A prospective survey of Pseudomonas aeruginosa colonization and infection in the intensive care unit
Cohen et al. Antimicrobial Resistance and Infection Control
A prospective survey of Pseudomonas aeruginosa colonization and infection in the intensive care unit
Regev Cohen 0 1
Ronen Ben Ami
0 Ruth and Bruce Rappaport Faculty of Medicine , Technion, Haifa , Israel
1 Head of Infectious diseases unit, Sanz Medical Center, Laniado hospital , Neytanya , Israel
Background: Pseudomonas aeruginosa (PA) surveillance may improve empiric antimicrobial therapy, since colonizing strains frequently cause infections. This colonization may be 'endogenous' or 'exogenous', and the source determines infection control measures. We prospectively investigated the sources of PA, the clinical impact of PA colonization upon admission and the dynamics of colonization at different body sites throughout the intensive care unit stay. Methods: Intensive care patients were screened on admission and weekly from the pharynx, endotracheal aspirate, rectum and urine. Molecular typing was performed using Enterobacterial Repetitive Intergenic Consensus Polymerase Chain reaction (ERIC-PCR). Results: Between November 2014 and January 2015, 34 patients were included. Thirteen (38%) were colonized on admission, and were at a higher risk for PA-related clinical infection (Hazard Ratio = 14.6, p = 0.0002). Strains were often patient-specific, site-specific and site-persistent. Sixteen out of 17 (94%) clinical isolates were identical to strains found concurrently or previously on screening cultures from the same patient, and none were unique. Ventilator associated pneumonia-related strains were identical to endotracheal aspirates and pharynx screening (87-75% of cases). No clinical case was found among patients with repeated negative screening. Conclusion: PA origin in this non-outbreak setting was mainly 'endogenous' and PA-strains were generally patientand site-specific, especially in the gastrointestinal tract. While prediction of ventilator associated pneumonia-related PA-strain by screening was fair, the negative predictive value of screening was very high.
Pseudomonas aeruginosa; Endogenous; Intensive Care unit; Surveillance; ERIC-PCR; Infection control
Pseudomonas aeruginosa (PA) is a leading cause of
healthcare-associated infections in intensive care units
(ICUs), mainly ventilator-associated pneumonia (VAP),
central line-associated bloodstream infection (CLABSI)
and surgical site infection (SSI). PA colonization typically
precedes infection . Colonization may be endogenous,
arising from the patient’s own microbial repertoire [2–4],
or exogenous if acquired from the hospital environment
or by cross-infection from other patients [5–11]. This
distinction has implications for the means needed for
infection control . Specifically, water fixtures and
piping colonized with PA have been implicated as
environmental reservoirs during outbreaks in ICUs [13, 14].
Use of point-of-care water filters was shown to effectively
reduce PA infections in surgical ICUs .
In previous work, we studied the genetic relatedness
of PA strains isolated from ventilated patients and
hospital faucets. We found a clear temporal and spatial
relation between patient and environmental strains .
In the present study we aimed to prospectively determine
the clinical impact of PA colonization on admission to the
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
ICU and the dynamics of colonization at different body
sites throughout the ICU stay.
The study was conducted at the Sanz Medical Center, a
400-bed community hospital located in central Israel.
The adult ICU is a combined medical and surgical unit
with ~250 admissions (~2000 patient days) per year. The
ICU is located in one room with 6 beds with no physical
barrier between patients. ICU staff members were
instructed to use tap water for patients bathing only,
whereas sterile water was used for drinking, moistening
and mouth treatment. All faucet aerators were
dismantled 23 months prior to initiation of this study .
All patients hospitalized in the ICU from November
2014 to January 2015 were included and underwent
prospective weekly PA surveillance cultures, as detailed
below. Patients staying in the unit for less than 72 h
were excluded from the analysis. The primary endpoint
was the development of clinical infection due to PA,
defined according to CDC/NHSN surveillance definitions
of healthcare-associated infections  and American
Thoracic Society criteria for VAP . Secondary aims
were identifying risk factors for PA colonization on
admission and during ICU stay, clonal analysis of strains
at each body site during the ICU stay and the
concordance between the strains related to infection and those
detected on weekly screening.
This study was approved by the hospital institutional
review board committee (0033-14-LND). As the study
was aimed for infection control and patient safety
purposes, the requirement for informed consent was waived.
The following baseline characteristics were collected from
electronic medical records: age, sex, place of residence
(home or long-term care facility [LCTF]), comorbidities,
hospitalization within 90 days prior to admission, surgery
in the previous 30 days and duration of hospitalization
before admission to the ICU. We recorded the dates of
hospitalization, ICU admission and discharge, Acute
Physiology and Chronic Health Evaluation (APACHE) II
on ICU admission, length of stay in the ICU and in the
hospital in general, ventilation duration, tracheostomy
date, death in ICU and within 90 days of hospitalization
and major diagnoses in ICU. We also documented the
dates and sources of PA cultures (screening and clinical),
and PA related diagnoses of VAP, CLABSI, SSI and
catheter-associated urinary tract infection (CAUTI).
Each patient was surveyed using standard bacterial
cultures on admission (within the first 72 h) and then
once a week until discharge. Cultures were collected using
swabs (Transsystem, Copan®, California, USA) from 4
sites: throat, rectum, endotracheal aspirate (EA) for
ventilated patients, and urine, and transferred to the laboratory
within 30 min. Faucet cultures were collected weekly from
the distal part of the faucet using a bacterial swab.
Swabs were inoculated on tryptic soy blood agar,
chocolate agar, MacConkey agar and fluid thioglycoate
medium (Hy-labs®, Rehovot, Israel). Cultures were
incubated at 35° C overnight. Broth samples were
subcultured to the same media plates whenever no growth
was detected on the initial plates.
Bacterial identification and antimicrobial susceptibility
testing were done using the VITEK 2 system (Biomerieux,
Marcy l’Etoile, France) and interpreted according to CLSI
PA isolates were stored at -70oc for molecular analysis.
Molecular typing was done by enterobacterial repetitive
intergenic consensus (ERIC)-PCR. DNA was extracted
using the easyMag® system (BioMerieux) and ERIC-PCR
was performed as previously described . PCR products
were resolved using the QIAxcel capillary gel
electrophoresis apparatus (QIAGEN, Hilden, Germany) ) and
compared visually. The discriminatory power of
ERICPCR was found to be similar to that of PFGE in PA .
Acquisition of PA was defined as the isolation of PA
from surveillance or clinical cultures from patients not
colonized within 72 h of admission. Colonization was
defined as the isolation of PA from specimens taken
from the rectum, catheter-urine, pharynx or EA, in the
absence of clinical infection.
Patient characteristics were presented using
descriptive statistics. Continuous variables were compared
using the Student t test or Mann Whitney test, and
two-tailed Fisher’s exact test was used for categorical
variables. Time to PA related infections was evaluated
with the Kaplan-Meier method, with the day of ICU
admission serving as day 0. Differences between
curves were calculated with the two-sided logrank
test. Death discharge from hospital, and PA related
infection were treated as competing events. In all
statistical analyses, a two-sided p-value less than 0.05
was considered significant.
Patient surveillance cultures
Fifty-six patients were admitted to the ICU during the
study period. Eleven patients were excluded (5
hospitalized < 72h and 6 discharged prior to screening). Out of the
remaining 45 patients, 34 patients were screened <72h
from admission and 11 were screened ≥72h from
admission. Four of the 11 patients screened late were found
negative and were regarded also as negative on admission,
and together comprised the study cohort of 38
patients (Fig. 1).
Of the 38 patients, 13 (34%) were colonized with PA on
admission (Table 1). Advanced age (>70 years) and
residency in a LTCF were significantly associated with PA
colonization on admission (odds ratio (OR) 7, 95%
confidence interval (CI) 1.2-38.3; p = 0.035; OR = 17, 95% CI
0.8–358, p = 0.033, respectively; Table 1). Diabetes mellitus
was negatively associated with PA colonization (OR = 0.06,
95% CI 0.007–0.58; p = 0.005).
Of the 38 patients in the study cohort, 21 were still
hospitalized on the next week, and 11 (52%) of them screened
positive for PA (Table 2). The proportion of patients with
positive PA screening increased with length of ICU stay,
reaching 71% after 3 weeks of ICU stay (Table 2). Three
(12%) of 25 patients who were negative on admission
screening acquired PA during their ICU stay. In two of
them, PA was also found in clinical cultures of sputum,
and in one VAP was diagnosed.
Of a total of 68 positive surveillance cultures, 33 (49%)
were rectal, 17 (25%) pharynx, 16 (23%) EA, and 2 (3%)
urine. Rectal screening identified 77% of colonized
patients upon admission, 91% after 1 week of ICU stay,
and nearly 100% thereafter.
During the entire ICU stay we found 20 clonal
ERICPCR genotypes (among 18 patients) and 11 unique
genotypes from 9 patients (two patients had 2 isolates).
In the clonal analysis we included cases that were
excluded because of being positive on late screening.
Overall, the clonal structure was diverse. There were
no dominant strains (related to many patients or to
clinical cultures). Twelve patients (patients 2, 4, 5, 7, 8, 10,
12, 14, 15, 16, 17, 18 in Fig. 2) had >1 screening culture
(on a following week) available for genotypic analysis
(range, 1 to 10 isolates per patient). In 11 of these
patients (92%) a serial identical isolate was identified on
the following week (all except patient 12, in which the
same genotype O was indeed found but only after 2 and
4 weeks, Fig. 2).
Fig. 1 Study population
P (OR, 95% CI)
Fig. 2 P. aeruginosa ERIC-PCR strains among 23 positive patients
Clonal persistence vs. replacement in sequential
Rectum screening: 10 patients had at least 1 sequential
rectal screening culture available for typing. In 9 of these
(90%) the same clone persisted at least once (range 1–3
weeks). In 3 patients (30%) other strains appeared.
Pharynx screening: 5 patients had at least 1
sequential pharynx screening culture available for typing and
in 4 (80%) the same clone persisted at least once (range
AE screening: 4 patients had at least 1 sequential
AE screening culture available for typing and in 3
(75%) the same clone persisted at least once (range
1–3 weeks). Cross-overs between sites and strains
occurred (Fig. 2).
Calculated together, in 16 out of 19 (84%) of patients
in which a sequential same-site screening cultures were
available for typing, the same clone persisted. Clonal
persistence was evident in all screening sites, but was
most prominent in the rectum (90% vs. 80% and 75% in
the pharynx and AE, respectively). Cross-overs between
sites and strains occurred (Fig. 2).
In 5 patients a spread from the original site of
identification to other screening sites was evident
(Fig. 2). On 3 occasions the same genotype (B, C, S)
was identified in different patients, indicating cross
Clinical impact of PA isolation in the ICU
Thirteen patients (29%) were diagnosed with PA infection:
10 with VAP, 4 with SSI and 1 with bloodstream infection.
Ten additional patients (22%) acquired PA colonization
Patients colonized with PA on admission were at a
higher risk of PA-related clinical infection, compared with
patients who were PA-negative on admission [8/13 (62%)
vs. 1/25 (4%), hazard ratio = 14.65, CI (3.07–47.39), p =
0.0002], and for PA-related VAP [hazard ratio = 7.381, CI
(1.39–36.41), p = 0.0047], Fig. 3). PA-colonized patients
also had significantly longer mean stay in the ICU (23.7
days versus 10.6 days; p = 0.033, Table 1). None of the 22
patients with repeated negative screening had a positive
clinical culture with PA throughout their ICU stay.
Genotyping was performed on 17 clinical isolates from
12 patients (patients 1, 2, 4, 6, 7, 8, 12, 13, 15, 16, 17, 18;
Fig. 2). Sixteen (94%) clinical strains were related to strains
found concurrently or previously on screening cultures
from the same patient and none were unique (Fig. 2,
Table 3). The one exceptional clinical isolate (C2 from
patient 4) was not unique since it was found in the screening
cultures of patient 5). In 6 patients (50%) the clinical PA
isolate could have been predicted from the screening
cultures between 1 and 4 weeks earlier (genotypes A, K2, G,
O, S1, S2); and in the other 6 patients the identification by
screening occurred on the same week (genotypes B1, D, I,
Fig. 3 Kaplan-Meier survival curves, comparing PA-related outcomes between positive and negative patients on admission
P, R, U). However, six patients (50%) had screening PA
isolates that were different from a concurrent or subsequent
clinical PA strain (patients 2, 4, 7, 12, 16, 17). The
accuracy of (any site) surveillance cultures to predict the same
genotype cultivation in a clinical sample was 76% (36/47),
75% (43/57) and 72% (45/62) when obtained on the same
week or within 1 week before, 2 week before and
throughout the ICU stay, respectively.
Table 4 shows the correlation of the site-specific
screening culture with the infective strain according to
the different diagnoses. Among 8 patients with VAP
(who had clinical AE cultures available for typing),
identical surveillance cultures were recovered from EA in 7
(87%), from the pharynx in 6 (75%), and from the
rectum in 4 (50%). Among 4 patients with SSI, identical
surveillance cultures were recovered from EA, pharynx
and rectum in 2 patients (50%) each.
We used systematic sequential screening to define the
dynamics of PA colonization and infection at a general
ICU. In a non-outbreak setting, we found a highly
diverse population of patient-unique PA strains. Strains
were often site-specific and site-persistent, particularly
with regards to rectal colonization, but could also
distribute between body sites, and be replaced frequently.
A positive screening culture for PA was associated with
an increased risk of PA related infection: there was a
50–70% likelihood of subsequent clinical infection with
the same strain, depending on the timing and site of
screening. Importantly, we found that when adequate
infection control standards are maintained, repeated
negative multi-site screening results were associated with a
very low rate of subsequent clinical infection with PA.
A third of our patients were carriers of PA on admission
to the ICU (26% rectal, 16% EA and 8% pharyngeal
carriage). Bonten et al. reported similar figures (34%) along
with striking similarities regarding the relative importance
of the sites of screening: the gastrointestinal being the most
sensitive (24% positivity), and pharynx and EA being
positive in only 9% and 7%, respectively . In a more recent
study, Zorilla et al. reported similar findings (27% PA
colonization on admission) . Advanced age and prior
hospital stay were risk factors for PA colonization on
admission. Similarly, we found advanced age and residence
in a LTCF as significant risk factors. Surprisingly, diabetes
mellitus was associated with a low rate of PA colonization
on ICU admission. In line with others [1, 22], we found that
colonization often preceded infection. Specifically, patients
colonized upon admission had a 14.65-fold risk of
developing infection as compared with non-colonized patients.
Early and accurate antibiotic coverage in patients
developing VAP in the ICU is critical to improve patient
outcomes [23, 24], but the increasing rates of multidrug
resistant (MDR) organisms (including PA) in ICU and
non-ICU patients pose an obstacle for appropriate empiric
therapy. Accurate prediction of antimicrobial resistance
patterns of organisms causing VAP by using surveillance
cultures in ICUs has been a matter of an ongoing debate
in the literature. A recent systematic review and a
metaTable 3 Concordance between screening and clinical ERIC-PCR strains
1 B1 EA, P B1 Wound Same week 2 4 6
Uncorrelated cases 2
1 week and same week
A-W – ERIC-PCR strain (a number denotes a clone subtype), Sg unique strain, EA endotracheal aspirate, P pharynx, Rc – rectum, U urine
analysis found high accuracy of surveillance cultures, with
pooled sensitivities of up to 0.75 and specificities up to
0.92 in culture-positive VAP . Our results support the
predictive value of surveillance cultures: among patients
who developed VAP, screening the EA or the pharynx
accurately predicted the VAP-related strain in 75–87% of
episodes. SSI-related strains were predicted by EA and
pharynx screening in 50% of cases.
None of the patients who had persistently negative
surveillance cultures had subsequent recovery of PA
from clinical cultures. Similar findings were reported in the
meta-analysis cited . Hence, screening two sites weekly
with negative results can provide reassurance for the
physician not to initiate empirical anti-pseudomonal antibiotics
in patients with suspected VAP or SSI, which are among
the most frequent infections in critically ill patients. This
finding may have implications for antibiotic stewardship, as
it provides an evidence-based framework for limiting the
use of wide-spectrum antibiotics in the ICU.
The current study is unique in providing a longitudinal
assessment of PA colonization dynamics in multiple body
sites throughout the ICU stay. Recently, Zorrilla et al. 
found high rates (87%) of genotypic concordance between
rectal surveillance cultures and infecting strains of PA. Our
Table 4 Prediction of clinical strain by screening sites according to diagnosis
Diagnosis Clinical clone Concordant screening sites
EA P Rc
VAP A + + +
Screen site utility for VAP (%)
Screen site utility for SSI (%)
Screen site utility for all infections (%)
VAP ventilator associated pneumonia, SSI surgical site infection, BSI blood stream infection, IAI intraabdominal infection, EA endotracheal aspirate, P pharynx, Rc
rectum, U urine
results underscore the limitations of rectal screening for
predicting respiratory strains, as further demonstrated in a
study performed among hematopoietic stem cell recipients
. The high efficacy of lower airways screening to predict
the strains that caused VAP is consistent with results of
previous studies [3, 10].
The limitations of this study are the relatively small
number of patients in a single center setting. Screening
was limited to PA colonization, whereas in clinical
practice empiric antimicrobial therapy often targets other
MDR bacteria such as MRSA, MDR-Acinetobacter spp.
and ESBL-producing Enterobacteriaceae. From a
practical perspective, screening 3 body sites for PA only, may
be expensive and labor intensive, and will miss other
important causes of VAP and SSI. Another limitation is
that antimicrobial susceptibility data of all screening
strains was not available for comparison. Therefore, the
utility of screening cultures to predict the susceptibility
patterns of clinical PA strains remains to be established.
In this study we showed that in a non-outbreak setting
of ICU, most strains were patient-unique, endogenous in
origin, and cross contamination was rare. Colonization
on admission was a significant risk factor for the
development of infection with PA. Detection of PA on
surveillance cultures may serve as a good predictor of PA
clinical infection and also of the infecting clone, while
negative screening is an excellent negative predictor for
clinical infection. VAP-related strains are better
predicted by upper airways screening than rectal screening.
APACHE: Acute physiology and chronic health evaluation; CAUTI:
Catheterassociated urinary tract infection; CDC: Centers for disease control and
prevention; CLABSI: Central line-associated blood stream infection;
EA: Endotracheal aspirate; ERIC: Enterobacterial repetitive Intergenic
Consensus; ICU: Intensive care unit; LCTF: Long-term care facility;
MDR: Multidrug resistant; NHSN: The national healthcare safety network;
PA: Pseudomonas aeruginosa; PCR: Polymerase chain reaction; SSI: Surgical
site infection; VAP: Ventilator associated pneumonia
Availability of data and materials
The datasets during and/or analysed during the current study available from
the corresponding author on reasonable request.
RC concepted the idea of the study, gathered the data, analyzed it and
wrote the manuscript. BF reviewed independently the clinical cases in order
to decide between infection and colonization states, and critically reviewed
the manuscript. CS and AF collected the clinical and screening cultures. SM
and UM treated the patients in the ICU and critically reviewed the
manuscript. KE and AA performed the ERIC-PCR assays and gathered the
molecular laboratory data. RBA critically reviewed the manuscript and made the
statisitcal analysis. He was major contributor in writing the manuscript. PS
made all the microbiology cultures and gathered all the microbiology data
All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
This study was approved by the hospital institutional review board
committee (0033-14-LND). As the study was aimed for infection control and
patient safety purposes, the requirement for informed consent was waived.
1. Gomez-Zorrilla S , Camoez M , Tubau F , Canizares R , Periche E , Dominguez MA , Ariza J , Pena C. Prospective observational study of prior rectal colonization status as a predictor for subsequent development of Pseudomonas aeruginosa clinical infections . Antimicrob Agents Chemother . 2015 ; 59 : 5213 - 9 .
2. Gruner E , Kropec A , Huebner J , Altwegg M , Daschner F. Ribotyping of Pseudomonas aeruginosa strains isolated from surgical intensive care patients . J Infect Dis . 1993 ; 167 : 1216 - 20 .
3. Bonten MJ , Bergmans DC , Speijer H , Stobberingh EE. Characteristics of polyclonal endemicity of Pseudomonas aeruginosa colonization in intensive care units . Implications for infection control . Am J Respir Crit Care Med . 1999 ; 160 : 1212 - 9 .
4. Trautmann M , Halder S , Hoegel J , Royer H , Haller M. Point-of-use water filtration reduces endemic Pseudomonas aeruginosa infections on a surgical intensive care unit . Am J Infect Control . 2008 ; 36 : 421 - 9 .
5. Blanc DS , Francioli P , Zanetti G . Molecular Epidemiology of Pseudomonas aeruginosa in the Intensive Care Units - A Review . Open Microbiol J . 2007 ; 1 : 8 - 11 .
6. Reuter S , Sigge A , Wiedeck H , Trautmann M. Analysis of transmission pathways of Pseudomonas aeruginosa between patients and tap water outlets . Crit Care Med . 2002 ; 30 : 2222 - 8 .
7. Thuong M , Arvaniti K , Ruimy R , de la Salmoniere P , Scanvic-Hameg A , Lucet JC , Regnier B. Epidemiology of Pseudomonas aeruginosa and risk factors for carriage acquisition in an intensive care unit . J Hosp Infect . 2003 ; 53 : 274 - 82 .
8. Trautmann M , Bauer C , Schumann C , Hahn P , Hoher M , Haller M , Lepper PM . Common RAPD pattern of Pseudomonas aeruginosa from patients and tap water in a medical intensive care unit . Int J Hyg Environ Health . 2006 ; 209 : 325 - 31 .
9. Trautmann M , Michalsky T , Wiedeck H , Radosavljevic V , Ruhnke M. Tap water colonization with Pseudomonas aeruginosa in a surgical intensive care unit (ICU) and relation to Pseudomonas infections of ICU patients . Infect Control Hosp Epidemiol . 2001 ; 22 : 49 - 52 .
10. Valles J , Mariscal D , Cortes P , Coll P , Villagra A , Diaz E , Artigas A , Rello J. Patterns of colonization by Pseudomonas aeruginosa in intubated patients: a 3-year prospective study of 1,607 isolates using pulsed-field gel electrophoresis with implications for prevention of ventilator-associated pneumonia . Intensive Care Med . 2004 ; 30 : 1768 - 75 .
11. Venier AG , Leroyer C , Slekovec C , Talon D , Bertrand X , Parer S , Alfandari S , Guerin JM , Megarbane B , Lawrence C , et al. Risk factors for Pseudomonas aeruginosa acquisition in intensive care units: a prospective multicentre study . J Hosp Infect . 2014 ; 88 : 103 - 8 .
12. Petignat C , Francioli P , Nahimana I , Wenger A , Bille J , Schaller MD , Revelly JP , Zanetti G , Blanc DS . Exogenous sources of Pseudomonas aeruginosa in intensive care unit patients: implementation of infection control measures and follow-up with molecular typing . Infect Control Hosp Epidemiol . 2006 ; 27 : 953 - 7 .
13. Bukholm G , Tannaes T , Kjelsberg AB , Smith-Erichsen N. An outbreak of multidrug-resistant Pseudomonas aeruginosa associated with increased risk of patient death in an intensive care unit . Infect Control Hosp Epidemiol . 2002 ; 23 : 441 - 6 .
14. Rogues AM , Boulestreau H , Lasheras A , Boyer A , Gruson D , Merle C , Castaing Y , Bebear CM , Gachie JP . Contribution of tap water to patient colonisation with Pseudomonas aeruginosa in a medical intensive care unit . J Hosp Infect . 2007 ; 67 : 72 - 8 .
15. Cohen R , Babushkin F , Shimoni Z , Cohen S , Litig E , Shapiro M , Adler A , Paikin S. Water faucets as a source of Pseudomonas aeruginosa infection and colonization in neonatal and adult intensive care unit patients . Am J Infect Control . 2016 .
16. Horan TC , Andrus M , Dudeck MA . CDC/NHSN surveillance definition of health care-associated infection and criteria for specific types of infections in the acute care setting . Am J Infect Control . 2008 ; 36 : 309 - 32 .
17. American Thoracic S , Infectious Diseases Society of A. Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia . Am J Respir Crit Care Med . 2005 ; 171 : 388 - 416 .
18. CLSI. M100-S25 performance standards for antimicrobial susceptibility testing; Twenty-fifth informational supplement . 2015 .
19. Wolska K , Szweda P. A comparative evaluation of PCR ribotyping and ERIC PCR for determining the diversity of clinical Pseudomonas aeruginosa isolates . Pol J Microbiol . 2008 ; 57 : 157 - 63 .
20. Kidd TJ , Grimwood K , Ramsay KA , Rainey PB , Bell SC . Comparison of three molecular techniques for typing Pseudomonas aeruginosa isolates in sputum samples from patients with cystic fibrosis . J Clin Microbiol . 2011 ; 49 : 263 - 8 .
21. Gomez-Zorrilla S , Camoez M , Tubau F , Periche E , Canizares R , Dominguez MA , Ariza J , Pena C. Antibiotic pressure is a major risk factor for rectal colonization by multidrug-resistant Pseudomonas aeruginosa in critically ill patients . Antimicrob Agents Chemother . 2014 ; 58 : 5863 - 70 .
22. Bertrand X , Thouverez M , Talon D , Boillot A , Capellier G , Floriot C , Helias JP . Endemicity, molecular diversity and colonisation routes of Pseudomonas aeruginosa in intensive care units . Intensive Care Med . 2001 ; 27 : 1263 - 8 .
23. Iregui M , Ward S , Sherman G , Fraser VJ , Kollef MH . Clinical importance of delays in the initiation of appropriate antibiotic treatment for ventilatorassociated pneumonia . Chest . 2002 ; 122 : 262 - 8 .
24. Kollef M. Appropriate empirical antibacterial therapy for nosocomial infections: getting it right the first time . Drugs . 2003 ; 63 : 2157 - 68 .
25. Brusselaers N , Labeau S , Vogelaers D , Blot S. Value of lower respiratory tract surveillance cultures to predict bacterial pathogens in ventilator-associated pneumonia: systematic review and diagnostic test accuracy meta-analysis . Intensive Care Med . 2013 ; 39 : 365 - 75 .
26. Nesher L , Rolston KV , Shah DP , Tarrand JT , Mulanovich V , Ariza-Heredia EJ , Chemaly RF . Fecal colonization and infection with Pseudomonas aeruginosa in recipients of allogeneic hematopoietic stem cell transplantation . Transpl Infect Dis . 2015 ; 17 : 33 - 8 .