Acquisition of Pseudomonas aeruginosa and its resistance phenotypes in critically ill medical patients: role of colonization pressure and antibiotic exposure
Cobos-Trigueros et al. Critical Care
Acquisition of Pseudomonas aeruginosa and its resistance phenotypes in critically ill medical patients: role of colonization pressure and antibiotic exposure
Nazaret Cobos-Trigueros 0 3
Mar Sol 2
Pedro Castro 1
Jorge Luis Torres 4
Cristina Hernndez 1
Mariano Rinaudo 1
Sara Fernndez 1
lex Soriano 0 3
Jos Mara Nicols 1
Josep Mensa 0 3
Jordi Vila 2 5
Jos Antonio Martnez 0 3
0 Department of Infectious Diseases, Hospital Clinic, Institut d'Investigacions Biomediques August Pi i Sunyer (IDIBAPS), University of Barcelona , Barcelona , Spain
1 Medical Intensive Care Unit, Hospital Clinic, Institut d'Investigacions Biomediques August Pi i Sunyer (IDIBAPS), University of Barcelona , Barcelona , Spain
2 ISGlobal, Barcelona Center for International Health Research (CRESIB), Hospital Clinic, University of Barcelona , Barcelona , Spain
3 Department of Infectious Diseases, Hospital Clinic, Institut d'Investigacions Biomediques August Pi i Sunyer (IDIBAPS), University of Barcelona , Barcelona , Spain
4 Department of Internal Medicine, University Hospital of Salamanca, Institute of Biomedical Research of Salamanca (IBSAL) , Salamanca , Spain
5 Department of Clinical Microbiology, Hospital Clinic, School of Medicine, University of Barcelona , Barcelona , Spain
Introduction: The objective of this work was to investigate the risk factors for the acquisition of Pseudomonas aeruginosa and its resistance phenotypes in critically ill patients, taking into account colonization pressure. Methods: We conducted a prospective cohort study in an 8-bed medical intensive care unit during a 35-month period. Nasopharyngeal and rectal swabs and respiratory secretions were obtained within 48 hours of admission and thrice weekly thereafter. During the study, a policy of consecutive mixing and cycling periods of three classes of antipseudomonal antibiotics was followed in the unit. Results: Of 850 patients admitted for 3 days, 751 (88.3%) received an antibiotic, 562 of which (66.1%) were antipseudomonal antibiotics. A total of 68 patients (8%) carried P. aeruginosa upon admission, and among the remaining 782, 104 (13%) acquired at least one strain of P. aeruginosa during their stay. Multivariate analysis selected shock (odds ratio (OR) =2.1; 95% confidence interval (CI), 1.2 to 3.7), intubation (OR =3.6; 95% CI, 1.7 to 7.5), enteral nutrition (OR =3.6; 95% CI, 1.8 to 7.6), parenteral nutrition (OR =3.9; 95% CI, 1.6 to 9.6), tracheostomy (OR =4.4; 95% CI, 2.3 to 8.3) and colonization pressure >0.43 (OR =4; 95% CI, 1.2 to 5) as independently associated with the acquisition of P. aeruginosa, whereas exposure to fluoroquinolones for >3 days (OR =0.4; 95% CI, 0.2 to 0.8) was protective. In the whole series, prior exposure to carbapenems was independently associated with carbapenem resistance, and prior amikacin use predicted piperacillin-tazobactam, fluoroquinolone and multiple-drug resistance. Conclusions: In critical care settings with a high rate of antibiotic use, colonization pressure and non-antibiotic exposures may be the crucial factors for P. aeruginosa acquisition, whereas fluoroquinolones may actually decrease its likelihood. For the acquisition of strains resistant to piperacillin-tazobactam, fluoroquinolones and multiple drugs, exposure to amikacin may be more relevant than previously recognized.
Previous exposure to antibiotics is considered an imperative
risk factor for the acquisition of Pseudomonas aeruginosa
and the subsequent development of infection . According
to the classical paradigm, non-antipseudomonal agents
would promote acquisition of any P. aeruginosa strain [2,3],
whereas drugs with antipseudomonal activity would select
those resistant to the particular class of antimicrobial drug
used . Resistance acquisition driven by exposure to
antipseudomonal agents can be reached by either selecting
mutants in patients previously colonized or infected by
susceptible phenotypes [5,6] or promoting selection of an
already resistant strain . Many study researchers have
reported that prior exposure to a given antipseudomonal
agent is associated with the acquisition of strains resistant
to it [4,8-16], to unrelated agents [8-10,14,17-20] or to
multiple drugs [13,21-27]. However, not enough data have
been provided to ascertain which of the above-mentioned
processes is preferentially involved [26,28].
There are discrepancies regarding the magnitude of
the risk of resistance acquisition associated with the
different antipseudomonal agents. In patients previously
colonized or infected by P. aeruginosa, carbapenems and
fluoroquinolones may have a greater tendency to select
resistant mutants than other agents [5,6,29,30]. In addition,
prior exposure to fluoroquinolones or carbapenems has
commonly been associated with the acquisition of
strains resistant to unrelated antibiotics and multiple
drugs [10,15,17-22,24,26,31]. However, there are some
exceptions. In a casecontrol study , cephalosporins
and aminoglycosides (but not quinolones) were the main
predictors of a multidrug-resistant (MDR) phenotype,
and, in a cohort study , quinolones were protective
against the acquisition of P. aeruginosa and had no role in
the acquisition of resistant phenotypes.
Part of the discrepancies among studies regarding
the role of previous use of antibiotics on P. aeruginosa
resistance may be due to local differences in transmission
rates, because exposure to antipseudomonal agents in
previously non-colonized patients necessarily requires
transmission from other patients or environmental sources
to foster the acquisition of resistant strains. However,
variables influencing transmission, such as colonization
pressure , have rarely been taken into account in studies
aimed at defining the influence of antibiotics on the
acquisition of any P. aeruginosa or of strains with
specific resistance phenotypes [2,3,14,33]. An accurate
picture of the most meaningful epidemiological and
exposure variables is essential to designing effective
control measures directed at curbing the increasing
incidence of the resistance of this important pathogen.
During a 3-year period, we were able to systematically
obtain multisite surveillance cultures from patients
admitted to a medical intensive care unit (ICU). This allowed us
to investigate in detail the factors associated with the
acquisition of P. aeruginosa and its different resistance
phenotypes, taking into account both significant exposures
(including antibiotics) and colonization pressure.
Materials and methods
From February 2006 to December 2008, all patients
admitted to an 8-bed adult medical ICU of a 700-bed
university hospital who stayed in the unit for at least
3 days (72 hours) were prospectively included in the study.
The study unit has two individual rooms and a central
space with six cubicles, and it is the reference unit for
critically ill medical patients from the internal medicine,
haematology, oncology and infectious diseases wards.
After a previous pilot experience , the director of
the study unit decided to implement a mixing and cycling
strategy of antibiotic use on a regular basis. To evaluate
this policy, a prospective study of systematic screening
for the detection of resistant or potentially resistant
microorganisms was carried out during the first 3 years of
its implementation. The present study was an analysis
using clinical and microbiological data collected during
the prospective screening program with the aim of
investigating the risk factors for P. aeruginosa acquisition.
The study protocol was approved by the Research Ethics
Committee of the University Hospital Clinic of Barcelona,
which waived the requirement of informed consent
(approval reference number 2616).
Swabbing of nares, pharynx and rectum, as well as
respiratory secretions (tracheobronchial aspirate, bronchoscopic
samples or sputum), were obtained within 48 hours of
admission and thrice weekly thereafter until discharge or
the first 2 months of the ICU stay. Other clinical samples
were obtained as deemed necessary by the attending
physician. Samples were cultured in conventional agar
media. No environmental cultures were taken. Susceptibility
testing was done by using a microdilution technique
according to Clinical and Laboratory Standards Institute
guidelines . For the purpose of analysis, intermediate
susceptibility was considered as resistance. Molecular typing
was performed by pulse-field gel electrophoresis as
previously described . Resistance to multiple antibiotics
was defined as MDR, extensively drug-resistant (XDR) or
pandrug-resistant (PDR) as described elsewhere .
Demographics, clinical variables, severity scores (Acute
Physiology and Chronic Health Evaluation (APACHE) II
and Sequential Organ Failure Assessment (SOFA))
upon admission and exposures during ICU stay were
prospectively collected from all admitted patients as
previously described. These data are shown in Table 1 .
For the duration of the study, a policy of consecutive mixing
and cycling periods of three classes of antipseudomonal
agents (meropenem, ceftazidime/piperacillin-tazobactam
and ciprofloxacin/levofloxacin) was implemented in the
study unit. Each period lasted 4.5 months. During mixing, a
different antipseudomonal antibiotic class was prescribed to
each consecutive patient. Cycling periods were divided in
three consecutive 6-week intervals in which a different
antibiotic class was given to every patient. The decision to
provide antipseudomonal antibiotics was made by the
attending physician based on clinical judgment. Amikacin
in a once-daily dose was the aminoglycoside favoured for
antipseudomonal antibiotic coverage, but its administration
as monotherapy or for >5 days was discouraged. The
Table 1 Patient characteristics on admission, exposures
during the ICU stay and outcomes of the entire
Table 1 Patient characteristics on admission, exposures
during the ICU stay and outcomes of the entire
Pre-ICU stay (days)
Prior antibiotic (1 mo)
Shock on admission
Reason for admission
Prior corticosteroids (1 mo)
Exposures during ICU stay
Central venous catheter
Renal replacement therapies
Packed red blood cell transfusion
Any non-antipseudomonal antibiotic
Any antipseudomonal antibiotic:
Length of stay (days)
aAPACHE II, Acute Physiology and Chronic Health Evaluation II; CNS, Central
nervous system; COPD, Chronic obstructive pulmonary disease; ICU, Intensive
care unit; SOFA, Sequential Organ Failure Assessment. bOthers include patients
with HIV infection, hepatic cirrhosis, renal failure and heart failure. Categorical
variables are expressed as number of patients (%) and continuous variables as
mean (standard deviation).
decision to administer combination treatment with a
-lactam and a fluoroquinolone or amikacin was also
made by the attending physician, and, in accordance with
current protocols, it was encouraged only for patients with
severe sepsis or septic shock.
The results of surveillance cultures were
communicated to the attending physician either when they
yielded a microorganism requiring contact precautions
according to current isolation practices in the hospital
(methicillin-resistant Staphylococcus aureus (MRSA);
vancomycin-resistant enterococci (VRE); enteric
Gramnegative bacilli producing extended-spectrum -lactamases;
P. aeruginosa resistant to at least three classes of
antipseudomonal agents, considering ceftazidime and
piperacillin-tazobactam or ciprofloxacin and levofloxacin
as single classes) or when an outbreak was suspected.
Contact precautions implied the transfer to an individual
room when available and, in any case, the wearing of
gowns and gloves when entering the cubicle or room.
Patients with prior MRSA, MDR Gram-negative bacilli
and VRE were automatically identified by an electronic
tag on admission, but preventive isolation based on risk
factors was never performed. Hand hygiene was primarily
based on alcohol-based hand rubs. Decolonization with
mupirocin was carried out only in patients with MRSA
present exclusively in nares. Chlorhexidine was used
for oral hygiene, but not for body bathing. Selective
decontamination of the digestive tract or any additional
practice, such as the use of extraordinary prophylactic
antibiotics (except as clinically recommended in neutropenic,
cirrhotic or HIV patients), was not performed during the
study. There were no changes in isolation or hand hygiene
practices during the study period.
Colonization was defined as the isolation of P. aeruginosa
from a surveillance culture or non-sterile clinical sample.
Patients with P. aeruginosa isolated within 48 hours of
ICU admission were considered to be colonized upon
admission. Organisms isolated 48 hours after admission in
patients with previous negative specimens were considered
as ICU-acquired. Infection was considered the reason for
admission when the organic failure leading to critical care
was understood to be a direct consequence of either the
dysfunction of the infected organ or sepsis. Acquisition of
resistance was defined as the isolation of a resistant
organism in a patient with a previous sensitive strain
or prior negative cultures. Emergence of resistance to
a given antibiotic refers to the conversion of a genotypically
defined strain from susceptible to non-susceptible; hence,
these isolates were also included in the previous definition.
Cross-transmission was considered to have occurred when
a patient acquired a pulsotype identical to that of an isolate
previously found in a patient who stayed in the unit during
the same period. Colonization pressure was estimated as
the average of the daily proportion of colonized patients
(number of patients colonized divided by number of
patients in the unit on a given day) from the day of
admission until the day before acquisition of the
microorganism or until discharge if the patient did
not acquire the microorganism . Time at risk was the
number of days until the detection of the microorganism
and/or resistance in patients who acquired it and the whole
length of ICU stay if the patient did not acquire it. Exposure
to antibiotics meant at least 24 hours of treatment.
For continuous variables, means (with standard deviations)
or medians (with interquartile ranges (IQRs)) were used as
measures of central tendency (dispersion). Denominators in
proportions were always number of patients. Proportions
were compared by using the 2 test or Fishers exact test,
and continuous variables were compared by using the t-test
or MannWhitney U test. Multivariable logistic regression
analysis (step-forward procedure) was used to evaluate
characteristics associated with the acquisition of P.
aeruginosa and acquisition of resistance to ceftazidime,
piperacillin-tazobactam, carbapenems and quinolones. For
the purpose of analysing the risk factors associated with
the acquisition of P. aeruginosa, patients colonized or
infected with this microorganism upon admission were
excluded, owing to uncertainty about the ability to detect
their new episodes of acquisition. However, the whole
cohort was considered when we analysed the risk
factors for the acquisition of resistance to the different
antipseudomonal antibiotics, because resistance could
emerge in strains of P. aeruginosa present upon admission.
Age, APACHE II score and SOFA score were introduced
into the models as dichotomous variables, taking the
median as the cutoff value, whereas colonization pressure
was dichotomized by the highest observed value (95th
percentile). In multivariate models predicting the
acquisition of strains resistant to each antipseudomonal agent
and multiple drugs, a cutoff of 72 hours was used to
dichotomize antibiotic exposure because in previous
studies it appeared to be the best time span for defining the
minimal duration of exposure associated with resistance
[39,40]. Variables with a P-value <0.3 in the univariate
analysis were introduced into the multivariate model.
Calculations were done using the IBM SPSS version 20.0
statistical software package (IBM, Armonk, NY, USA).
During the 35-month study period, a total of 850 patients
were hospitalized in the unit for 72 hours or more. Patient
characteristics and exposures are shown in Table 1, and a
detailed description of the reasons for admission is
provided in Additional file 1.
In regard to antibiotic exposure, 751 patients (88.3%)
received an antibiotic, 562 (66.1%) of which were
antipseudomonal agents. The median daily dosages of
antipseudomonal antibiotics were 6 g for ceftazidime,
3 g for carbapenems (meropenem or imipenem; there
was no exposure to ertapenem), 12 g for
piperacillintazobactam, 1,200 mg for ciprofloxacin, 500 mg for
levofloxacin and 1 g for amikacin. The median days
(IQR) of exposure to antipseudomonal antibiotics were 6
(3 to 11) for ceftazidime, 6 (3 to 10) for carbapenems,
5 (3 to 8) for piperacillin-tazobactam, 6 (3 to 10) for
ciprofloxacin, 4 (3 to 8) for levofloxacin and 4 (2 to 10) for
amikacin. The median (IQR) length of ICU stay was
5 (4 to 10) days.
During the study period, a total of 9,561 surveillance
samples were obtained and cultured, of which 1,646
proceeded from the lower respiratory tract, 2,664
from the pharynx, 2,690 from the nares and 2,561
from the rectum. The mean numbers per included
patient was 3.2 for nasal swabs, 3.1 for pharyngeal swabs,
3 for rectal swabs and 1.9 for respiratory samples (3.3 in
A total of 68 patients (8%) were colonized with P.
aeruginosa upon admission, and of the remaining 782,
104 (13.3%) acquired at least one strain of P. aeruginosa
during their ICU stay (4 patients acquired 2 different
strains). Acquired isolates belonged to 79 distinct
pulsotypes, of which 7 were obtained from more than 1 patient
(from 2 to 20). The more numerous cluster, which
included 20 patients, corresponded to a strain that had a
XDR phenotype on 16 occasions. Of the 104 patients who
acquired P. aeruginosa, in 20 (19.2%) acquisition was due
to cross-transmission (13 of the XDR genotype) and in
the remaining patients the origin was unknown. The sites
of primary detection were the rectum in 57 cases (54.8%),
the nares or pharynx in 16 (15.3%), the lower respiratory
tract in 10 (9.6%), more than one of these sites in 19
(18.2%) and other sites in 2 (1.9%). Of the 57 patients
with initial unique rectal colonization, 18 (31.5%) had
subsequent nasopharyngeal (n =3) or lower respiratory
tract colonization (n =15). In all, 48 patients (46.1%)
eventually had P. aeruginosa isolated from a lower
respiratory sample (in 24 as a first site of colonization and in 24
as a secondary one), of whom 13 (27%) had pneumonia
(11 ventilator-associated). Detection in tracheal aspirates
or sputum preceded pneumonia in seven patients for a
median of 3 days (range, 1 to 14), and it was coincidental
with its clinical diagnosis in the remaining six patients.
Pneumonia occurred more frequently in patients with first
detection of P. aeruginosa in the lower respiratory (8 of 24
(33.3%)) than in other sites (5 of 80 (6.2%)) (P =0.002).
Other infections diagnosed in the 104 patients who
acquired P. aeruginosa were ventilator-associated
tracheobronchitis in 9, catheter-related bacteraemia in 3,
primary bacteraemia in 2 and a surgical wound infection in
1. Tracheobronchitis was equally common in patients with
first detection of P. aeruginosa in the lower respiratory
tract (2 of 24 (8.3%)) than in other sites (7 of 80 (8.7%)
(P =1), whereas the 6 other infections occurred in patients
in whom P. aeruginosa was first detected outside the
lower respiratory tract. Sites of primary and secondary
acquisition are shown in detail in Additional file 2.
Risk factors for acquisition of Pseudomonas aeruginosa
The acquired strains were susceptible to all
antipseudomonal antibiotics in 56 patients (53%), PDR in 1 (1%), XDR in
17 (16%), MDR in 4 (4%) and resistant to 1 or 2 groups of
antipseudomonal antibiotics in 27 (26%). Upon acquisition,
resistance to carbapenems, piperacillin-tazobactam,
ceftazidime, fluoroquinolones and amikacin was observed in 39
(37%), 19 (18%), 30 (29%), 29 (28%) and 1 (1%) strains,
The univariate analysis of the relationship between
patients characteristics or exposures and the acquisition
of P. aeruginosa is shown in Table 2. Multivariate analysis
selected shock, orotracheal intubation, enteral nutrition
for 3 days, parenteral nutrition for 3 days, tracheostomy
and colonization pressure >0.43 as being independently
associated with the acquisition of P. aeruginosa, whereas
exposure to fluoroquinolones for >3 days was protective.
The complete model is shown in Table 3.
Risk factors for the acquisition of resistance to
The number of patients in whom P. aeruginosa resistant
to the different antipseudomonal antibiotics was isolated
during the ICU stay, the resistance status when first
isolated and the number of acquisitions due to
crosstransmissions are stated in Table 4. In most cases, the
resistance phenotype was acquired as such and did not
emerge from a susceptible one. Only one strain acquired
as susceptible was the result of cross-transmission, whereas
this was observed in 41% to 58% of the strains acquired
as resistant to the different antipseudomonal -lactams or
Univariate analysis of the association of prior exposure to
carbapenems, ceftazidime, piperacillin-tazobactam,
fluoroquinolones and amikacin with acquisition of resistance to
the different antipseudomonal antibiotics and multiple
drugs is shown in Additional file 3.
In multivariate analysis, carbapenem exposure for more
than 3 days was associated with acquisition of resistance
to itself, and amikacin exposure for more than 3 days was
associated with acquisition of resistance to
piperacillintazobactam and fluoroquinolones as well as MDR. Exposure
to fluoroquinolones, piperacillin-tazobactam or ceftazidime
was not associated with acquisition of resistance to
themselves or to other antipseudomonal agents. Complete
models are shown in Additional file 4.
Emergence of resistance to a given antipseudomonal
agent from a previous susceptible strain after exposure
to itself occurred in 4 (20%) of 20 patients exposed to
ceftazidime (vs 8 (8%) of 106 non-exposed; P =0.1), 6
(46%) of 13 exposed to carbapenems (vs 1 (3%) of 95
nonexposed; P <0.001), 3 (15%) of 20 exposed to
piperacillintazobactam (vs 9 (8%) of 117 non-exposed; P =0.3) and 8
(29%) of 28 exposed to fluoroquinolones (vs 2 (3%) of 94
non-exposed; P <0.001).
The main findings of this study are the following: (1)
colonization pressure and several patient conditions
or instrumentations seem to be more relevant risk
factors than exposure to antibiotics for the acquisition
of P. aeruginosa; (2) exposure to fluoroquinolones
(levofloxacin or ciprofloxacin) for >3 days was
protective against the acquisition of this pathogen; (3)
exposure to carbapenems predicted resistance to
themselves; and (4) amikacin exposure was associated with
the acquisition of resistance to piperacillin-tazobactam,
quinolones and multiple drugs.
Whenever cross-transmission is involved in the
acquisition of a given microorganism, it is expected that
colonization pressure should be a relevant risk factor.
Although defined in different ways, colonization pressure
has been independently associated, in the ICU setting,
with the acquisition of MRSA , VRE , Clostridium
difficile , Acinetobacter baumannii [33,43] and P.
aeruginosa [16,33,44]. However, in none of the MRSA
studies, and in only some on A. baumannii [43,45] or
P. aeruginosa , was adjustment for prior antibiotic
exposure performed. Some reports indicate that there is an
interaction between colonization pressure and antibiotics.
In one study in which investigators searched for predictors
of P. aeruginosa acquisition in the ICU , prior exposure
to 3 days of non-antipseudomonal antibiotics was a
significant risk factor only when there was at least one
Table 2 Relationship between Pseudomonas aeruginosa acquisition, characteristics on admission and exposures in the ICUa
Any antipseudomonal antibiotic
Any non-antipseudomonal antibiotic
0.6 (0.3 to 1.3)
0.9 (0.5 to 1.5)
1.2 (0.6 to 2.4)
1.7 (1.1 to 2.8)
1.8 (0.9 to 3.7)
1.2 (0.6 to 2.7)
1.4 (0.8 to 2.5)
1.7 (1.1 to 2.8)
2.5 (1.4 to 4.2)
0.5 (0.1 to 3.9)
2.6 (0.8 to 8.6)
8.82 (2.1 to 36.3)
aAPACHE II, Acute Physiology and Chronic Health Evaluation II; CI, Confidence interval; CVC, Central venous catheter; ICU, Intensive care unit; IQR, Interquartile
range; OR, Odds ratio; SOFA, Sequential Organ Failure Assessment. Variables are expressed in terms of frequency as number of patients (%) and in terms of
duration as median days (IQR). bValue corresponding to the 95th percentile. Variables with P 0.3 introduced in the multivariate analysis and not shown include
the following: infections on admission (pneumonia, urinary tract infection and primary bacteraemia), arterial catheter, nasogastric tube, corticosteroids,
glycopeptides, clindamycin, macrolide, trimethoprim-sulphamethoxazole, linezolid, fluconazole, other penicillins and other cephalosporins. Variables with a
P-value >0.3 are not shown and include the following: age; bone marrow transplant; solid organ transplant; solid organ cancer; haemodialysis; HIV infection; heart failure;
chronic obstructive pulmonary disease; diabetes; prior corticosteroid and immunosuppressive therapy; admission within the previous year; respiratory, cardiovascular,
central nervous system and other diseases as reasons for admission; catheter-related bacteraemia as prevalent infection; and renal replacement therapy.
colonized patient in the unit. This fact supports the notion
that, in previously non-colonized patients, antibiotics
cannot promote acquisition of resistance without relying
on transmission. In another study on imipenem-resistant
A. baumannii acquisition, antimicrobials were found to be
a risk factor only for patients admitted during periods in
which colonization pressure was low , suggesting that
the role of antibiotics may be relatively more important
when there are fewer opportunities for patient-to-patient
transmission. Our data indicate that colonization pressure,
measured as originally described , was an independent
risk factor for the acquisition of P. aeruginosa in a critical
care setting where most patients were exposed to antibiotics
(87% to any drug and 64.7% to an antipseudomonal
antibiotic) and 19% of the acquisition episodes were
due to cross-transmission. We think that having a daily
colonization pressure chart for the main pathogens of
interest in a given ICU may therefore be useful for
quantifying the risk of new acquisitions and establish
the appropriate control measures aimed at preventing
this untoward event.
In regard to the role of antibiotics, the most striking
finding of the present study was that fluoroquinolones
were actually protective for the acquisition of P. aeruginosa
and rather neutral for the acquisition of its resistance
phenotypes. In critically ill patients, in the few previous
Table 3 Multivariate analysis of factors associated with
Pseudomonas aeruginosa acquisition during ICU staya
Colonization pressure >0.43
Prior exposure to fluoroquinolones
4 (1.2 to 12.8)
3.6 (1.7 to 7.5)
3.6 (1.8 to 7.6)
4.4 (2.3 to 8.3)
3.9 (1.6 to 9.6)
1.1 (0.6 to 2.2)
0.5 (0.2 to 1.2)
0.4 (0.2 to 0.8)
aCI, Confidence interval; ICU, Intensive care unit; OR, Odds ratio.
Hosmer-Lemeshow goodness-of-fit test value of 9.7 (P =0.2).
studies in which researchers have specifically investigated
P. aeruginosa acquisition, findings have been that
fluoroquinolones are protective against it in the pharynx  or in
any site . In hospital-wide studies, levofloxacin (but not
ciprofloxacin) has been reported to be protective against
nosocomial infection due to fluoroquinolone-susceptible
P. aeruginosa  and also against Gram-negative
bacilli (including P. aeruginosa) colonization or infection
with chromosomally mediated cephalosporin resistance
. All these data, including ours, suggest that
fluoroquinolones, when administered to critically ill patients not
previously colonized by P. aeruginosa, may decrease the
burden of new acquisition, even in settings where the
prevalence of resistance to fluoroquinolones is around
Ceftazidime (n =40)
Carbapenems (n =46)
Quinolones (n =39)
Amikacin (n =1)
MDR (n =31)
29%. Even more surprising is the persistent inability of our
group to find an independent association between prior
exposure to antipseudomonal quinolones and the
acquisition of a particular resistant phenotype or MDR . A
plethora of previous casecontrol or cohort studies have
linked this antibiotic class with resistance to themselves
[11-13], to antipseudomonal -lactams [10,17-19] or
to multiple drugs [21,23,25-27]. We do not have a
satisfactory explanation of this discrepancy, but the
fact that we found such association (of prior use of
quinolones with resistance to themselves and MDR)
in the univariate analysis, but not in the multivariate
analysis, enhances the absolute need of careful consideration
of potential confounders. In contrast to fluoroquinolones,
the present data confirm the involvement of prior
carbapenem use in acquisition of resistance to itself [9,14-16]. It
is of note that, in our experience, both quinolones and
carbapenems had a higher propensity than ceftazidime or
piperacillin-tazobactam to select resistance to themselves
when administered to patients previously colonized with
susceptible strains. These data suggest that, in critically ill
patients not colonized by P. aeruginosa or at low risk of
carriage, quinolones may be safer than carbapenems in
terms of risk of acquisition of resistant strains and
may even lower the burden of P. aeruginosa. In other
circumstances, ceftazidime and piperacillin-tazobactam
can be associated with a lower risk of resistance
acquisition because they apparently have a lesser tendency than
carbapenems to select resistance in patients previously
colonized. However, when trying to select an appropriate
empirical antibiotic regimen in patients with severe sepsis,
it remains appropriate to avoid the administration of
a recently used antipseudomonal antibiotic, to take into
consideration the local rates of P. aeruginosa resistance if
this pathogen is an issue and to consider the risk of other
MDR microorganisms .
In the present study, previous administration of
amikacin was associated with the acquisition of resistance
to fluoroquinolone, piperacillin-tazobactam and multiple
drugs in the multivariate analysis. A small number of prior
studies have also noted an association between amikacin or
aminoglycosides and acquisition of P. aeruginosa resistant
Table 4 Pseudomonas aeruginosa resistance to antibiotics when first detected and cross-transmission casesa
aICU, Intensive care unit; MDR, Multidrug-resistant. Number of patients in whom P. aeruginosa resistant to the different antipseudomonal antibiotics was isolated
during ICU stay, resistance status of the strains when first detected (percentage) and number of cases due to cross-transmission [in brackets].
to imipenem, [9,14] ceftazidime , piperacillin-tazobactam
 and multiple drugs [13,24]. We consider that such an
association cannot be attributed to the selection of MexXY
overproducers , because no increase in amikacin
minimum inhibitory concentrations were observed.
Although relying on multivariate analysis, there is still
room for non-casual associations; hence, these should
be validated in other studies.
Many of the non-antibiotic-related confounders in our
study denoting exposure to medical devices, severity of
the underlying condition and duration of critical status
have previously been reported as risk factors for the
acquisition of any P. aeruginosa or strains with single-drug
resistance or MDR phenotypes [3,28,50].
This study has several limitations, the most obvious
being that it was conducted in a single medical ICU
whose results may not be applicable to other settings with
different epidemiological characteristics. In addition, the
number of outcomes regarding the different resistance
phenotypes and MDR were scarce, thus increasing
uncertainty about the quality of the multivariate models. On the
other hand, as sensitivity of surveillance cultures was
probably not complete, the colonization status of some
patients could have been misclassified. Finally,
information about compliance with infection control measures
during the study period and surveillance cultures from the
environment was not available.
In ICU settings with a high rate of antibiotic use,
colonization pressure and non-antibiotic exposures may be
more important than antibiotics as determinants of P.
aeruginosa acquisition; antipseudomonal quinolones may
actually prevent it; and, regarding the acquisition of
resistance to selected -lactams (piperacillin-tazobactam),
fluoroquinolones and multiple drugs, exposure to amikacin may
be a more crucial risk factor than previously recognized.
In ICU settings with a high rate of antibiotic use,
colonization pressure and non-antibiotic exposures
are the main determinants of P. aeruginosa
Fluoroquinolones may prevent the acquisition of P.
Carbapenem exposure is associated with acquisition
of carbapenem resistance, and amikacin exposure is
associated with acquisition of resistance to
piperacillintazobactam and fluoroquinolones and MDR.
In previously sensitive strains of P. aeruginosa,
emergence of resistance occurs more frequently
after exposure to carbapenems and fluoroquinolones
than to ceftazidime or piperacillin-tazobactam.
Additional file 3: Relationship between acquisition of resistance to
antipseudomonal antibiotics and prior exposure to different agents.
Additional file 4: Multivariate analysis of factors associated with
acquisition of resistance to each antipseudomonal agent and MDR.
APACHE: Acute Physiology and Chronic Health Evaluation; CI: Confidence
interval; CNS: Central nervous system; COPD: Chronic obstructive pulmonary
disease; CVC: Central venous catheter; ICU: Intensive care unit; IQR: Interquartile
range; MDR: Multidrug-resistant; MRSA: Methicillin-resistant Staphylococcus
aureus; OR: Odds ratio; PDR: Pandrug-resistant; SOFA: Sequential Organ
Failure Assessment; VRE: Vancomycin-resistant enterococci; XDR: Extensively
The authors declare that they have no competing interests.
JAM, PC, JMN, JV, AS and JM participated in the conception, design, analysis
and interpretation of the data and drafted the manuscript. NC participated in
the collection, analysis and interpretation of the data and drafted the
manuscript. MS performed microbiological analysis and participated in
analysis and interpretation of the data. CH, MR, SF and JLT participated in
acquisition and interpretation of the data and revised the manuscript
critically. All authors read and approved the final manuscript.
This study was supported by a grant from the Fondo de Investigacin
Sanitaria, Subdireccin General de Evaluacin y Fomento de la Investigacin,
Ministerio de Ciencia e Innovacin, Gobierno de Espaa (PI050167 to JAM);
by a grant from the Departament dUniversitats, Recerca i Societat de
la Informaci de la Generalitat de Catalunya (2014SGR653 to MS); and
by funding from the European Community (SATURN, contract
HEALTH-F3-2009241796 to MS). NC is the recipient of a Ro Hortega
grant (CM12/00155) from the Instituto de Salud Carlos III and has also
been supported by Fundacin Privada Mximo Soriano Jimnez.
1. Kollef MH , Chastre J , Fagon JY , Franois B , Niederman MS , Rello J , et al. Global prospective epidemiologic and surveillance study of ventilator-associated pneumonia due to Pseudomonas aeruginosa . Crit Care Med . 2014 ; 42 : 2178 - 87 .
2. 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 .
3. Venier AG , Leroyer C , Slekovec C , Talon D , Bertrand X , Parer S , et al. Risk factors for Pseudomonas aeruginosa acquisition in intensive care units: a prospective multicentre study . J Hosp Infect . 2014 ; 88 : 103 - 8 .
4. El Amari EB , Chamot E , Auckenthaler R , Pechre JC , Van Delden C. Influence of previous exposure to antibiotic therapy on the susceptibility pattern of Pseudomonas aeruginosa bacteremic isolates . Clin Infect Dis . 2001 ; 33 : 1859 - 64 .
5. Carmeli Y , Troillet N , Eliopoulos GM , Samore MH . Emergence of antibiotic-resistant Pseudomonas aeruginosa: comparison of risks associated with different antipseudomonal agents . Antimicrob Agents Chemother . 1999 ; 43 : 1379 - 82 .
6. Riou M , Carbonnelle S , Avrain L , Mesaros N , Pirnay JP , Bilocq F , et al. In vivo development of antimicrobial resistance in Pseudomonas aeruginosa strains isolated from the lower respiratory tract of Intensive Care Unit patients with nosocomial pneumonia and receiving antipseudomonal therapy . Int J Antimicrob Agents . 2010 ; 36 : 513 - 22 .
7. Lipsitch M , Bergstrom CT , Levin BR . The epidemiology of antibiotic resistance in hospitals: paradoxes and prescriptions . Proc Natl Acad Sci U S A . 2000 ; 97 : 1938 - 43 .
8. Harris AD , Perencevich E , Roghmann MC , Morris G , Kaye KS , Johnson JA . Risk factors for piperacillin-tazobactam-resistant Pseudomonas aeruginosa among hospitalized patients . Antimicrob Agents Chemother . 2002 ; 46 : 854 - 8 .
9. Fortaleza CMCB , Freire MP , de Carvalho Moreira Filho D , de Carvalho Ramos M. Risk factors for recovery of imipenem- or ceftazidime-resistant Pseudomonas aeruginosa among patients admitted to a teaching hospital in Brazil . Infect Control Hosp Epidemiol . 2006 ; 27 : 901 - 6 .
10. Akhabue E , Synnestvedt M , Weiner MG , Bilker WB , Lautenbach E. Cefepime-resistant Pseudomonas aeruginosa . Emerg Infect Dis . 2011 ; 17 : 1037 - 43 .
11. Richard P , Delangle MH , Raffi F , Espaze E , Richet H. Impact of fluoroquinolone administration on the emergence of fluoroquinolone-resistant Gram-negative bacilli from gastrointestinal flora . Clin Infect Dis . 2001 ; 32 : 162 - 6 .
12. Kaye KS , Kanafani ZA , Dodds AE , Engemann JJ , Weber SG , Carmeli Y. Differential effects of levofloxacin and ciprofloxacin on the risk for isolation of quinolone-resistant Pseudomonas aeruginosa . Antimicrob Agents Chemother . 2006 ; 50 : 2192 - 6 .
13. D'Agata EMC , Cataldo MA , Cauda R , Tacconelli E. The importance of addressing multidrug resistance and not assuming single-drug resistance in case-control studies . Infect Control Hosp Epidemiol . 2006 ; 27 : 670 - 4 .
14. Harris AD , Smith D , Johnson JA , Bradham DD , Roghmann MC . Risk factors for imipenem-resistant Pseudomonas aeruginosa among hospitalized patients . Clin Infect Dis . 2002 ; 34 : 340 - 5 .
15. Lautenbach E , Synnestvedt M , Weiner MG , Bilker WB , Vo L , Schein J , et al. Imipenem resistance in Pseudomonas aeruginosa: emergence, epidemiology, and impact on clinical and economic outcomes . Infect Control Hosp Epidemiol . 2010 ; 31 : 47 - 53 .
16. Harris AD , Johnson JK , Thom KA , Morgan DJ , McGregor JC , Ajao AO , et al. Risk factors for development of intestinal colonization with imipenemresistant Pseudomonas aeruginosa in the intensive care unit setting . Infect Control Hosp Epidemiol . 2011 ; 32 : 719 - 22 .
17. Trouillet JL , Vuagnat A , Combes A , Kassis N , Chastre J , Gibert C. Pseudomonas aeruginosa ventilator-associated pneumonia: comparison of episodes due to piperacillin-resistant versus piperacillin-susceptible organisms . Clin Infect Dis . 2002 ; 34 : 1047 - 54 .
18. Gasink LB , Fishman NO , Nachamkin I , Bilker WB , Lautenbach E. Risk factors for and impact of infection or colonization with aztreonam-resistant Pseudomonas aeruginosa . Infect Control Hosp Epidemiol . 2007 ; 28 : 1175 - 80 .
19. Lin KY , Lauderdale TL , Wang JT , Chang SC . Carbapenem-resistant Pseudomonas aeruginosa in Taiwan: prevalence, risk factors, and impact on outcome of infections . J Microbiol Immunol Infect . In press. doi: 10.1016/j.jmii. 2014 .01.005.
20. Lpez-Dupla M , Martnez JA , Vidal F , Almela M , Soriano A , Marco F , et al. Previous ciprofloxacin exposure is associated with resistance to -lactam antibiotics in subsequent Pseudomonas aeruginosa bacteremic isolates . Am J Infect Control . 2009 ; 37 : 753 - 8 .
21. Defez C , Fabbro-Peray P , Bouziges N , Gouby A , Mahamat A , Daurs JP , et al. Risk factors for multidrug-resistant Pseudomonas aeruginosa nosocomial infection . J Hosp Infect . 2004 ; 57 : 209 - 16 .
22. Cao B , Wang H , Sun H , Zhu Y , Chen M. Risk factors and clinical outcomes of nosocomial multi-drug resistant Pseudomonas aeruginosa infections . J Hosp Infect . 2004 ; 57 : 112 - 8 .
23. Paramythiotou E , Lucet J , Timsit JF , Vanjak D , Paugam-Burtz C , Trouillet J , et al. Acquisition of multidrug-resistant Pseudomonas aeruginosa in patients in intensive care units: role of antibiotics with antipseudomonal activity . Clin Infect Dis . 2004 ; 38 : 670 - 7 .
24. Aloush V , Navon-Venezia S , Seigman-Igra Y , Cabili S , Carmeli Y. Multidrug-resistant Pseudomonas aeruginosa: risk factors and clinical impact . Antimicrob Agents Chemother . 2006 ; 50 : 43 - 8 .
25. Lodise TP , Miller CD , Graves J , Furuno JP , McGregor JC , Lomaestro B , et al. Clinical prediction tool to identify patients with Pseudomonas aeruginosa respiratory tract infections at greatest risk for multidrug resistance . Antimicrob Agents Chemother . 2007 ; 51 : 417 - 22 .
26. Gmez-Zorrilla S , Camoez M , Tubau F , Periche E , Caizares R , Dominguez MA , et al. 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 .
27. Montero M , Sala M , Riu M , Belvis F , Salvado M , Grau S , et al. Risk factors for multidrug-resistant Pseudomonas aeruginosa acquisition: impact of antibiotic use in a double case-control study . Eur J Clin Microbiol Infect Dis . 2010 ; 29 : 335 - 9 .
28. Martnez JA , Delgado E , Mart S , Marco F , Vila J , Mensa J , et al. Influence of antipseudomonal agents on Pseudomonas aeruginosa colonization and acquisition of resistance in critically ill medical patients . Intensive Care Med . 2009 ; 35 : 439 - 47 .
29. Chastre J , Wunderink R , Prokocimer P , Lee M , Kaniga K , Friedland I. Efficacy and safety of intravenous infusion of doripenem versus imipenem in ventilator-associated pneumonia: a multicenter, randomized study . Crit Care Med . 2008 ; 36 : 1089 - 96 .
30. Ong DSY , Jongerden IP , Buiting AG , Leverstein-van Hall MA , Speelberg B , Kesecioglu J , et al. Antibiotic exposure and resistance development in Pseudomonas aeruginosa and Enterobacter species in intensive care units . Crit Care Med . 2011 ; 39 : 2458 - 63 .
31. Lodise TP , Miller C , Patel N , Graves J , McNutt LA . Identification of patients with Pseudomonas aeruginosa respiratory tract infections at greatest risk of infection with carbapenem-resistant isolates . Infect Control Hosp Epidemiol . 2007 ; 28 : 959 - 65 .
32. Ajao AO , Harris AD , Roghmann MC , Johnson JK , Zhan M , McGregor JC , et al. Systematic review of measurement and adjustment for colonization pressure in studies of methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and Clostridium difficile acquisition . Infect Control Hosp Epidemiol . 2011 ; 32 : 481 - 9 .
33. DalBen MF , Basso M , Garcia CP , Costa SF , Toscano CM , Jarvis WR , et al. Colonization pressure as a risk factor for colonization by multiresistant Acinetobacter spp and carbapenem-resistant Pseudomonas aeruginosa in an intensive care unit . Clinics (Sao Paulo) . 2013 ; 68 : 1128 - 33 .
34. Martnez JA , Nicols JM , Marco F , Horcajada JP , Garcia-Segarra G , Trilla A , et al. Comparison of antimicrobial cycling and mixing strategies in two medical intensive care units . Crit Care Med . 2006 ; 34 : 329 - 36 .
35. Clinical and Laboratory Standards Institute (CLSI). Performance standards for antimicrobial susceptibility testing: 19th informational supplement . CLSI Document M100-S19 . Wayne, PA: CLSI; 2009 .
36. Durmaz R , Otlu B , Koksal F , Hosoglu S , Ozturk R , Ersoy Y , et al. The optimization of a rapid pulsed-field gel electrophoresis protocol for the typing of Acinetobacter baumannii, Escherichia coli and Klebsiella spp . Jpn J Infect Dis . 2009 ; 62 : 372 - 7 .
37. Magiorakos A , Srinivasan A , Carey RB , Carmeli Y , Falagas ME , Giske CG , et al. Multidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance . Clin Microbiol Infect . 2012 ; 18 : 268 - 81 .
38. Bonten MJ , Slaughter S , Ambergen AW , Hayden MK , van Voorhis J , Nathan C , et al. The role of colonization pressure in the spread of vancomycin-resistant enterococci: an important infection control variable . Arch Intern Med . 1998 ; 158 : 1127 - 32 .
39. Hyle EP , Gasink LB , Linkin DR , Bilker WB , Lautenbach E. Use of different thresholds of prior antimicrobial use in defining exposure: impact on the association between antimicrobial use and antimicrobial resistance . J Infect . 2007 ; 55 : 414 - 8 .
40. Patel N , McNutt LA , Lodise TP . Relationship between various definitions of prior antibiotic exposure and piperacillin-tazobactam resistance among patients with respiratory tract infections caused by Pseudomonas aeruginosa . Antimicrob Agents Chemother . 2008 ; 52 : 2933 - 6 .
41. Merrer J , Santoli F , Appr de Vecchi C , Tran B , De Jonghe B , Outin H. Colonization pressure and risk of acquisition of methicillin-resistant Staphylococcus aureus in a medical intensive care unit . Infect Control Hosp Epidemiol . 2000 ; 21 : 718 - 23 .
42. Lawrence SJ , Puzniak LA , Shadel BN , Gillespie KN , Kollef MH , Mundy LM . Clostridium difficile in the intensive care unit: epidemiology, costs, and colonization pressure . Infect Control Hosp Epidemiol . 2007 ; 28 : 123 - 30 .
43. Playford EG , Craig JC , Iredell JR . Carbapenem-resistant Acinetobacter baumannii in intensive care unit patients: risk factors for acquisition, infection and their consequences . J Hosp Infect . 2007 ; 65 : 204 - 11 .
44. Boyer A , Doussau A , Thibault R , Venier AG , Tran V , Boulestreau H , et al. Pseudomonas aeruginosa acquisition on an intensive care unit: relationship between antibiotic selective pressure and patients' environment . Crit Care . 2011 ; 15 :R55.
45. Castelo Branco Fortaleza CM , Moreira de Freitas F , da Paz Lauterbach G . Colonization pressure and risk factors for acquisition of imipenem-resistant Acinetobacter baumannii in a medical surgical intensive care unit in Brazil . Am J Infect Control . 2013 ; 41 : 263 - 5 .
46. Fortaleza CMCB , Figueiredo LC , Beraldo CC , de Melo EC , Pla PMS , Arago VDN . Risk factors of oropharyngeal carriage of Pseudomonas aeruginosa among patients from a medical-surgical intensive care unit . Braz J Infect Dis . 2009 ; 13 : 173 - 6 .
47. Schwaber MJ , Cosgrove SE , Gold HS , Kaye KS , Carmeli Y. Fluoroquinolones protective against cephalosporin resistance in Gram-negative nosocomial pathogens . Emerg Infect Dis . 2004 ; 10 : 94 - 9 .
48. Bhat S , Fujitani S , Potoski BA , Capitano B , Linden PK , Shutt K , et al. Pseudomonas aeruginosa infections in the intensive care unit: can the adequacy of empirical -lactam antibiotic therapy be improved? Int J Antimicrob Agents . 2007 ; 30 : 458 - 62 .
49. Masuda N , Sakagawa E , Ohya S , Gotoh N , Tsujimoto H , Nishino T. Substrate specificities of MexAB-OprM, MexCD-OprJ, and MexXY-OprM efflux pumps in Pseudomonas aeruginosa . Antimicrob Agents Chemother . 2000 ; 44 : 3322 - 7 .
50. Voor in 't holt AF, Severin JA , Lesaffre EMEH , Vos MC . A systematic review and meta-analyses show that carbapenem use and medical devices are the leading risk factors for carbapenem-resistant Pseudomonas aeruginosa . Antimicrob Agents Chemother . 2014 ; 58 : 2626 - 37 .