ
O2þH2O2!OH þOHþO2ð5Þ
O2þHþ!OOH ð6Þ
OH þOrganic þO2!CO2;H2Oð7Þ
There are three main polymorphs of TiO
2
: anatase, rutile
and brookite. The majority of studies show that anatase was
the most effective photocatalyst and that rutile was less
active; the differences are probably due to differences in the
extent of recombination of electron and hole between the
two forms (Miyagi et al. 2004). However, studies have
shown that mixtures of anatase and rutile were more
effective photocatalysts than 100% anatase (Miyagi et al.
2004) and were more efficient for killing coliphage MS2
(Sato and Taya 2006a). One active commercially available
preparations of TiO
2
is Degussa P25 (Degussa Ltd.,
Germany) which contains approx. 80% anatase and 20%
rutile. The increased activity is generally ascribed to
interactions between the two forms, reducing bulk recom-
bination. Brookite has been relatively little studied, but a
recent paper showed that a brookiteanatase mixture was
more active than anatase alone (Shah et al. 2008). A silver-
doped multiphase catalyst was shown to have increased
photocatalytic activity, but its antimicrobial activity was not
reported (Yu et al. 2005a). Indoor use of photocatalytic
disinfection is limited by the requirement for UVA
irradiation. Modified catalysts can reduce the band gap so
that visible light activates the photocatalysis. This has been
shown for TiO
2
combined with C, N and S, metals such as
Sn, Pd, and Cu, and dyes (Fujishima and Zhang 2006), but
activity is generally lower than when activated with UVA.
This area is currently the subject of much research.
The antimicrobial activity of UVA-activated TiO
2
was first
demonstrated by Matsunaga and coworkers (Matusunga
1985; Matsunaga et al. 1985). Since then, there have been
reports on the use of photocatalysis for the destruction of
bacteria, fungi, algae, protozoa and viruses as well as
microbial toxins. TiO
2
can be used in suspension in liquids
or immobilised on surfaces (Kikuchi et al. 1997; Sunada et
al. 1998;Kühnetal.2003;Yuetal.2003a; Brook et al.
2007; Yates et al. 2008a,b; Ditta et al. 2008). The ability to
eliminate microorganisms on photocatalytic self-cleaning/
self-disinfecting surfaces may provide a useful additional
mechanism in the control of transmission of diseases along
with conventional disinfection methods. Copper and silver
ions are well characterised for their antimicrobial activities
and can also enhance the photocatalytic activity. Combina-
tions of Cu
2+
and Ag
+
with TiO
2
therefore provide dual
function surfaces (see below).
Photocatalytic action on microorganisms
Photocatalysis has been shown to be capable of killing a
wide range of organisms including Gram-negative and
Gram-positive bacteria, including endospores, fungi, algae,
protozoa and viruses, and has also been shown to be
capable of inactivating prions (Paspaltsis et al. 2006).
Photocatalysis has also been shown to destroy microbial
toxins. As far as the authors are aware, only Acanthamoeba
cysts and Trichoderma asperellum coniodiospores have
been reported to be resistant (see below), but these have not
been extensively studied. The ability to kill all other groups
of microorganisms suggests that the surfaces have the
potential to be self-sterilising, particularly when combined
Fig. 1 Number of publications on photocatalytic disinfection
Appl Microbiol Biotechnol (2011) 90:18471868 1849
with Cu or Ag. However, for the present, it is correct to
refer to photocatalytic surfaces or suspensions as being self-
disinfecting rather than self-sterilising. Many studies have
used pure cultures, although there are reports of photo-
catalytic activity against mixed cultures (van Grieken et al.
2010) and of natural communities (Armon et al. 1998;
Araña et al. 2002; Cho et al. 2007a).
Gram-negative bacteria
The great majority of studies have been performed with
Escherichia coli, and there are far too many to give a
complete list in this review. Some examples of different
strains used and applications are shown in Table 2.
Examples of other Gram-negative bacteria that are suscep-
tible to PCD are shown in Table 3. They include cocci,
straight and curved rods, and filamentous forms from 19
different genera.
Gram-positive bacteria
Most studies showed that Gram-positive bacteria were more
resistant to photocatalytic disinfection than Gram-negative
bacteria (Kim et al. 2003; Liu and Yang 2003; Erkan et al.
2006; Pal et al. 2005,2007; Muszkat et al. 2005; Hu et al.
2007; Sheel et al. 2008; Skorb et al. 2008). The difference
is usually ascribed to the difference in cell wall structure
between Gram-positive and Gram-negative bacteria. Gram-
negative bacteria have a triple-layer cell wall with an inner
membrane (IM), a thin peptidoglycan layer (PG) and an
outer membrane (OM), whereas Gram-positive bacteria
have a thicker PG and no OM. However, a few studies
show that Gram-positive bacteria were more sensitive.
Lactobacillus was more sensitive than E. coli on a Pt-
doped TiO
2
catalyst (Matsunaga et al. 1985). methicillin-
resistant Staphylococcus aureus (MRSA) and E. coli were
more resistant than Micrococcus luteus (Kangwansupamon-
kon et al. 2009). Dunlop et al. (2010) showed that MRSA
were more sensitive than an extended spectrum β-
lactamase (ESBL)-producing E. coli strain, but less sensi-
tive than E. coli K12. Enterococcus faecalis was more
resistant than E. coli, but more sensitive than Pseudomonas
aeruginosa (Luo et al. 2008). Conversely, Kubacka et al.
(2008a) showed no difference in sensitivity between
clinical isolates of P. aeruginosa and E. faecalis. Van
Grieken et al. (2010) saw no difference in disinfection time
for E. coli and E. faecalis in natural waters, but E. faecalis
was more resistant in distilled water. These differences may
relate to different affinities for TiO
2
(close contact between
the cells and the TiO
2
is required for optimal activitysee
below) as well as cell wall structure.
Gram-positive bacteria that have been shown to be killed
by PCD are shown in Table 4and include species of 17
different genera, including aerobic and anaerobic endospore
formers. The endospores were uniformly more resistant
than the vegetative cells to PCD.
Fungi, algae and protozoa
Fungi, algae and protozoa that have been shown to be
susceptible to PCD are shown in Tables 5and 6. These
include 11 genera of filamentous fungi, 3 yeasts, 2
amoebae, 1 Apicomplexan, 1 diplomonad, 1 ciliate and 7
algae, including 1 diatom. Fungal spores were generally
more resistant than vegetative forms, and Trichoderma
harzianum spores in particular were resistant to killing
under the conditions tested (Giannantonio et al. 2009).
Cysts of Acanthamoeba showed only a 50% reduction
during the treatment time and may have been killed if the
treatment time had been extended (Sökmen et al. 2008).
Viruses
Viruses that have been shown to be killed by PCD are
shown in Table 7.
Most studies were on E. coli bacteriophages in suspen-
sion, which have been demonstrated for icosahedral ssRNA
viruses (MS2 and Qβ), filamentous ssRNA virus (fr),
ssDNA (phi-X174) and dsDNA viruses (λand T4). Other
bacteriophages include Salmonella typhimurium phage
PRD-1, Lactobacillus phage PL1 and an unspecified
Bacteroides fragilis phage. Mammalian viruses include
poliovirus 1, avian and human influenza viruses, and SARS
coronavirus (Table 7).
Bacterial toxins
Photocatalytic activity has been shown to be capable of
inactivating bacterial toxins including Gram-negative en-
dotoxin and algal and cyanobacterial toxins (Table 8).
Mechanism of killing of bacteria
The mode of action of photoactivated TiO
2
against bacteria
has been studied with both Gram-positive and Gram-
negative bacteria. The killing action was originally pro-
posed to be via depletion of coenzyme A by dimerization
and subsequent inhibition of respiration (Matsunaga et al.
1985,1988). However, there is overwhelming evidence that
the lethal action is due to membrane and cell wall damage.
These studies include microscopy, detection of lipid
peroxidation products, leakage of intercellular components,
e.g. cations, RNA and protein, permeability to low-
molecular-weight labels, e.g. o-nitrophenyl-galactoside
(ONPG), and spectroscopic studies.
1850 Appl Microbiol Biotechnol (2011) 90:18471868
Table 2 Examples of E. coli strains shown to be killed by photocatalytic disinfection on TiO
2
Organism Notes Reference
Escherichia coli WO
3
nanoparticle doped TiO
2
Tatsuma et al. (2003)
Escherichia coli Degussa P25 inpregnated cloth filter Vohra et al. (2006)
Escherichia coli ATCC 8739 Degussa P25 suspension Cho et al. (2005)
Escherichia coli ATCC 11229 Degussa P25 coated plexiglass Kühn et al. (2003)
Escherichia coli ATCC 13706 Degussa P25 immobilised on glass substrate Rodriguez et al. (2007)
Escherichia coli ATCC 10536 Ag and CuO TiO
2
hybrid catalysts Brook et al. (2007), Ditta
et al. (2008)
Escherichia coli ATCC 15153 Degussa P25 suspension Ibáñez et al. (2003)
Escherichia coli ATCC 23505 Rfc sputter was used to deposit films of
120 nm thickness onto glass and steel
substrates
Shieh et al. (2006)
Escherichia coli ATCC 23631 Degussa P25 applied to a plastic support Sichel et al. (2007a)
Escherichia coli ATCC 25922 Aldrich TiO
2
99.9% pure anatase Sökmen et al. (2001)
Escherichia coli ATCC 25922 Aerosol deposited nanocrystalline film Ryu et al. (2008)
Escherichia coli ATCC 27325 Degussa P25, suspension Huang et al. (2000)
Maness et al. (1999)
Escherichia coli ATCC-39713 Aerosil P25 suspension Matsunaga et al. (1995)
Escherichia coli CAH57 (ESBL) Thin film TiO
2
Dunlop et al. (2010)
Escherichia coli CCRC 10675 TiO
2
and ZnO suspension Liu and Yang (2003)
Escherichia coli CECT 101 Solgel microemulsion with an Ag overlayer Kubacka et al. (2008b)
Escherichia coli DH 4αDegussa P25 suspension Lan et al. (2007)
Escherichia coli DH5αFlow through reactor Belhácová et al. (1999)
Anatase thin film on glass Yu et al. (2002,2003b)
Escherichia coli HB101 Degussa P25 suspension Bekbölet and Araz (1996), Bekbölet (1997)
Escherichia coli HB101 Degussa P25 and Ag/P25 mixed suspension Coleman et al. (2005)
Escherichia coli IFO 3301 Silica coated lime glass plates dip coated
with TiO
2
Kikuchi et al. (1997)
Sunada et al. (2003b)
Escherichia coli IM303 TiO
2
coated air filter Sato et al. (2003)
Escherichia coli JM109 Anatase thin film on glass Yu et al. (2002)
Escherichia coli K12 ATCC10798 Degussa P25 suspension Duffy et al. (2004)
McLoughlin et al. (2004a,b)
Pal et al. (2007)
Escherichia coli K12 ATCC10798 Degussa P25 coated glass fibre air filter Pal et al. (2008)
Escherichia coli K12 (ATCC 23716) Degussa P25 Rincon and Pulgarin
(2003,2004a)
Escherichia coli K12 (ATCC 2363) Degussa P25 suspension Marugan et al. (2008)
Escherichia coli K12 Degussa P25 suspension Fernandez et al. (2005)
Gumy et al. (2006a,b)
Quisenberry et al. (2009)
Escherichia coli K12 Thin film TiO
2
Dunlop et al. (2002)
Escherichia coli MG1655 Degussa P25 suspension Gogniat and Dukan (2007)
Escherichia coli MM294 Degussa P25 suspension Kim et al. (2004)
Escherichia coli NCIMB-4481 Immobilised TiO
2
Butterfield et al. (1997)
Escherichia coli PHL1273 Degussa P25 suspension Benabbou et al. (2007)
Escherichia coli PHL1273 Degussa P25 and millennium PC500 Guillard et al. (2008)
Escherichia coli S1400/95 Degussa P25 suspension Robertson et al. (2005)
Escherichia coli 078 Thin films on glass substrate Choi et al. (2004)
Escherichia coli XL1 Blue MRF Anatase thin film on glass Yu et al. (2002)
Appl Microbiol Biotechnol (2011) 90:18471868 1851
Changes in cell permeability
Indirect evidence for membrane damage comes from
studies of leakage of cellular components. Saito et al.
(1992) showed that there was a rapid leakage of K
+
from
treated cells of Streptococcus sobrinus AHT which
occurred within 1 min of exposure and paralleled the loss
of viability. This was followed by a slower release of RNA
Table 3 Other Gram-negative bacteria shown to be killed by photocatalytic disinfection
Organism Notes Reference
Acinetobacter TiO
2
suspension Kashyout et al. (2006)
Acinetobacter baumanii C doped TiO
2
Cheng et al. (2009)
Aeromonas hydrophila AWWX1 TiO
2
pellets Kersters et al. (1998)
Anabaena TiO
2
-coated glass beads Kim and Lee (2005)
Bacteroides fragilis TiO
2
on orthopaedic implants Tsuang et al. (2008)
Coliforms Degussa P25 suspension Araña et al. (2002)
Coliforms Anatase suspension Watts et al. (1995)
Edwardsiella tarda Sol/gel-coated glass slides Cheng et al. (2008)
Enterobacter aerogenes Degussa P25 suspension Ibáñez et al. (2003)
Enterobacter cloacae SM1 Anatase, spin-coated glass plates Yao et al. (2007a)
Erwinia carotovora subsp. carotovora Degussa P25 suspension Muszkat et al. (2005)
Erwinia carotovora subsp. carotovora ZL1,
subsp. Carotovora 3, subsp. Carotovora 7
Anatase, spin-coated glass lates Yao et al. (2007a,b,2008a,b)
Faecal colifoms Anatase suspension Watts et al. (1995)
Flavobacterium sp. TiO
2
suspension and coated glass beads Cohen-Yaniv et al. (2008)
Fusobacterium nucleatum Thin film of anatase on titanium Suketa et al. (2005), Bai et al. (2007)
Legionella pneumophila ATCC 33153 Degussa P25 suspension Cheng et al. (2007)
Legionella pneumophila CCRC 16084 TiO
2
air filter + UVC Li et al. (2003)
Legionella pneumophila GIFU-9888 Ultrasonic activated suspension of TiO
2
Dadjour et al. (2005,2006)
Microcystis TiO
2
-coated glass beads Kim and Lee (2005)
Porphyromonas gingivalis TiO
2
sol/gel-coated orthodontic wires Chun et al. (2007)
Prevotella intermedia AghydroxyapatiteTiO
2
catalyst Mo et al. (2007)
Proteus vulgaris P25 (10% Pt),0.25 g/L slurry Matsunaga et al. (1985)
P. aeruginosa Surfaces Kühn et al. (2003)
P. aeruginosa environmental isolate Spray-coated soda lime glass and silica tubing Amezaga-Madrid et al. (2002,2003)
P. aeruginosa PA01 Thin film Gage et al. (2005)
P. aeruginosa Coated Al fibres Luo et al. (2008)
P. aeruginosa Catheters Yao et al. (2008c)
P. fluorescens R2F TiO
2
pellets Kersters et al. (1998)
P. fluorescens B22 Sigma-Aldrich TiO
2
thin films Skorb et al. (2008)
Pseudomonas sp. Anodized titanium alloy Muraleedharan et al. (2003)
Pseudomonas stutzeri NCIMB11358 TiO
2
suspension Biguzzi and Shama (1994)
Pseudomonas syringae pv tomato Degussa P25 suspension Muszkat et al. (2005)
Pseudomonas tolaasi TiO
2
suspension Sawada et al. (2005)
Salmonella choleraesuis Anatase suspension Kim et al. (2003)
Salmonella enteriditis Typhimurium Degussa P25 suspension Ibáñez et al. (2003), Cushnie et al. (2009)
Salmonella enteriditis Typhimurium TiO
2
film on quartz rods with UVC Cho et al. (2007a,b)
Serratia marcescens Degussa P25 suspension Block et al. (1997)
Goswami et al. (1999)
Shigella flexneri C-doped TiO
2
Cheng et al. (2009)
Vibrio parahaemolyticus Anatase suspension Kim et al. (2003)
Vibrio parahaemolyticus VP 144 Anatase TiO
2
dip coated on open porcelain filter cell Hara-Kudo et al. (2006)
Vibrio vulnificus TiO
2
-impregnated steel fibres for water treatment Song et al. (2008)
1852 Appl Microbiol Biotechnol (2011) 90:18471868
Table 4 Gram-positive bacteria shown to be killed by photocatalytic disinfection
Organism Notes Reference
Actinobacillus actinomycetemcomitans TiO
2
coating on titanium Suketa et al. (2005)
Actinomyces viscosus Kobe Steel TiO
2
99.98% anatase Nagame et al. (1989)
Bacillus cereus TiO
2
suspension Cho et al. (2007a)
Bacillus cereus spores TiO
2
suspension Armon et al. (2004)
Bacillus megaterium QM B1551 Colloidal suspension of TiO
2
Fu et al. (2005)
Bacillus pumilis spores ATCC 27142 TiO
2
anatase 99.9% slurry in Petri dish Pham et al. (1995,1997)
Bacillus sp. Degussa P-25 immobilised on Pyrex glass Rincón and Pulgarin (2005)
Bacillus subtilis vegetative cells and endospores Degussa P25-coated quartz discs Wolfrum et al. (2002)
Bacillus subtilis endospores Aluminium foil coated with TiO
2
Greist et al. (2002)
Bacillus thuringiensis 100% anatase thin film ± Pt doping Kozlova et al. (2010)
Clavibacter micheganensis Solar + H
2
O
2
Muszkat et al. (2005)
Clostridium difficile Evonik Aeroxide P25 thin fim Dunlop et al. (2010)
Clostridium perfringens spores NCIMB 6125 TiO
2
film on metal electrode Butterfield et al. (1997)
Clostridium perfringens spores Degussa P-25 + UVC Guimarães and Barretto (2003)
Deinococcus radiophilus TiO
2
suspension Laot et al. (1999)
Enterococcus (Streptococcus) faecalis Degussa P25 suspension Herrera Melián et al. (2000)
Enterococcus (Streptococcus) faecalis Immobilised TiO
2
Singh et al. (2005)
Enterococcus faecalis CECT 481 Degussa P25 suspension Vidal et al. (1999)
Enterococcus faecium Degussa P25-coated Plexiglass Kühn et al. (2003)
Enterococcus hirae TiO
2
on orthopaedic implants Tsuang et al. (2008)
Enterococcus sp. Degussa P-25 suspension Rincón and Pulgarin (2005)
Lactobacillus acidophilus Degussa P25 suspension Matsunaga et al. (1985), Choi et al. (2007a)
Lactobacillus helveticus CCRC 13936 TiO
2
suspension Liu and Yang (2003)
Lactococcus lactis 411 Sigma-Aldrich TiO
2
thin films Skorb et al. (2008)
Listeria monocytogenes TiO
2
(Yakuri Pure Chemical Company,
Japan) suspension
Kim et al. (2003)
Microbacterium sp. Microbacteriaceae str. W7 Degussa P25 immobilised on membrane Pal et al. (2007)
Micrococcus luteus Degussa P25 thick film Wolfrum et al. (2002)
Micrococcus lylae TiO
2
suspension Yu et al. (2005b)
MRSA Fe
3
O
4
TiO
2
core/shell magnetic nanoparticles
in suspension
Chen et al. (2008)
MRSA TiO
2
thin film on titanium Oka et al. (2008)
Mycobacterium smegmatis 100% anatase thin film ± Pt doping Kozlova et al. (2010)
Porphyromonas gingivalis TiO
2
thin film on steel and titanium Shiraishi et al. (1999)
Paenibacillus sp SAFN-007 Degussa P25 immobilised on membrane Pal et al. (2007)
Staphylococcus aureus Degussa P25 suspension Block et al. (1997)
Staphylococcus aureus TiO
2
thin film on steel and titanium Shiraishi et al. (1999)
Staphylococcus epidermidis NCTC11047 Ag-TiO
2
catalyst Sheel et al. (2008)
Staphylococcus saprophyticus Fe
3
O
4
TiO
2
core/shell magnetic nanoparticles
in suspension
Chen et al. (2008)
Streptococcus cricetus Kobe Steel TiO
2
99.98% anatase Nagame et al. (1989)
Streptococcus iniae Sol/gel-coated glass slides Cheng et al. (2008)
Streptococcus mutans TiO
2
sol/gel-coated orthodontic wires Chun et al. (2007)
Streptococcus mutans GS5, LM7, OMZ175 P25 aerosil, 70% anatase suspension Saito et al. (1992)
Streptococcus pyogenes ery
r
cam
r
Fe
3
O
4
TiO
2
core/shell magnetic nanoparticles
in suspension
Chen et al. (2008)
Streptococcus rattus FA-1 P25 aerosil, 70% anatase suspension Saito et al. (1992)
Streptococcus sobrinus AHT P25 suspension Saito et al. (1992)
Appl Microbiol Biotechnol (2011) 90:18471868 1853
and protein. Leakage of K
+
was also shown to parallel cell
death of E. coli (Hu et al. 2007; Kambala and Naidu 2009).
Huang et al. (2000) showed an initial increase in permeability
to small molecules such as ONPG which was followed by
leakage of large molecules such as β-D-galactosidase from
treated cells of E. coli, suggesting a progressive increase in
membrane permeability. Membrane damage has been shown
with cells labelled with the LIVE-DEAD® BacLight
Bacterial Viability Kit which uses the fluorescent dyes Cyto
9, which stains all cells green, and propidium iodide, which
only penetrates cells with damaged membranes and stains
cells red. Gogniat et al. (2006) showed that permeability
changes occurred in the membrane soon after attachment of E.
coli to the TiO
2
, and we have seen similar changes (Ditta and
Foster, unpublished). However, no damage was detected on
a visible light active PdO/TiON catalyst until the catalyst had
been irradiated (Wu et al. 2010b). SEM clearly showed
membrane damage after irradiation on this catalyst (Wu
et al. 2008,2009a,b,2010b;seeFig.2).
Microscopic changes during PCD
TEM images of treated cells of S. sobrinus showed clearly
that the cell wall was partially broken after cells had
undergone TiO
2
photocatalytic treatment for 60 min, with
further disruption after 120 min (Saito et al. 1992). The
authors suggested that cell death was caused by alterations
in cell permeability and the decomposition of the cell wall.
SEM images of S. aureus, MRSA, E. coli and M. luteus
showed morphological changes suggestive of cell wall
disruption after UVA irradiation on apatite-coated TiO
2
on
cotton fabrics (Kangwansupamonkon et al. 2009).
Damage to the cell wall of P. aeruginosa was shown by
SEM and TEM, which showed changes in membrane
structure such as bubble-like protuberances which ex-
pelled cellular material(Fig. 3; Amezaga-Madrid et al.
2002,2003). They suggested that leakage of cellular
material, and possibly abnormal cell division, was occur-
ring, although the bubbles may have been due to localised
Table 5 Fungi shown to be killed by photocatalytic disinfection
Organism Notes Reference
Aspergillus niger AS3315 Wood coated with TiO
2
Chen et al. (2009)
A. niger spores Degussa P25 film on quartz discs Wolfrum et al. (2002)
Aspergillus niger Thin films of TiO
2
on glass plates Erkan et al. (2006)
Candida albicans ATCC 10231 Degussa P25 suspension Lonnen et al. (2005)
Candida albicans TiO
2
-coated surfaces Kühn et al. (2003)
Candida famata TiO
2
coated catheters Yao et al. (2008c)
Candida vini TiO
2
thin film Veselá et al. (2008)
Cladobotryum varium TiO
2
suspension Sawada et al. (2005)
Cladosporium cladospoiroides TiO
2
-coated concrete Giannantonio et al. (2009)
Diaporthe actinidae TiO
2
immobilised on alumina spheres Hur et al. (2005)
Erysiphe cichoracearum Degussa P25 and Ce
3+
doped catalysts Lu et al. (2006)
Epicoccum nigrum TiO
2
coated concrete Giannantonio et al. (2009)
Fungi from spinach Plastic fruit containers with TiO
2
coating Koide and Nonami (2007)
Fusarium mucor TiO
2
-coated concrete Giannantonio et al. (2009)
Fusarium solani ATCC 36031 Degussa P25 suspension Lonnen et al. (2005)
Fusarium spp. (equisetii, oxypartan, anthophilum,
verticilloides, solani)
TiO
2
suspension, solar irradiation Sichel et al. (2007b,c)
Hanseula anomala CCY-138-30 TiO
2
- and Ag-doped Veselá et al. (2008)
Peronophythora litchii Degussa P25- and Ce
3+
-doped catalysts Lu et al. (2006)
Penicillium citrinum TiO
2
-coated air filter Lin and Li (2003a,b)
Penicillium expansum TiO
2
spray coated on polypropylene film Maneerat and Hayata (2006)
Penicillium oxalicum TiO
2
-coated concrete Giannantonio et al. (2009)
Pestaotiopsis maculans TiO
2
-coated concrete Giannantonio et al. (2009)
Saccharomyces cerevisiae Aerosil P25 suspension Matsunaga et al. (1985)
Sacchararomyces cerevisiae Pd-doped TiO
2
Erkan et al. (2006)
Spicellum roseum TiO
2
suspension Sawada et al. (2005)
Trichoderma asperellum TiO
2
-coated concrete Giannantonio et al. (2009)
Trichoderma harzianum TiO
2
suspension Sawada et al. (2005)
1854 Appl Microbiol Biotechnol (2011) 90:18471868
damage to the peptidoglycan layer allowing the inner
membrane to bulge through the peptidoglycan layer.
Sunada et al. (2003b) studied killing of E. coli on thin
films of TiO
2
and showed that the outer membrane was
damaged first and then the cytoplasmic membrane followed
by complete degradation. Photocatalytic killing occurred
without substantial visible degradation of peptidoglycan.
Atomic force microscopy measurements of cells on
Table 7 Viruses shown to be killed by photocatalytic disinfection
Host Virus Reference
Bacteroides fragilis Not specified Armon et al. (1998)
Birds Influenza (avian) A/H5N2 Guillard et al. (2008)
E. coli Coliphage Guimarães and Barretto (2003)
E. coli fr Gerrity et al. (2008)
E. coli T4 Ditta et al. (2008), Sheel et al. (2008)
E. coli λvir Yu et al. (2008)
E. coli λNM1149 Belhácová et al. (1999)
E. coli φX174 Gerrity et al. (2008)
E. coli MS2 Sjogren and Sierka (1994), Greist et al. (2002), Cho et al. (2004,2005),
Sato and Taya (2006a,b), Vohra et al. (2006), Gerrity et al. (2008)
E. coli QβLee et al. (1997), Otaki et al. (2000)
Human Hepatitis B virus surface antigen HBsAg Zan et al. (2007)
Human Influenza A/H1N1 Lin et al. (2006)
Human Influenza A/H3N2 Kozlova et al. (2010)
Human Norovirus Kato et al. (2005)
Human Poliovirus type 1 (ATCC VFR-192) Watts et al. (1995)
Human SARS coronavirus Han et al. (2004)
Human Vaccinia Kozlova et al. (2010)
Lactobacillus casei PL-1 Kakita et al. (1997,20000, Kashige et al. (2001)
Salmonella typhimurium PRD1 Gerrity et al. (2008)
Table 6 Protozoa and algae shown to be killed by photocatalytic disinfection
Organism Notes Reference
Protozoa
Acanthamoeba castellanii Degussa P25 suspension Sökmen et al. (2008)
Only 50% kill for cysts, trophozoites were sensitive
Acanthamoeba polyphaga environmental isolate Degussa P25 suspension Lonnen et al. (2005)
Cryptosporidium parvum UVC + TiO
2
Ryu et al. (2008)
Cryptosporidium parvum Solgel and thermal TiO
2
thin films applied to Petri
dish with a counter electrode Pt mesh
Curtis et al. (2002)
Giardia sp. Fibrous ceramic TiO
2
filter Navalon et al. (2009)
Giardia intestinalis cysts TiO
2
(anatase 99.9%) + Ag
+
Sökmen et al. (2008)
Giardia lamblia TiO
2
thin film catalyst Lee et al. (2004)
Tetrahymena pyriformis TiO
2
suspension Peng et al. (2010)
Algae
Amphidinium corterae AgTiO
2
catalyst Rodriguez-Gonzalez et al. (2010)
Chlorella vulgaris TiO
2
Pt catalyst Matsunaga et al. (1985)
Cladophora sp. TiO
2
-covered glass beads Peller et al. (2007)
Chroococcus sp. 27269 Anatase, fluorescent light Hong et al. (2005)
Melosira sp. TiO
2
-coated glass beads Kim and Lee (2005)
Oedogonium sp. TiO
2
-coated concrete Linkous et al. (2000)
Tetraselmis suecica AgTiO
2
catalyst Rodriguez-Gonzalez et al. (2010)
Appl Microbiol Biotechnol (2011) 90:18471868 1855
illuminated TiO
2
film showed that the outer membrane
decomposed first (Sunada et al. 2003b).
TEM images showed progressive destruction of E. coli
cells on Ag/AgBr/TiO
2
in suspension (Hu et al. 2006). Cell
membrane was degraded first followed by penetration of
TiO
2
particles into the cell and further damage. TEM of E.
coli showed that there were changes to the nucleoid which
became condensed, possibly due to leakage of ions out of
the cell (Chung et al. 2009).
TEM of thin sections of treated cells of E. coli on a
visible light-activated TiO
2
showed various degrees of cell
disruption including plasmolysis, intracellular vacuoles
ghost and cell debris (Vacaroiu et al. 2009). SEM and
TEM studies showed initial swelling and rough appearance
of the cells followed by scars and holes in the OM,
especially where the TiO
2
particles were in contact with the
cells. Erdem et al. (2006) showed damage by SEM on E.
coli and production of membrane breakdown products.
SEM has shown changes to the outer membrane of E. coli
(Li et al. 2008; Shah et al. 2008; Gartner et al. 2009). TEM
of thin sections of treated cells of E. coli on a visible light-
activated TiO
2
showed various degrees of cell disruption
including plasmolysis, intracellular vacuoles ghost and cell
debris (Vacaroiu et al. 2009).
Atomic force microscopy was used to show membrane
damage to E. coli,S. aureus and Diplococcus (Streptococ-
cus) pneumoniae on thin films of TiO
2
(Miron et al. 2005).
Changes to treated cells of S. aureus seen by TEM included
separation of cytoplasmic membrane from the peptidogly-
can layer (Chung et al. 2009). Distortion of treated cells of
both MRSA and methicillin-sensitive S. aureus was seen by
SEM on anatasebrookite (Shah et al. 2008), again
suggesting cell wall damage.
Lipid peroxidation by ROS was demonstrated by the
release of MDA as a breakdown product, and there was a
concurrent loss of membrane respiratory activity measured
by reduction of 2,3,5-triphenyltetrazolium chloride (Maness
et al. 1999). The demonstration of degradation of E. coli
endotoxin without substantial degradation of peptidoglycan
(Sunada et al. 1998) suggested that in the case of Gram-
negative bacteria, cell disruption occurred in the order of
OMPGIM. However, alterations to the peptidoglycan
layer may not be obvious in electron micrographs as
peptidoglycan is a highly cross-linked structure and
Fig. 2 Scanning electron micro-
graphs of photocatalytically
treated E. coli.aUntreated cells.
b,cCells after 240 min. dCells
after 30 min. Catalyst TiON thin
film. From Wu et al. (2010a,b)
Table 8 Microbial toxins inactivated by photocatalysis
Toxin Publication
Brevetoxins Khan et al. (2010)
Cylindrospermopsin Senogles et al. (2000,2001)
Lipopolysaccharide endotoxin Sunada et al. (1998)
Microcystin-LR Lawton et al. (1999,2003)
Cornish et al. (2000)
Feitz and Waite (2003)
Choi et al. (2007b)
Microcystins LR, YA and YR Shephard et al. (1998)
Nodularin Liu et al. (2005)
1856 Appl Microbiol Biotechnol (2011) 90:18471868
appreciable damage may occur without destruction of its
overall appearance. Localised destruction may occur where
TiO
2
particles are in contact with the cell. This may allow
protrusion of inner membrane through the cell wall as seen
by Amezaga-Madrid et al. (2003), followed by total rupture
of the cell wall.
Yao et al. (2007c) showed damage to cells of Erwinia
carotovora and DNA damage, which suggested that damage
to DNA was responsible for cell death. However, our own
data showed that there was no DNA damage seen by
COMET assay on plain TiO
2
surfaces even when 97% of
the cells were non-viable (Varghese and Foster, unpublished
data; Fig. 4). Damage to DNA does occur on TiO
2
(Wamer et
al. 1997; Hirakawa et al. 2004; Wang and Yang 2005; Wang
et al. 2005; Gogniat and Dukan 2007;Shenetal.2008;Yao
et al. 2007c; Yang and Wang 2008), but is probably a late
event after rupture of the membrane and cell death.
Killing of other microorganisms
There have been fewer studies on the mechanism of killing
of eukaryotes. Linkous et al. (2000) suggested that death of
the alga Oedogonium sp. was due to nonspecific break-
down of cellular structures. Microscopy has shown mem-
brane damage to the alga Chroococcus sp. (Hong et al.
2005). Light microscopy and SEM showed damage to cell
walls of Candida albicans suspended over a thin film of
TiO
2
(Kühn et al. 2003) and on TiO
2
-coated tissue
conditioner (Akiba et al. 2005). Cell wall and membrane
damage to cysts were seen with light microscopy of
photocatalytically treated Giardia lamblia (Sökmen et al.
2008). Membrane damage was also shown to occur on
Fig. 3 Transmission electron
micrographs of photocatalyti-
cally treated P. aeruginosa. Un-
treated cells transverse section
showing normal thickness and
shape cell wall (arrows). bd
Cells after 240 min treatment
showing abnormal wavy cell
wall (arrows)(b), cytoplasmic
material escaping from the cell
with damaged cell wall (arrows)
(c) and cell showing two
bubblesof cellular material
with cell wall (arrows)(d).
Catalyst TiO
2
thin film. Bar
marker= 200 nm. From
Amezaga-Madrid et al. (2003b)
Fig. 4 Comet assay of DNA from cells of E. coli on photoirradiated
TiO
2
and CuOTiO
2
catalysts. Upper photographs show fragmented
DNA entering the gel like the tail of a comet. The graph shows viability
(control, open circle;TiO
2
catalyst, closed circle;TiO
2
CuO dual
catalyst, downturned triangle) and tail moment (TM = Tail length × %
DNA in tail/100; Olive et al. 1990) as the measure of the extent of DNA
damage (TiO
2
catalyst, black square;TiO
2
CuO dual catalyst, gray
square) against time
Appl Microbiol Biotechnol (2011) 90:18471868 1857
treatment of the ciliate protozoan Tetrahymena pyriformis
(Peng et al. 2010).
Killing of Lactobacillus phage PL1 by thin films of TiO
2
suspended in liquid was reported to be via initial damage to
protein of the capsid by OH, followed by damage to the
phage DNA inside the particles (Kashige et al. 2001). SEM
showed ghost particles and empty heads. Damage to the H
and N projections of influenza virus A/H1N1 occurred on
PCD and was followed by total mineralisation (Lin et al.
2006).
Spectroscopic studies
The activity of titanium dioxide on isolated phospholipid
bilayers has been shown to result in disruption of the
bilayer structure using X-ray diffraction (Suwalsky et al.
2005), laser kinetic spectroscopy and attenuated total
reflection Fourier transform infrared spectroscopy (FTIR).
Disruption was shown to be due to lipid peroxidation (Kiwi
and Nadtochenko 2004; Nadtochenko et al. 2006) measured
by production of malondialdehyde (MDA). Lipid perox-
idation occurs when polyunsaturated fatty acids such as
linoleic acid are attacked by ROS (Kiwi and Nadtochenko
2005).
FTIR spectra of treated E. coli confirmed the production
of carboxylic acids such as MDA as products of membrane
degradation. MDA was further degraded by longer irradi-
ation times (Hu et al. 2007).
The electron decay on TiO
2
was studied using laser
kinetic spectroscopy in the presence of phosphatidyl
ethanolamine, lipopolysaccharide and E. coli (Nadtochenko
et al. 2006). Spectrosopic studies using FTIR spectrosco-
py suggested that organic components bound to the TiO
2
were directly oxidised by reduction of the electron holes
(Nadtochenko et al. 2006,2008). This work suggested that
direct oxidation of cellular components could occur
without the production of ROS, but only if cells were in
direct contact with the surface of the TiO
2
.Thisiswholly
consistent with the greater effectiveness of PCD when the
cells are in contact with the TiO
2
rather than in
suspension. Overall, the spectroscopic studies support the
light microscopic studies and confirm the order of
destruction being OMIMPG. Details of kinetic mod-
els of the killing mechanism are presented by Dalrymple et
al. (2010).
The role of ROS in killing of bacteria is summarised in
Fig. 5.
Role of ROS in the killing mechanism
Most studies show that ROS are responsible for the killing,
and various authors propose that OH are responsible
(Ireland et al. 1993; Kikuchi et al. 1997; Maness et al.
1999; Salih 2002; Cho et al. 2004,2005; Cho and Yoon
2008). Lipid peroxidation by ROS was demonstrated by the
release of MDA as a breakdown product, and there was a
concurrent loss of membrane respiratory activity measured
by reduction of 2,3,5-triphenyltetrazolium chloride (Maness
et al. 1999). The OH scavengers, dimethylsulphoxide and
cysteamine, eliminated the PCD activity of suspensions of
TiO
2
in water (Salih 2002). However, OH are short-lived
and will probably not diffuse further than 1 μm from the
surface of the TiO
2
, especially in the presence of organic
matter (Pryor 1986; Kikuchi et al. 1997). Kikuchi et al.
(1997) showed that killing of E. coli still occurred even
when the bacteria were separated from the surface by a
50-μm-thick porous membrane. However, the free radical
scavenger mannitol only inhibited killing without the
membrane, whereas catalase, which would degrade H
2
O
2
,
decreased killing both with and without the membrane.
This suggested that OH and H
2
O
2
were responsible for
killing close to the TiO
2
,withH
2
O
2
acting at a distance.
TheroleofotherROS,e.g.O
2
was not considered.
However, no killing was seen when cells were separated
from the TiO
2
by a dialysis membrane in a separate study
(Guillard et al. 2008). Hydrogen peroxide may act at a
distance if ferrous ions are present by producing OH via
the Fenton reaction (8and 9).
Fe3þþO2!Fe2þþO2ð8Þ
Fe2þþH2O2!Fe3þþOHþOH ð9Þ
Fig. 5 Role of ROS in photocatalytic killing of bacteria. Direct
oxidation of cell components can occur when cells are in direct
contact with the catalyst. Hydroxyl radicals and H
2
O
2
are involved
close to and distant from the catalyst, respectively. Furthermore, OH
can be generated from reduction of metal ions, e.g. Cu
2+
by H
2
O
2
(Sato and Taya 2006c)
1858 Appl Microbiol Biotechnol (2011) 90:18471868
A study of the roles of H
2
O
2
and OH in an immobilised
TiO
2
thin film reactor activated by UVC using electron spin
resonance suggested that OH were produced by direct
photolysis of H
2
O
2
as well as by Eqs. 3and 4(Yan et al.
2009).
A role for OH in sonocatalysis on TiO
2
(where the
energy to bridge the band gap is provided by sound waves)
was suggested by the work of Ogino et al. 2006 who
showed that the killing was inhibited by the OH scavenger
glutathione. Hydroxyl radicals produced by microwave
irradiation of TiO
2
were shown to enhance the killing of
E. coli (Takashima et al. 2007).
Hydroxyl radicals were shown to be the major ROS
involved in killing of C. parvum cysts, although other ROS
were also involved (Cho and Yoon 2008).
Studies with hydroxyl radical scavengers suggested that
inactivation of phage in suspensions of TiO
2
also occurred
due to bulk phase OH, whereas inactivation of bacteria
occurred with both bulk phase and surface OH (Cho et al.
2004,2005). The rate of inactivation of E. coli correlated
with the concentration of OH. A role for other ROS such
as H
2
O
2
and O
2
was also suggested.
Studies on superoxide dismutase (SOD)-defective E. coli
have shown that oxidative damage to the membrane
combined with the turgor pressure inside the cell initially
permeabilizes the cell envelope, allowing critical metabo-
lites to escape (Imlay and Fridovich 1992). Studies on
oxidative damage caused by TiO
2
in SOD mutants of E.
coli showed that the inactivation rate was inversely
proportional to SOD activity (Koizumi et al. 2002; Kim et
al. 2004).
Kinetic models and further details of the chemistry of the
killing mechanism are presented by Dalrymple et al.
(2010). The role of h
vb+
and ROS in killing of bacteria is
summarised in Fig. 5.
Importance of contact between bacteria and TiO
2
Many studies have shown that close contact between the
bacteria and the TiO
2
increases the extent of oxidative
damage. Studies on the disinfection of water have shown
that suspended TiO
2
is more active than TiO
2
immobilised
on surfaces, e.g. on thin films (Lee et al. 1997; Otaki et al.
2000;Sunetal.2003;Gumyetal.2006b; Marugan et al.
2006,2008; Cohen-Yaniv et al. 2008). This is probably
due to increased contact between the TiO
2
particles and
the bacterial cells in suspension as well as an increased
surface area for ROS production. A number of studies
confirm the importance of such contact (Horie et al.
1996a,b,1998;Gumyetal.2006a; Pratap Reddy et al.
2008; Caballero et al. 2009;Chengetal.2009). Co-
precipitation of cells and TiO
2
particles from suspension
by alum enhanced killing of E. coli (Salih 2004). Certain
ionic species have been shown to inhibit PCD, e.g. PO
43
(Araña et al. 2002; Koizumi and Taya 2002a,b;Christensen
et al. 2003; Rincón and Pulgarin 2004b; Egerton et al.
2006; Xiong et al. 2006; Marugan et al. 2008)andHCO
3
(Rincón and Pulgarin 2004b; Coleman et al. 2005;
Gogniatetal.2006),andtherateofadsorptionontothe
TiO
2
in the presence of different ions correlated with the
rate of inactivation, suggesting that the inhibition was due
to the prevention of binding of the bacteria to the TiO
2
particles. Light micrographs (Nadtochenko et al. 2005;
Gumy et al. 2006b; Gogniat et al. 2006) and electron
micrographs clearly show binding of the titania particles to
bacterial cells (Gumy et al. 2006a,b; Saito et al. 1992;
Cheng et al. 2007;Shahetal.2008). A micrograph
showing particles of TiO
2
attached to an E. coli cell is
showninFig.6. Contact with highly crystalline TiO
2
may
also cause physical damage to the cells (Liu et al. 2007c;
Caballero et al. 2009).
Although differences in binding of isolated O antigens to
TiO
2
have been shown (E. coli O8 and Citrobacter freundii
O antigens bound strongly to TiO
2
, whereas that from
Stenotrophomonas maltophilia had a low affinity for TiO
2
;
Jucker et al. 1997), differences in the susceptibility of
bacteria with different O antigens have not been studied.
Differences in the susceptibility of different strains of
Legionella pneumophila correlated with the amount of
saturated 16C branched chain fatty acids in the membrane
(Cheng et al. 2007). The more hydrophobic cells of
Fig. 6 Transmission electron micrograph of E. coli showing adhesion
betwen cells and TiO
2
in suspension. Catalyst Degussa P25 pH 6.0.
From Gumy et al. (2006b)
Appl Microbiol Biotechnol (2011) 90:18471868 1859
Flavobacterium sp. were more easily killed by PCD than E.
coli (Cohen-Yaniv et al. 2008), which may also have been
due to altered interactions with the TiO
2
.
In an attempt to increase contact between the cells,
Benabbou et al. (2007) studied the PCD of a strain of E.
coli overexpressing curli, pili, which enhance adhesion to
abiotic surfaces. However, the strain was more resistant
than the non-piliated control, and evidence of protein
degradation suggested that the pili were being degraded
before the membrane was damaged and therefore protected
the membrane from damage. The presence of extracellular
polysaccharides interfered with PCD of biofilms of P.
aeruginosa (Gage et al. 2005) and a natural biofilm (Liu et
al. 2007a), but killing was seen throughout a biofilm of
Staphylococcus epidermidis on a TiO
2
catalyst (Dunlop et
al. 2010). The different biofilms and catalysts may explain
these anomalies.
The inhibition of close contact between coliphage MS2
and TiO
2
by certain cations was shown by Koizumi and
Taya (2002a,b), and the rate of inactivation was propor-
tional to adsorption of the phage onto the TiO
2
. Sato and
Tay a (2006a,b) showed that the presence of organic
materials protected the phage by adsorbing to the surface
of the TiO
2
, preventing phage binding.
Cell mineralisation
Following initial cell damage and cell death, photocatalysis
has been shown to be capable of complete mineralisation of
bacteria on air filters using
14
C-labelled cells (Jacoby et al.
1998; Wolfrum et al. 2002) and for cells suspended in water
(Cooper et al. 1997;Sökmenetal.2001). The total
oxidation of Legionella by PCO was measured by total
organic carbon analysis (Cheng et al. 2007). An almost
complete degradation of E. coli was demonstrated on
prolonged treatment on a TiO
2
-activated charcoal catalyst
(Li et al. 2008). Nadtochenko et al. (2008) showed total
oxidation of cell organic matter by total internal reflection/
FTIR. Removal of microorganisms during regeneration of
photocatalytic TiO
2
-coated air filters by complete removal
of contaminants has also been shown by SEM (Goswami et
al. 1999; Ortiz López and Jacoby 2002). Penetration of
TiO
2
particles into the cells was shown using an Ag/AgBr/
TiO
2
catalyst (Hu et al. 2006).
A scheme for the killing mechanism of TiO
2
on
bacteria is shown in Fig. 7. We suggest that there may be
initial damage on contact between the cells and TiO
2
which affects membrane permeability, but is reversible.
This is followed by increased damage to all cell wall
layers, allowing leakage of small molecules such as ions.
Damage at this stage may be irreversible, and this
accompanies cell death. As the peptidoglycan is a highly
cross-linked molecule, damage may not be visibly evident
at this stage or may be localised if the TiO
2
is in contact
with the cells. Further membrane damage allows leakage
of higher molecular weight components such as proteins.
This may be followed by protrusion of the cytoplasmic
membrane into the surrounding medium through degraded
areas of the peptidoglycan and, eventually, lysis of the
cell. Degradation of the internal components of the cell
can then occur followed by complete mineralisation.
Dual function materials
Copper-deposited films show enhanced PCD activity
(Sunada et al. 2003a; Foster et al. 2010; Wu et al. 2010a;
Yates et al. 2008a,b). A clear synergy in photokilling of E.
coli on Cu-containing TiO
2
films was shown by Sato and
Taya (2006c) who showed that H
2
O
2
was produced from
the photocatalyst and Cu
2+
leached from the surface, but
neither reached high enough concentrations to kill the E.
coli directly. They suggested that the Cu
2+
was reduced to
Cu
+
(10) which reacted with the H
2
O
2
to produce OH via a
Fenton-type reaction (11), which was responsible for killing
cells in suspension and explaining why catalase reduced
this activity. Inclusion of Cu also gave higher PC activity,
hence the enhanced killing of cells bound to the TiO
2
.In
our own work, we have seen DNA damage when TiO
2
/
Fig. 7 Scheme for photocatalytic killing and destruction of bacteria
on TiO
2
. Contact between the cells and TiO
2
may affects membrane
permeability, but is reversible. This is followed by increased damage
to all cell wall layers, allowing leakage of small molecules such as
ions. Damage at this stage may be irreversible, and this accompanies
cell death. Furthermore, membrane damage allows leakage of higher
molecular weight components such as proteins, which may be
followed by protrusion of the cytoplasmic membrane into the
surrounding medium through degraded areas of the peptidoglycan
and lysis of the cell. Degradation of the internal components of the
cell then occurs, followed by complete mineralisation. The degrada-
tion process may occur progressively from the side of the cell in
contact with the catalyst
1860 Appl Microbiol Biotechnol (2011) 90:18471868
CuO surfaces were used (Fig. 4). Thus, Cu may also kill
cells by DNA damage as well as membrane damage. This is
consistent with the observed enhancement of damage to
DNA and protein caused by ROS (Cervantes-Cervantes et
al. 2005).
Cu2þþecb !Cuþð10Þ
H2O2þCuþ!HOþOH þCu2þð11Þ
Similar synergy has been shown between Ag and
TiO
2
. Ag enhances photocatalysis by enhancing charge
separation at the surface of the TiO
2
(Sökmen et al. 2001;
He et al. 2002; Hirakawa and Kamat 2005; Kubacka et al.
2008b;Liuetal.2007b; Musil et al. 2009). Ag
+
is
antimicrobial and can also enhance generation of ROS
(Eqs. 12,13 and 14).
AgþþO2!Ag0þO2ð12Þ
Ag0þO2!AgþþO22ð13Þ
H2O2þAg0!HOþOH þAgþð14Þ
Conclusions
Generation of ROS by photocatalysis on TiO
2
is capable of
killing a wide range of organisms including bacteria
endospores in water, in air and on surfaces, including
various materials. The technology has the potential to
provide a powerful weapon in the fight against transmission
of infectious diseases, particularly in view of the develop-
ment of visible light-activated catalysts.
One of the problems is that until relatively recently, there
has not been an accepted standard method for the testing of
the antimicrobial efficiency of photocatalytic processes. For
example, many different strains of E. coli have been used
(Table 2) with different growth media and test conditions.
This makes it very difficult to compare results from
different research groups. In the second part of this review,
we will investigate the evaluation of photocatalytic killing
activity.
Acknowledgements The authors are grateful to Professor David
Sheel and Dr. Heather Yates of the Centre for Physics and Materials
Research, University of Salford and to CVD Technologies Ltd. for
production of titania films and for their comments on the manuscript.
We would also like to thank Mr. Roger Bickley for his advice on the
early literature on TiO
2
. This work was partly supported by EEC
Framework 7 grant CP-IP 214134-2 N2P "Nano-to Production".
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