Cough-generated aerosols of Pseudomonas aeruginosa and other Gram-negative bacteria from patients with cystic fibrosis.
ABSTRACT Pseudomonas aeruginosa is the most common bacterial pathogen in patients with cystic fibrosis (CF). Current infection control guidelines aim to prevent transmission via contact and respiratory droplet routes and do not consider the possibility of airborne transmission. It was hypothesised that subjects with CF produce viable respirable bacterial aerosols with coughing.
A cross-sectional study was undertaken of 15 children and 13 adults with CF, 26 chronically infected with P aeruginosa. A cough aerosol sampling system enabled fractioning of respiratory particles of different sizes and culture of viable Gram-negative non-fermentative bacteria. Cough aerosols were collected during 5 min of voluntary coughing and during a sputum induction procedure when tolerated. Standardised quantitative culture and genotyping techniques were used.
P aeruginosa was isolated in cough aerosols of 25 subjects (89%), 22 of whom produced sputum samples. P aeruginosa from sputum and paired cough aerosols were indistinguishable by molecular typing. In four cases the same genotype was isolated from ambient room air. Approximately 70% of viable aerosols collected during voluntary coughing were of particles <or=3.3 microm aerodynamic diameter. P aeruginosa, Burkholderia cenocepacia, Stenotrophomonas maltophilia and Achromobacter xylosoxidans were cultivated from respiratory particles in this size range. Positive room air samples were associated with high total counts in cough aerosols (p = 0.003). The magnitude of cough aerosols was associated with higher forced expiratory volume in 1 s (r = 0.45, p = 0.02) and higher quantitative sputum culture results (r = 0.58, p = 0.008).
During coughing, patients with CF produce viable aerosols of P aeruginosa and other Gram-negative bacteria of respirable size range, suggesting the potential for airborne transmission.
- SourceAvailable from: John J Lipuma08/2014; 35(S1):S1-S67.
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ABSTRACT: Influenza remains a significant threat to public health, yet there is significant uncertainty about the routes of influenza transmission from an infectious source through the environment to a receptor, and their relative risks. Herein, data pertaining to factors that influence the environmental mediation of influenza transmission are critically reviewed, including: frequency, magnitude and size distribution and virus expiration, inactivation rates, environmental and self-contact rates, and viral transfer efficiencies during contacts. Where appropriate, two-stage Monte Carlo uncertainty analysis is used to characterize variability and uncertainty in the reported data. Significant uncertainties are present in most factors, due to: limitations in instrumentation or study realism; lack of documentation of data variability; or lack of study. These analyses, and future experimental work, will improve parameterization of influenza transmission and risk models, facilitating more robust characterization of the magnitude and uncertainty in infection risk.Risk Analysis 03/2011; 31(8):1226-42. · 2.28 Impact Factor
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ABSTRACT: Aerosol transmission routes of respiratory viruses have been classified by the WHO on the basis of equilibrium particle size. Droplet transmission is associated with particles sized >5 µm in diameter and airborne transmission is associated with particles sized ≤5 µm in diameter. Current infection control measures for respiratory viruses are directed at preventing droplet transmission, although epidemiological evidence suggests concurrent airborne transmission also occurs. Understanding the size of particles carrying viruses can be used to inform infection control procedures and therefore reduce virus transmission. This study determined the size of particles carrying respiratory viral RNA produced on coughing and breathing by 12 adults and 41 children with symptomatic respiratory infections. A modified six-stage Andersen Sampler collected expelled particles. Each stage was washed to recover samples for viral RNA extraction. Influenza A and B, parainfluenza 1, 2 and 3, respiratory syncytial virus (RSV), human metapneumovirus and human rhinoviruses (hRV) were detected using RT-PCR. On breathing, 58% of participants produced large particles (>5 µm) containing viral RNA and 80% produced small particles (≤5 µm) carrying viral RNA. On coughing, 57% of participants produced large particles containing viral RNA and 82% produced small particles containing viral RNA. Forty five percent of participants produced samples positive for hRV viral RNA and 26% of participants produced samples positive for viral RNA from parainfluenza viruses. This study demonstrates that individuals with symptomatic respiratory viral infections produce both large and small particles carrying viral RNA on coughing and breathing. J. Med. Virol. © 2013 Wiley Periodicals, Inc.Journal of Medical Virology 08/2013; · 2.37 Impact Factor
Cough-generated aerosols of Pseudomonas
aeruginosa and other Gram-negative bacteria from
patients with cystic fibrosis
C E Wainwright,1,2M W France,3P O’Rourke,4S Anuj,2,5T J Kidd,5,6,7M D Nissen,1,2,5,6
T P Sloots,2,5C Coulter,8Z Ristovski,9M Hargreaves,9B R Rose,10C Harbour,10
S C Bell,3,7K P Fennelly11
See Editorial, p 921
c Additional details are
published online only at http://
1Royal Children’s Hospital and
Health Service District, Brisbane,
Paediatrics and Child Health,
University of Queensland,
Medicine, The Prince Charles
Hospital, Brisbane, Australia;
4Queensland Institute of
Medical Research, Brisbane,
Australia;5Qpid Laboratory, Sir
Albert Sakzewski Virus Research
Centre, Herston, Australia;
Herston, Australia;7School of
Medicine, University of
Queensland, Brisbane, Australia;
8Infectious Diseases, The Prince
Charles Hospital, Brisbane,
University Technology, Brisbane,
Infectious Diseases, University
of Sydney, Sydney, Australia;
11UMDNJ-New Jersey Medical
School, New Jersey, USA
Dr C E Wainwright, Department
of Respiratory Medicine, Royal
Children’s Hospital, Brisbane
SCB and KPF contributed equally
to this study.
Received 15 December 2008
Accepted 15 June 2009
This paper is freely available
online under the BMJ Journals
unlocked scheme, see http://
Background: Pseudomonas aeruginosa is the most
common bacterial pathogen in patients with cystic fibrosis
(CF). Current infection control guidelines aim to prevent
transmission via contact and respiratory droplet routes
and do not consider the possibility of airborne transmis-
sion. It was hypothesised that subjects with CF produce
viable respirable bacterial aerosols with coughing.
Methods: A cross-sectional study was undertaken of 15
children and 13 adults with CF, 26 chronically infected
with P aeruginosa. A cough aerosol sampling system
enabled fractioning of respiratory particles of different
sizes and culture of viable Gram-negative non-fermenta-
tive bacteria. Cough aerosols were collected during 5 min
of voluntary coughing and during a sputum induction
procedure when tolerated. Standardised quantitative
culture and genotyping techniques were used.
Results: P aeruginosa was isolated in cough aerosols of
25 subjects (89%), 22 of whom produced sputum
samples. P aeruginosa from sputum and paired cough
aerosols were indistinguishable by molecular typing. In
four cases the same genotype was isolated from ambient
room air. Approximately 70% of viable aerosols collected
during voluntary coughing were of particles (3.3 mm
aerodynamic diameter. P aeruginosa, Burkholderia ceno-
cepacia, Stenotrophomonas maltophilia and
Achromobacter xylosoxidans were cultivated from
respiratory particles in this size range. Positive room air
samples were associated with high total counts in cough
aerosols (p=0.003). The magnitude of cough aerosols
was associated with higher forced expiratory volume in
1 s (r=0.45, p=0.02) and higher quantitative sputum
culture results (r=0.58, p=0.008).
Conclusion: During coughing, patients with CF produce
viable aerosols of P aeruginosa and other Gram-negative
bacteria of respirable size range, suggesting the potential
for airborne transmission.
Pseudomonas aeruginosa is the most common
bacterial pathogen in patients with cystic fibrosis
(CF).1The prevalence of chronic P aeruginosa
increases with age, and is a major predictor of
mortality and morbidity.2It is unclear to what
extent cross-infection of P aeruginosa between
patients with CF occurs.3 4While siblings with
CF can harbour the same P aeruginosa strain, it was
thought until recently that most patients had their
own individual strain acquired from the environ-
ment.5With the advent of molecular typing
methods there is now convincing evidence of
clonal P aeruginosa infection in patients attending
some paediatric and adult CF centres.6 7P aerugi-
nosa has been cultured from soap holders held at up
to 40 cm from the mouth of coughing patients
with CF, supporting large respiratory droplet
spread.8The exact mechanisms involved in the
spread of bacteria in CF clinics remain unclear.6 7
Two studies have isolated clonal P aeruginosa
during environmental air sampling up to 10 m
from patients with CF infected with clonal strains
while performing physiotherapy and lung function
testing, suggesting the potential of person-to-
person spread via the airborne route.9 10
Current guidelines for infection control for
patients with CF recommend only contact and
droplet precautions—that is, focusing on hand
hygiene and avoiding close contact between patients
withCFwho are advised tomaintain a distanceof at
least 1 m from other patients.11 12It is possible that
airborne transmission of P aeruginosa, Burkholderia
cepacia complex and other bacteria may occur in
addition to other modes of transmission.13The
relative contribution of the airborne route may be
opportunistic in nature and occur in certain circum-
stances, such as in enclosed spaces with favourable
ambient temperature and humidity as may occur in
hospital, clinic and congregate settings.
Particle size distribution of aerosols is a key
determinant for both deposition in the respiratory
tract and for the ability of particles to remain
airborne. To our knowledge, the particle size
distribution of aerosols from patients with CF has
never been reported. We hypothesised that, during
voluntary coughing and during sputum induction,
subjects with CF produce viable bacterial aerosols
that are respirable. To test this hypothesis we
modified a cough aerosol sampling system (CASS)
recently developed to measure cough-generated aero-
sols from patients with Mycobacterium tuberculosis.14
Our primary aim was to determine the concen-
tration and particle size distribution of cough
aerosols containing culturable P aeruginosa and
other Gram-negative bacteria from children and
adults with CF. We also sought to determine
whether concentrations of cough aerosols detected
were related to clinical parameters and clonality of
P aeruginosa strains.
Subjects with CF were recruited from both the
inpatient and outpatient services at the Royal
926Thorax 2009;64:926–931. doi:10.1136/thx.2008.112466
Children’s Hospital and The Prince Charles Hospital in
Brisbane, Australia. Inclusion criteria were age .9 years, a
confirmed diagnosis of CF and culture of P aeruginosa or B
cepacia complex from sputum on at least one occasion within
the previous 12 months. Exclusion criteria included known
pregnancy, pneumothorax within the previous 6 months,
history of cough syncope or vomiting associated with coughing.
After the first subject experienced recurrence of mild haemo-
ptysis during the cough study, we excluded those with
haemoptysis in the previous 7 days. Subjects were excluded
from hypertonic saline inhalation if there was a history of
intolerance of hypertonic saline, presence of asthma symptoms
or a forced expiratory volume in 1 s (FEV1) (40% predicted and
no previous trials of hypertonic saline. Subjects were asked to
withhold all nebulised therapy for 12 h prior to testing.
Cough aerosol sampling system (CASS)
The equipment used was a modification of that developed
previously.14In brief, a subject coughs through a mouthpiece
connected to afferent tubing into a chamber whereupon a
vacuum pump draws exhaled air and generated respiratory
particles through one of two Anderson six-stage impactors. Each
stage has 400 holes of decreasing diameter through which
appropriately-sized aerosolised particles will penetrate and
deposit on an agar plate. A ‘‘settle plate’’ of the same agar
was placed inside the chamber to capture larger droplets. Larger
particles (droplets) would be expected to deposit in the afferent
limb tubing, the settle plate and the walls of the chamber.
Additional details are provided in the online data supplement.
The Andersen impactors were loaded with agar plates at room
temperature. Thetubingfrom the vacuumpump wasattached to
the port for the first six-stage impactor in the CASS. After the
first session of coughing, the tubing was moved to the second
sampler. All unused ports were occluded with plastic tape.
Subjects were instructed to cough into the CASS as
frequently and as strongly as was comfortable for 5 min. At
the onset of coughing the timer (set for 5 min) controlling the
power to the vacuum pump was started. Cough strength was
assessed as strong, moderate or weak and cough frequency was
If hypertonic saline could be tolerated, the first sampling was
done during voluntary coughing and the second 5-minute
sample was collected during inhalation of 5 ml 4.5% saline
delivered bya handheldultrasonic
Allersearch distributed by Becton Dickinson, North Ryde,
Australia). Subjects were pretreated with albuterol metered
dose inhaler (88 mg per puff), 4 puffs via spacer (Volumatic,
Allen & Hanburys, UK). If hypertonic saline was not considered
safe, sampling was done with the subject using tidal breathing
for 5 min. Sputum samples were collected if produced.
Pulmonary function testing
Forced expiratory volume in 1 s (FEV1) and forced vital capacity
(FVC) were obtained according to standard guidelines prior to
the cough study.15Respiratory muscle strength was assessed
using maximum inspiratory pressure (MIP) and maximum
expiratory pressure (MEP) (Morgan Pmax) at the paediatric
centre and using a Micro Medical Respiratory Pressure Meter
(Micro Medical, Rochester, UK) at the adult centre.
Age, gender, presence of current exacerbation of disease, height,
weight and body mass index were recorded.
Room air sampling and air exchange
Using a centrifugal air sampler, two samples were obtained
before each cough aerosol study: one during the subjects’
performance of spirometry and one during the cough aerosol
study. The indoor air temperature and relative humidity were
measured with a thermohygrometer (Rotronic HygroPalm 2,
Rotronic Instrument Corp, Huntington, New York, USA) at the
beginning of each study. Effective air exchange rates in the
consultation rooms used for CASS testing and in the pulmonary
function laboratory at the adult centre were determined using
carbon dioxide as a tracer gas. Further details are provided in the
CASS aerosol samples and chamber settle plate
Cultures were performed using chocolate bacitracin (300 mg/ml)
agar in aerobic conditions at 35uC. After 48 h and 72 h
incubation, a colony forming unit (CFU) count was performed
on each plate including individual colonial P aeruginosa
morphotypes and the combined total CFU count of P aeruginosa
and other Gram-negative bacteria. Following presumptive
screening (characteristic colonial appearance, presence of oxi-
dase and growth at 42uC), the identity of each P aeruginosa
isolate was confirmed by species-specific oprL gene PCR.16Other
non-fermenting Gram-negative bacteria detected throughout
the study were identified using a combination of API 20NE
(bioMerieux), amplified rDNA restriction analysis (ARDRA)
and recA-based PCR analysis.17 18
Each Andersen sampler stage contains 400 holes and each
CFU is regarded as the result of an infectious particle within a
specific size range impacting on the agar. Colony counts
exceeding 400 have been interpreted in two ways: an accepted
‘‘positive-hole’’ correction model taking into account the
probability of multiple hits through each hole and a conserva-
tive model of a maximum count of 400 only.19 20The total sum
of P aeruginosa or B cepacia complex colonies counted (total
count) in all the Andersen stages for 5 min of voluntary
coughing and for 5 min hypertonic saline study or tidal
breathing was calculated, as was the sum of the colonies from
stages 4, 5 and 6 (,3.3 mm, termed ‘‘small aerosol fraction’’).
Sputum samples, afferent limb cultures and air samples
Standard quantitative culture methods were used.21For air
samples and afferent limb cultures, only Gram-negative non-
fermentative bacteria were assessed. Isolates were identified as
above with molecular strain typing of P aeruginosa isolates.
Further details are provided in the online data supplement.
Analysis of data
Counts for individual components and the totals for Andersen
stages 1–6 (total) and for Andersen stages 4–6 (small fraction)
were logarithmically transformed before analysis to correct for
skewness. Means and 95% confidence limits (95% CI) were back
transformed from log to linear scales for presentation. The
paired differences between counts during voluntary coughing
and each of the hypertonic and tidal breathing studies were
analysed by paired t tests and mean differences were also back
transformed from log to linear scales to calculate the ratios of
counts during voluntary coughing and each of the hypertonic
Thorax 2009;64:926–931. doi:10.1136/thx.2008.112466 927
and tidal breathing studies. Correlation coefficients were
estimated between logarithmically transformed total counts
and clinical and demographic factors where available for all
subjects. The Fisher exact test was used for the association
between positive air samples and high total counts. All reported
p values are two-sided. Linear regression was used to estimate
the slope of the relationship between FEV1and total count. All
analyses were performed with SPSS software Version 15.
Twenty-eight subjects (15 children, 13 adults) were consecu-
tively recruited and completed 5 min of voluntary coughing.
Twenty subjects were administered nebulised hypertonic saline
and seven subjects had measurements during tidal breathing.
One subject performed the voluntary cough only. Thirteen
subjects were studied during a pulmonary exacerbation (table 1).
In the 12 months before the study, 27 subjects had sputum that
cultured positive for P aeruginosa and one subject had cultured B
cenocepacia (table 2). Of the 27 patients with P aeruginosa
infection, all adults (n=12) and 14 children had chronic
infection based on the Leeds criteria.22One child had recently
cleared a new infection with P aeruginosa following an
eradication course of antibiotic therapy and cultured normal
respiratory flora from a sputum sample collected on the day of
testing. The patient with B cenocepacia had chronic infection
based on the Leeds criteria (table 2).22On the study day, 23
subjects provided expectorated sputum samples. Of these, one
subject grew B cenocepacia as expected and P aeruginosa was
cultured in 21. In six subjects Staphylococcus aureus was cultured
and in two methicillin-resistant S aureus was cultured. Other
organisms cultured from sputum included a-haemolytic strep-
tococci, Aspergillus species and yeasts. Molecular strain typing
demonstrated a common clone corresponding to the previously
described Australian Epidemic Strain 2 (AES2) in 16 subjects (6
adults, 10 children) and 5 had unique strains (4 adults, 1 child)
Of the 28 subjects, 25 had cough aerosols that grew P aeruginosa.
One subject cultured P aeruginosa from cough aerosols only with
the hypertonic saline study and not from voluntary coughing.
One subject cultured B cenocepacia from cough aerosols. Two
subjects had no Gram-negative bacteria cultured from cough
aerosols. In five subjects with cough aerosols with P aeruginosa,
(Stenotrophomonas maltophilia in four and Achromobacter xylosox-
idans in one, table 2). Two of the subjects who co-cultured S
maltophilia did not produce sputum and sputum culture was
negative for S maltophilia for the other two. Three of the four
subjects cultured S maltophilia intermittently from sputum at
other times. The subject with A xylosoxidans in the cough
aerosol culture did not culture the organism in the sputum
sample on this occasion, although the subject was known to be
chronically infected with this organism which had been
cultured repeatedly from previous sputum samples.
The corrected total count of CFUs obtained from generated
aerosols varied widely among subjects and was log normally
distributed (voluntary cough: range 0–13 485 CFU, fig 1). All
subjects but one who cultured P aeruginosa in sputum also
cultured P aeruginosa of identical genotype in the CASS cough
aerosols. The total count from sputum correlated with the total
corrected count for voluntary coughing from the aerosols
(r=0.58, p=0.008). Three of the seven subjects who had tidal
breathing studies had positive CASS aerosol cultures, with P
aeruginosa cultured in low numbers (total aerosol counts from
tidal breathing 1, 5 and 137 CFU).
Settle plate and air sampling microbiology
The chamber settle plate and afferent limb equipment was not
changed between the two components of the study for
individual subjects with quantitative culture, reflecting large
droplet deposition for both components of the study combined.
The mean total count for the settle plate was 6 CFU (95% CI 3
wash fluid (95% CI 10 to 303).
Mean (SE) air exchange rates ranged between 9.77 (0.06) and
19.40 (0.70) exchanges per hour in the testing rooms. A total of
101 air samples were collected before and during testing. Sixteen
samples cultured unique strains of P aeruginosa during testing of
14 patients. The unique strains isolated did not match any
sputum or CASS isolates. Five air samples cultured AES2 strain
during testing of four subjects with AES2 strain of P aeruginosa.
For these four subjects, sputum, cough aerosol and air samples
all cultivated the same strain. Three of the AES2 positive air
samples were collected during pulmonary function testing and
two during background testing in the CASS study rooms.
Positive air samples were associated with a high concentration
in cough aerosols. If only subjects with AES2 were considered,
four out of five subjects with total cough aerosol counts
Demographic and baseline clinical factors of study subjects
Median (range) age (years)
Mean (SD) BMI (kg/m2)
Mean (SD) Z score for weight
Mean (SD) Z score for height
Mean (SD) FEV1(% predicted)
Mean (SD) FVC (% predicted)
Mean (SD) peak flow (l/s)
Mean (SD) MIP (cm H2O)
Mean (SD) MEP (cm H2O)
BMI, body mass index; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; MEP, maximum expiratory pressure;
MIP, maximum inspiratory pressure.
928Thorax 2009;64:926–931. doi:10.1136/thx.2008.112466
.1000 CFU had positive air samples and no subjects out of the
10 with lower total CFU counts had positive air samples
(p=0.003). Temperature and humidity did not vary signifi-
cantly between study sites or study days at each site (data not
CASS microbiology: voluntary cough, hypertonic cough and tidal
The infective particle size distribution of cough aerosols of P
aeruginosa or B cenocepacia during voluntary coughing is shown
in fig 2. Using the corrected total counts, 71.8% of particles
(95% CI 66.8% to 76.8%) containing culturable aerosol isolated
from voluntary coughing were on Andersen stages 4, 5 and 6 of
the Andersen samplers (small aerosol fraction (3.3 mm). The
conservative model gave similar results with 69.9% (95% CI
64.6% to 75.4%) in the small aerosol fraction. Mean total
corrected counts were much lower during tidal breathing (2,
95% CI 20.5 to 15) than during voluntary coughing (85, 95% CI
28 to 238; p,0.001) or hypertonic saline (68, 95% CI 21 to 215).
There was no significant difference in total corrected counts
between voluntary coughing and hypertonic saline (p=0.12).
The pattern of differences was unaffected by using the
conservative model (data not shown).
CASS microbiology: clinical correlates
FEV1 correlated with the total corrected count from the
voluntary cough aerosol (r=0.45, p=0.019; fig 3) and also
with the corrected small aerosol fraction (r=0.45, p=0.018).
Similarly, FEV1 correlated with the conservative total count
(r=0.39, p=0.044) and conservative small aerosol fraction
(r=0.39, p=0.047) during voluntary coughing. There was no
significant association between cough aerosol CFU counts and
any other clinical factor including gender, age, testing at
paediatric or adult centre, current exacerbation status, FVC,
MIP, MEP, percentage predicted FEV1, presence of clonal P
aeruginosa, quality or actual number of coughs counted (data
not shown). There was a trend for an association of peak
expiratory flow (r=0.36, p=0.079) and of body mass index
(r=0.37, p=0.058) with total corrected count for voluntary
This is the first study to report the magnitude, variability and
particle size distribution of culturable aerosols of Gram-negative
bacteria produced by coughing in patients with CF. Although
there is evidence of culturable Gram-negative bacteria in the
large droplets within the afferent tubing and settle plates in the
cough chamber, a large proportion of culturable particles were
found to be in a size range that is likely to deposit in the lower
respiratory tract. Genetically indistinguishable bacteria were
identified in expectorated sputum and in the cough-generated
aerosols, and in four experiments the same organisms were also
isolated from the ambient room air. This supports the assertion
that the sources of the bacteria are the patients rather than the
hospital or nearby environment.
Aerosolisation of respiratory tract particles during coughing
and sneezing and even during tidal breathing is a well
recognised phenomenon associated with the spread of many
infections including measles, influenza and tuberculosis.24–26The
majority of respiratory pathogens have been thought to be
spread by large droplets that settle within an approximate 1 m
range of an individual, providing a low risk of airborne infection.
Infection control practices for most CF centres reflect recently
published infection control guidelines suggesting that patients
should maintain a distance of at least 1 m to reduce the risk of
cross-infection.12The risk of acquisition of infection from
respiratory aerosols is complex and probably relates to the
pathogen type, concentration of the organism in the aerosol, the
susceptibility of exposed individuals and the environment (air
movement, relative humidity, temperature, etc). Limited studies
have examined particle size distribution of respiratory aerosols
and most have reported large droplet formation, predominantly
particles with a diameter of .8 mm.27 28More recently, Papineni
and Rosenthal reported that 85% of particles were ,1 mm and
that coughing produced more aerosol particles than did breath-
ing or talking.29The first published study to use a CASS
examined patients with tuberculosis and, like our study, found
that most of the respiratory particles were ,3.3 mm.14Our
study shows that patients with CF produce culturable aerosols
in a wide range of particle sizes including both respiratory
Microbiology of sputum and CASS samples
Infection status prior to study
Voluntary cough (n=28)/
hypertonic saline (n=20)
P aeruginosa not isolated
P aeruginosa isolated
Cleared P aeruginosa (n=1)
Chronic P aeruginosa (n=12 adults,
P aeruginosa (unique)
P aeruginosa (AES2)
Chronic B cenocepacia (n=1)
3 21 25*
1 B cenocepacia isolated
AES2, Australian Epidemic Strain 2; CASS, cough aerosol sampling system.
*Five subjects also cultured additional Gram-negative bacteria in cough aerosols (4 Stenotrophomonas maltophilia and 1 Achromobacter xylosoxidans).
B, subject with Burkholderia cenocepacia; CFU, colony forming unit;
+, positive ambient air samples isolated.
Distribution of total corrected voluntary cough aerosols.
Thorax 2009;64:926–931. doi:10.1136/thx.2008.112466929
droplets and infectious droplet nuclei. We have shown this
predominantly for clonal P aeruginosa and, in a small number of
patients, for other non-fermentative Gram-negative organisms
including B cenocepacia. We do not know the ideal site of
deposition in the respiratory tract for P aeruginosa to establish
infection in patients with CF, and either large or small droplets
or both may be important in the pathogenesis.
Until relatively recently cross-infection with P aeruginosa was
believed to be uncommon and limited to siblings with CF and
cohorts attending the same residential CF camps.30The
identification of genetically related P aeruginosa strains in many
CF centres in the UK, Europe and Australia has suggested cross-
infection between patients.23 31Clonal strains of P aeruginosa
contaminating the air close to patients with the same infection
during physiotherapy or lung function testing have been
reported.9Our results provide further evidence that cross-
infection may result from direct inhalation of aerosolised
This study demonstrates widely varying bacterial counts in
cough aerosols with a log normal distribution. Such a
distribution is consistent with descriptions of highly infectious
patients as ‘‘disseminators’’ (eg, in tuberculosis) or ‘‘super-
spreaders’’ (eg, in severe acute respiratory syndrome).32 33
Factors influencing the extent of isolation of Gram-negative
bacteria in cough aerosols are likely to be complex, including
both host factors and bacterial factors such as enhanced survival
in air. Our data show that the concentration of bacteria in the
sputum and the forced expiratory flow rates were related to
cough aerosol concentration, with a trend for association with
higher peak flow and higher body mass index. These data
suggest that patients with milder lung disease, perhaps as a
result of stronger cough, may have an increased risk of
producing infectious aerosols. This warrants further investiga-
tion as the improvement in clinical outcomes in patients with
CF may potentially increase the risk of spread of clonal strains
of P aeruginosa and other Gram-negative bacteria.
The only air samples that cultured P aeruginosa which
matched clinical samples from sputum or CASS samples were
clonal AES2 strains. Given that patients who had positive air
samples also had high total aerosol counts, we were unable to
determine if the density of infection on its own—or whether, in
addition, the nature of the specific infection—contributed to
the positive air samples. The source of the P aeruginosa air
isolates that did not match any clinical samples is unknown and
environmental sampling of surfaces was not undertaken. It is
possible that environmental sources such as sinks may have
been involved as hand washing occurred during testing. The
measured air exchange rates in the study rooms provide an
important perspective as air sampling was performed for 12 min
on each occasion and, during this period, 2–4 complete air
exchanges occurred. Higher rates and density of positive air
samples may be anticipated in less well ventilated rooms.
Nebulised hypertonic saline is now recognised as improving
mucociliary clearance.34We sought to determine if hypertonic
saline-induced cough further enhanced the production of
bacterial aerosols, but we found similar results to those seen
with voluntary coughing and much greater than those obtained
during tidal breathing. Treatments such as physiotherapy,
mucolytic agents and even nebulised antibiotics which can
induce coughing are likely to result in similar cough-induced
aerosols as with voluntary coughing. Although only seen in
three of seven patients tested during tidal breathing, the
presence of P aeruginosa in the cough aerosols and in the small
aerosol fraction from two patients warrants further study as
any reassessment of infection control recommendations to
incorporate the role of airborne transmission may not only
apply to coughing patients.
The significance of aerosol-positive sputum-negative results
for isolation in low numbers of S maltophila (n=2) and A
xyloxidans (n=1) is uncertain, but it is possible that separating
respiratory particles by size negates the obscuring of individual
colony morphotypes by other flora which may occur with direct
There are several limitations to this study. First, the study
was not powered to examine the effects of many of the clinical
variables such as exacerbations or strain of P aeruginosa on the
production of cough aerosols. In particular, there were few
patients with unique strains of P aeruginosa and none who
produced high concentrations of cough aerosols. The association
between a specific strain and obtaining a positive air sample
could not therefore be determined. Second, the media in the
Andersen plates was selective for Gram-negative organisms and
thus it is not possible to generalise these results to patients with
CF infected with Gram-positive bacteria, mycobacteria or fungi.
Third, we did not perform reproducibility or efficiency studies
of the CASS. Fourth, the studies of tidal breathing were in a
aerosol counts in colony forming units (CFU) with 95% confidence
intervals during voluntary coughing according to Andersen stage.
Particle size distribution of logarithmic corrected total cough
with logarithmic total corrected count from cough aerosols during
voluntary coughing. CFU, colony forming unit; +, positive ambient air
Correlation of baseline forced expiratory volume in 1 s (FEV1)
930Thorax 2009;64:926–931. doi:10.1136/thx.2008.112466
select group of patients who did not undertake hypertonic
saline-induced cough studies and further work is required to
evaluate the extent to which tidal breathing is associated with
the generation of potentially infective particles. Finally, while
this study provides evidence that patients with CF and Gram-
negative infection can produce potentially infectious cough
aerosols, we cannot draw conclusions about transmission to
susceptible individuals. A recent study examined the survival of
P aeruginosa in vitro and found bacterial survival, at least for a
limited time period of ,90 s, to be favoured by lower
temperature and mucoid phenotype.35While providing further
evidence that airborne transmission is plausible, transmission by
this route is yet to be proved beyond doubt.
In conclusion, this study shows that patients with CF
infected with P aeruginosa can produce respirable infectious
cough aerosols in a wide range of concentrations of a log normal
distribution. We also detected other non-fermenting Gram-
negative bacteria including B cenocepacia in the small aerosol
fraction, suggesting that airborne transmission of such organ-
isms is biologically plausible. Further studies of potential
airborne transmission of bacterial pathogens in patients with
CF are warranted to provide a scientific basis for infection
control recommendations to prevent the spread of multidrug-
resistant or clonal strains of P aeruginosa and other Gram-
negative bacteria in this patient population.
Funding: Supported by Royal Children’s Hospital Foundation, Brisbane, Australian
Cystic Fibrosis Research Trust and a University of Queensland Travel Grant Award.
Competing interests: None.
Ethics approval: The study was approved by the ethics committees of both CF
centres and the University of Queensland and the Institutional Review Board of
UMDNJ. Informed consent was obtained from all subjects and in addition from the
parents or guardians of all young people under 18 years of age.
Provenance and peer review: Not commissioned; externally peer reviewed.
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