The importance of colonization pressure in multiresistant Acinetobacter baumannii acquisition in a Greek intensive care unit
Arvaniti et al. Critical Care
The importance of colonization pressure in multiresistant Acinetobacter baumannii acquisition in a Greek intensive care unit
Introduction: We investigated the role of colonization pressure on multiresistant Acinetobacter baumannii acquisition and defined patient-related predictors for carriage at admission and acquisition during hospitalization in intensive care unit (ICU) patients. Methods: This was a 12-month, prospective, cohort study of all patients admitted to a single ICU of a tertiary hospital. Screening samples were collected at ICU admission to identify imported carriers, and weekly during hospitalization to identify acquisition. Colonization pressure (carriers' patient-days 100/all patients' patient-days) and the absolute number of carriers were calculated weekly, and the statistical correlation between these parameters and acquisition was explored. Multivariable analysis was performed to identify predictors for A. baumannii carriage at admission and acquisition during hospitalization. A. baumannii isolates were genotyped by repetitive-extragenic-palindromic polymerase chain reaction (PCR; rep-PCR). Results: At ICU admission, 284 patients were screened for carriage. A. baumannii was imported in 16 patients (5.6%), and acquisition occurred in 32 patients (15.7%). Acquisition was significantly correlated to weekly colonization pressure (correlation coefficient, 0.379; P = 0.004) and to the number of carriers per week (correlation coefficient, 0.499; P <0.001). More than one carrier per week significantly increased acquisition risk (two to three carriers, odds ratio (OR), 12.66; P = 0.028; more than four carriers, OR, 25.33; P = 0.004). Predictors of carriage at admission were infection at admission (OR, 11.03; confidence interval (CI), 3.56 to 34.18; P < 0.01) and hospitalization days before ICU (OR, 1.09; CI, 1.01 to 1.16; P = 0.02). Predictors of acquisition were a medical reason for ICU admission (OR, 5.11; CI, 1.31 to 19.93; P = 0.02), duration of antibiotic administration in the unit (OR, 1.24; CI, 1.12 to 1.38; P < 0.001), and duration of mechanical ventilation (OR, 1.08; CI, 1.04 to 1.13; P = 0.001). All strains were multiresistant. Rep-PCR analysis showed one dominant cluster. Conclusions: Acquisition of multiresistant A. baumannii in ICU patients is strongly correlated to colonization pressure. High levels of colonization pressure and more than two carriers per week independently increase acquisition risk. Patient-related factors, such as infection at admission and long hospitalization before the ICU, can identify imported A. baumannii carriers. Medical patients with extended administration of antibiotics and long duration of mechanical ventilation in the ICU were the most vulnerable to acquisition.
Acinetobacter baumannii has become a troublesome
emerging pathogen because of its wide range of
resistance determinants and its environmental resilience
[1,2]. It is responsible for severe nosocomial infections
in intensive care unit (ICU) patients, mainly
ventilatorassociated pneumonia (VAP), bacteremia, and central
nervous system infections [1,3-6]. Most of A. baumannii
strains are resistant to major antimicrobial drugs
(multidrug-resistant (MDRs)) [1,7,8]. MDR strains have
become endemic in several health care environments
and often cause sustained outbreaks [1,9-12]. Infected
and colonized patients (carriers) represent major
reservoirs for horizontal transmission and spread in
nosocomial environment, through the hands of healthcare
Acquisition of MDR bacteria in an ICU depends on
ICU-related variables (nurse-to-patient ratios,
compliance with hand hygiene) and patient-related factors
(severity of illness, prior hospitalization, invasive
procedures, antibiotic use) [13-18]. Colonization pressure
created by carriers (or colonized patients) has been
indicated as a contributing factor for acquisition of
MDR bacteria (methicillin-resistant Staphylococcus
aureus (MRSA) and vancomycin-resistant Enterococcus sp.
(VRE)) [19,20], but not yet extensively for A. baumannii
Our hypothesis was that the acquisition of
multiresistant A. baumannii in the ICU setting is strongly related
to high levels of colonization pressure and to
patientrelated factors. To determine the role of these factors,
we studied the effect of colonization pressure,
nurse-topatient ratios, severity, duration of invasive procedures,
and use of antibiotics on A. baumannii acquisition.
Additionally, we searched for predictors of carriage at
ICU admission, such as comorbidities, severity and type
of illness, prior hospitalization, and antibiotic use,
assuming that they can help physicians promptly
identify imported reservoirs of the pathogen.
Materials and methods
The study was conducted in a general (medical, surgical,
and trauma) adult, 15-bed ICU of a 750-bed,
tertiarycare, university-affiliated hospital in Thessaloniki,
Greece. The ICU had two subunits, one with 10 beds
and another with five beds. Subunits had only two-bed
rooms. Each nurse was responsible for two to three
patients and had 8-hour shifts. Six physicians and five
physicians-in-training worked in the unit every day.
Standard hygiene precautions  were applied. Hand
antisepsis with alcohol hand rubs, before and after each
contact with a patient and the surrounding surfaces, was
implemented. Screening sampling at ICU admission and
on a weekly basis was performed for A. baumannii and
MRSA. Contact isolation precautions with protective
gowns and gloves were applied for proven cases of
carriage or infection with A. baumannii or MRSA, as well
as for patients infected with carbapenem-resistant
Pseudomonas aeruginosa or Klebsiella pneumoniae . For
all those patients, confinement (isolation) was not
performed, because the unit was composed of two-bed
rooms, whereas nurse cohorting was implemented when
daily nurse-to-patient ratios were below or equal to 1:2.
For patients transferred from other institutions or other
ICUs, contact precautions were applied until results of
screening samples were collected at ICU admission.
Chlorhexidine was used for daily body washing of
Hand-hygiene compliance was not monitored.
Educational meetings were being held every 3 months to
emphasize the importance of hand hygiene and contact
precautions to ICU staff and to provide essential
feedback. Infection-control policy included continuous
surveillance of nosocomial infections and was revised yearly
by an ICU physician specialized in infectious diseases and
the Institutes Infection Control Committee. Nosocomial
infections in the ICU (that is, infections occurring after
the first 48 hours in the unit) were evaluated according
to CDC/NHSN definitions [24,25]. Antibiotic treatment
was based on epidemiologic data and guidelines for each
type of infection [26-30]. No selective digestive
decontamination or antibiotic rotation was performed.
Deescalation was routinely effectuated after culture results
[31,32]. The most common empiric antibiotic regimen in
case of clinical suspicion of severe nosocomial sepsis was
imipenem for the coverage of A. baumannii and P.
aeruginosa. An antibiotic with activity against MRSA was
initially prescribed in case of septic shock, presence of a
prosthetic endovascular or an orthopedic device, transfer
from wards with a high prevalence of MRSA, or
hospitalization in the ICU of more than one patient with
MRSApositive samples (screening or clinical samples).
This was a prospective cohort study that included all
consecutive patients admitted into the unit between
August 2008 and July 2009. Only the first ICU
admission per patient was included in the analysis. Patients
were evaluated for (a) carriage at admission and (b)
acquisition during hospitalization. Patients without
screening samples at admission were excluded. Imported
carriers and those without follow-up samples (due to
discharge or death before the scheduled follow-up
sampling) were excluded from acquisition analysis. Only the
initial isolate from each patient was counted and
underwent molecular typing.
The study was approved by the Institutes Ethics
committee. No written consent was obtained from patients,
because no therapeutic intervention was undertaken;
carrier screening was part of the routine practice in the
unit; and all data were anonymous.
Demographic data, underlying diseases, severity scores,
invasive procedures, antibiotic use, weekly
nurse-topatient ratios, and weekly levels of colonization pressure
were collected prospectively by using a standard written
questionnaire. The accuracy and completion of each
questionnaire was evaluated every week by two
More specifically, the following data were collected:
age, sex, date of hospital and ICU admission, diagnosis
on ICU admission, comorbidities, McCabe score,
immunodeficiency (hematologic malignancy, metastatic
cancer, AIDS, immunosuppressive therapy or
chemotherapy, corticosteroids (daily dose 1 mg/kg of
prednisolone for 1 month or any daily dose for > 1
month), neutropenia), Acute Physiology and Chronic
Health Evaluation-APACHE II, Simplified Acute
Physiologic Score-SAPS II, and Organ Dysfunction, and/or
Infection-ODIN on ICU admission, length of ICU stay,
and ICU outcome (death or not)) [33-36]. Imported and
acquired cases and time to A. baumannii acquisition
were recorded. During ICU hospitalization, duration of
antibiotic administration, of invasive procedures
(mechanical ventilation (MV), central venous (CVC) and
arterial catheterization, enteral feeding tube, urinary
tract catheterization) and duration of propofol infusion
were recorded in the questionnaire.
The unit of time chosen for measurements was the
week (from Monday through Sunday). Nurse-to-patient
ratios (N/P) per week were monitored, and weekly
colonization pressure created by all (imported and acquired)
A. baumannii carriers was calculated, as described by
Merrer et al. :
number of (admitted + acquired)carriers patient days in
the week before acquisition 100
total number of patient days in the week before
Samples for carriage were collected with cotton swabs
(Transwab for aerobic and anaerobic; Medical Wire and
Equipment Ltd., Corsham, England) from pharynx, skin,
and rectum (screening samples), at ICU admission
(initial sample, that is, within the first 48 hours of
admission) and afterward on a weekly basis (every
Monday), (follow-up samples) until discharge or death.
Swabs were processed in specific media, as previously
described . Identification, confirmation, and
susceptibility testing were performed with the Wider automated
system, according to the National Committee for
Clinical Laboratory Standards (NCCLS) criteria and
breakpoints . Microbiologic data including cultures
and antibiotic susceptibility tests of screening samples
were interpreted by a senior microbiologist on a daily
schedule. All isolates were kept at -35C for molecular
analysis, which was effectuated and interpreted blindly
by a senior specialized microbiologist, after completion
of all other data collection. Bacteria from the frozen
stock were inoculated onto Muller-Hinton agar
(SanofiPasteur, Marne la Coquette, France). PCR amplification
of repetitive extragenic palindromic sequences
(RepPCR) was performed for unique isolates by using the
primers and the protocol described by Snelling et al.
A patient was considered a carrier if he or she had at
least one positive screening sample. Carriage at ICU
admission (imported carriage) and acquisition during
hospitalization in the unit (acquired carriage) were
defined as isolation of A. baumannii from at least one
screening sample obtained within or later than the first
48 hours of admission, respectively. Imported carriers
were considered to be carriers throughout their entire
stay, and acquired carriers were considered to be
carriers from the date of the first positive sample until
discharge or death. An infection was considered to be
present at ICU admission if it was diagnosed within the
first 48 hours in the unit. Multiresistance was defined as
resistance to more than two of the following five drug
classes: antipseudomonal cephalosporins (ceftazidime or
cefepime), antipseudomonal carbapenems (imipenem or
meropenem), ampicillin-sulbactam, fluoroquinolones
(ciprofloxacin or levofloxacin), and aminoglycosides
(gentamicin, tobramycin, or amikacin) .
Descriptive analysis of the data was performed.
Continuous variables were presented as median (interquartile
range [IQR]) values, as they were not normally distributed
(if not, mean SD was added), and categoric variables, as
counts and percentages. Logistic regression analyses were
performed to assess predictors of A. baumannii carriage at
(a) admission and (b) acquisition during hospitalization.
All variables with a P value less than 0.05 in univariable
analysis were included in a logistic regression model for
multivariable analysis. To preserve the sufficient events
per variable, from the significant univariable parameters,
all possible two-variable models were determined for
imported A. baumannii carriers (n = 16) and all possible
three-variable models were determined for acquired
carriers (n = 32). The selection of the final multivariable
model presented was the one with the better fit in terms
of deviance, the one with the smallest -2-log likelihood.
Also, the goodness of fit was evaluated with the
HosmerLemeshow test, in which an insignificant result indicated a
good fit . Predictor variables that were highly
correlated were not entered together in the model to preserve
for collinearity problems. Odds ratios were presented with
the corresponding 95% confidence intervals (OR, CI 95%).
Correlation between acquisition and weekly values of
parameters (colonization pressure, total A. baumannii
carriers, and nurse-to-patient ratio) was calculated by using
the Pearson correlation coefficient. OR and 95% CI were
used to evaluate the risk for acquisition at different
numbers of total A. baumannii carriers hospitalized per week
in the ICU.
The Rep-PCR patterns were analyzed by using the
Bionumerics software (V1.5; Applied Maths, Ghent,
Belgium) in a blind fashion (that is, without knowledge of
the clinical data). Distance matrices were determined by
using the Pearson correlation coefficient, and
dendrograms were created by using the unweighted pair group
method with arithmetic averages (UPGMA).
All tests were two-sided, and a P value less than 0.05
was considered statistically significant in the
multivariable analyses. The analyses were conducted by using
PASW Statistics (version 18; SPSS, Chicago, IL, USA).
During the study period, 301 patients were admitted in
the ICU and assessed for eligibility. Forty-five (15%)
patients had an infection at ICU admission; 120 (40%)
had received previously antibiotics, and 168 (56%) were
hospitalized in the preceding 12 months. For 185 (61%)
patients, the main reason for ICU admission was
medical; 126 (42%) patients were transferred from the
emergency department; 143 (50%) from other hospital wards;
and 15 (5%) from ICUs of other institutions. Seventy-six
(25%) patients died in the ICU.
Seventeen (5.6%) patients 301 were excluded from
analysis for A. baumannii carrier status because
sampling was not performed (four died in less than 2 hours;
for eight patients, the sampling was neglected, and five
patients were readmitted). Finally, 284 (94.3%) of 301
patients were evaluated for A. baumannii carriage
Sixteen (5.6%) of 284 patients were imported carriers
at ICU admission and were excluded from acquisition
analysis. Two hundred sixty-eight (94.4%) of 284
patients were evaluated for A. baumannii acquisition.
Of these, 64 (23.9%) patients were excluded, as they had
no follow-up sample (48 were discharged, and 16 died
before the scheduled sampling).
Finally, 204 (76%) patients were evaluated for A.
baumannii acquisition. Thirty-two (15.7%) of 204 patients
acquired A. baumannii in the ICU after a delay of a
median 12 days (IQR, 10 to 17.5) (Figure 1).
The pharynx was the most frequent isolation site for
both imported (11 of 16, 69%) and acquired (19 of 32,
59%) carriers. Skin carriage was identified in four (25%)
of 16 imported and nine (28%) of 32 acquired carriers,
whereas the rectum was found to be positive in only
one (6%) of 16 imported and four (13%) of 32 acquired
The median calculated weekly colonization pressure
was 25.6% (IQR, 9.3 to 47.8) and ranged from 0 to 96%.
During 28 (54% of the study period) of 52 study weeks,
colonization pressure exceeded the median value. The
median number of all A. baumannii carriers per week
was 3.5 (IQR, 1 to7) and during 30 (58% of the study
period) of 52 study weeks, more than 3.5 carriers were
concomitantly hospitalized in the unit. The mean value
( SD) of the weekly nurse-to-patient ratio was 1:2.7 (
0.3) (range, 1:1.8 to 1:3.3) and median 1:2.7 (IQR, 1:2.5
to 1:2.7), whereas, for 19 (36.5% of the study period) of
52 weeks, ratios were below these values. Weekly
colonization pressure (correlation coefficient, 0.379; P =
0.004) and total number of A. baumannii carriers per
week (correlation coefficient, 0.499; P = 0.0001) were
significantly associated with A. baumannii acquisition,
but not the nurse-to-patient ratio (correlation
coefficient, -0.064; P = 0.064). The presence of more than
one A. baumannii carrier in the unit per week, either
imported or acquired, strongly predicted increased
acquisition risk (two to three carriers per week, OR,
12.66; P = 0.028; and more than four carriers, OR,
25.33; P = 0.004).
ODIN score, infection at ICU admission,
hospitalization before ICU admission, hospital days before ICU
admission, and antibiotic administration before ICU
admission were identified as risk factors for imported
carriage in univariable analysis (Table 1). A medical
reason for ICU admission, ICU days before acquisition,
antibiotic and propofol administration in the unit, days
of antibiotics and propofol use in the unit, and duration
of invasive procedures (mechanical ventilation, enteral
tube feeding, CVC, arterial and urinary tract
catheterization) were associated with acquisition in univariable
analysis (Table 2).
Infection at ICU admission (OR, 11.03; 95% CI, 3.56
to 34.18; P < 0.01) and duration of prior hospitalization
were independently associated with carriage at
admission in multivariable analysis (OR, 1.09; 95% CI, 1.01 to
1.16; P = 0.02) (Table 3). Hospitalization before ICU
admission and hospital days before ICU were not
entered together in the logistic regression models
because of collinearity.
In multivariable analysis for the acquisition, medical
reason for ICU admission (OR, 5.11; CI, 1.31 to 19.93; P
= 0.02), antibiotic days in the ICU (OR, 1.24; CI, 1.12 to
1.38; P < 0.001), and mechanical ventilation days (OR,
Figure 1 Flow chart of the patients.
1.08; CI, 1.04 to 1.13; P = 0.001) were found to be
significant risk factors (Table 3). The following factors
were not analyzed in the multivariable analysis for
acquisition: duration of enteral tube feeding, and of
arterial and urinary tract catheterization due to
collinearity with duration of mechanical ventilation.
Antibiotic use and mechanical ventilation before
acquisition were excluded, as they were found in all acquired
carriers. Propofol use, as a categoric variable, was not
analyzed in the acquisition multivariable analysis
Table 1 Univariable analysis of risk factors for multiresistant Acetinobacter baumannii carriage at ICU admission
A. baumannii carriers at ICU admission Noncarriers at ICU admission (n OR (95% CI)
(n = 16) = 268)
APACHE II, Acute Physiologic And Chronic Health Evaluation; CVC, central venous catheter; ICU, intensive care unit; IQR, interquartile range; ODIN, Organ
Dysfunctions and/or Infection; OR, odds ratio; Other, postsurgery follow-up, trauma, cancer, and immunodeficiency; SAPS II, Simplified Acute Physiology Score II.
because of clinical criteria. Although it was clearly used
in fewer acquired carriers than in nonacquired ones, we
considered that this result was found by chance, as our
study was not randomized, and propofol cannot be
considered a protective factor (OR, 0.188; CI, 0.085 to
0.417; P < 0.001) for multiresistant A. baumannii
acquisition. Duration of mechanical ventilation and of CVC
catheterization was not analyzed in the same logistic
regression model because of collinearity. When
mechanical ventilation days were replaced in the regression
model by CVC days, similar results were obtained, but
the model that contained the duration of mechanical
ventilation had a better fit (-2 log likelihood, 132.4),
than did the one with duration of CVC catheterization
(-2 log likelihood, 140.4).
All isolates were multiresistant (Table 4).
Nonsusceptibility to imipenem was noted in 6 (37.5%) of 16
imported and 15 (46.8%) of 32 acquired isolates.
Nonsusceptibility to colistin was rarely found (6%), and
this was exclusively among acquired isolates. High rates
of nonsusceptibility to other antimicrobials were
recorded. All recovered A. baumannii isolates were
genotyped. Rep-PCR patterns revealed clusters of isolates
(Figure 2 dendrogram). Among imported strains, 10
(62.5%) of 16 belonged to cluster III, and six (37.5%) of
16 to clusters I, II, and V. Among the 32 acquired
strains, 23 (72%) belonged to cluster III, and nine (28%),
to cluster II.
This prospective cohort study, performed in a general
ICU with severely ill patients, demonstrated the impact
of colonization pressure and patient-related variables
(medical reason for ICU admissions, antibiotic exposure,
and invasive procedures) on multiresistant A. baumannii
acquisition during ICU hospitalization and identified
predictors of multiresistant A. baumannii carriage at
0.60 (0.13-2.76) 0.52
10.41 (3.57-30.34) 0.001
23.29 (3.03-178.7) 0.002
Table 2 Univariable analysis of risk factors for multiresistant Acetinobacter baumannii acquisition during ICU
Days of mechanical ventilation, 11.5 (8.25-16)
CVC in the ICU before acquisition n (%) 26 (81)
Days of CVC, median [IQR]
Propofol use before acquisition n (%)
Days of propofol, median [IQR] 5 (3.75-7)
Arterial catheter before acquisition n (%) 20 (63)
Days of arterial catheter, median [IQR] 11 (8-14.7)
Enteral feeding catheter before 31 (97)
acquisition n (%)
Days of enteral feeding tube, median 12 (9.2-19)
Urinary catheter before acquisition n (%) 32 (100)
Days of urinary tract catheterization, 12.5 (9-18.25)
APACHE II, Acute Physiologic And Chronic Health Evaluation; CI, confidence interval; CVC, central venous catheter; ICU, intensive care unit; IQR, interquartile
range; ODIN, Organ Dysfunctions and/or Infection; OR, odds ratio; other, postsurgery follow-up, trauma, cancer, and immunodeficiency; SAPS II, Simplified Acute
Physiology Score II.
The present study is the first to correlate A.
baumannii acquisition with colonization pressure, signifying the
important role of this parameter in ICU setting. The
study also showed that the absolute number of all
(imported and acquired) multiresistant A. baumannii
carriers hospitalized in an ICU during a specific period
(a week), could be related to the increased likelihood for
new A. baumannii acquisitions. Previous studies already
3.85 (1.29-11.48) 0.016
Table 3 Multivariable analysis for multiresistant Acetinobacter baumannii carriage at ICU admission and acquisition
I, intermediate; R, resistant.
for A. baumannii
aThe best two-variable model with deviance 92.9 and Hosmer-Lemeshow test, c2 = 0.92; P = 0.1. bThe best three-variable model with deviance 132.3 and
Hosmer-Lemeshow test, c2 = 11.62; P = 0.17. cFactors not analyzed in the multivariable analysis for multiresistant A. baumannii acquisition: (a) duration of enteral
feeding tube, of arterial and urinary tract catheterization during hospitalization due to collinearity with duration of mechanical ventilation, and (b) antibiotic use
and mechanical ventilation before acquisition, as they were found in all patients with acquisition. Propofol administration was not analyzed in the multivariable
analysis for acquisition, as categoric variable, because of clinical criteria. CI, confidence interval; CVC, central venous catheter; ICU, intensive care unit; ODIN,
Organ Dysfunctions and/or Infection; OR, odds ratio.
presented similar results for VRE and MRSA [19,20],
probably because of the same transmission mode,
mainly via contacts with the hands of personnel. Two
other trials supported the influence of a high density of
A. baumannii infections on dissemination of the
pathogen, especially during outbreaks of carbapenem-resistant
strains [21,41]. For carbapenem-resistant K. pneumoniae,
a direct relation between colonization pressure and
nosocomial acquisition of the pathogen also was
Absence of correlation between acquisition and
nurseto-patient ratios in the current study certainly does not
underestimate the influence of understaffing and
overcrowding on multiresistant A. baumannii acquisition in
ICUs, already shown by others [21,43,44]. However, in
settings with a high incidence of MDR bacteria,
colonization pressure could be considered more sensitive than
staff ratios or patient-related variables and should be
used to alert health care personnel of increased
multiresistant A. baumannii acquisition risk [19,20].
Multiresistant A. baumannii carriers represented an
essential reservoir of the pathogen in the unit. Imported
carriers as initial sources and acquired carriers as
secondary sources of the pathogen generated a constant
colonization pressure that facilitated new acquisitions in
the ICU. Each A. baumannii carrier considerably
increased the acquisition risk for other patients. During
the study period, cross-transmission probably occurred
because of lapses in infection-control measures (hand
hygiene, contact precautions) in repeated circumstances
with high levels of colonization pressure. The
architectural design of the unit (only two-bed rooms available)
precluded patient isolation, which could possibly help
reduce A. baumannii dissemination. High
patientTable 4 Nonsusceptibility of Acetinobacter baumannii isolates to antibiotics
Imported A. baumannii isolates
Acquired A. baumannii isolates
Figure 2 Dendrogram illustrating genetic diversity among
multiresistant A. baumannii strains. Molecular analysis revealed
one predominant cluster (III) of A. baumannii isolates among other
minor ones (I, II, V). Cluster III included study isolates A1-2-3-5-6, B2-5-7,
C1-2-5-6-7, D2-3-4-5-6, E6-7-8, F1-2-4-5-6, and G1-2-4-6-7-8. Ten (62.5%) of 16
imported strains and 23 (72%) of 32 acquired strains belonged to
severity scores, prior hospitalization, and prolonged
exposure to invasive procedures and antibiotics
increased, for each ICU patient that was previously
exposed to high levels of colonization pressure, the
probability of acquiring one of the circulating clusters.
Imported A. baumannii carriers were admitted in the
unit from other hospital wards or other ICUs in town,
demonstrating dissemination in the entire hospital and
implying interinstitutional or even regional transmission
of the pathogen [1,9-12].
In accordance with other reports, we found that
duration of hospitalization before ICU admission was
independently associated with imported carriage [16,45]. For
each previous day in hospital, the risk of A. baumannii
carriage at ICU admission increased by 8% (P = 0.034).
Prior antibiotic use and duration of antibiotic
exposure had already been identified as predictors of
acquisition of MDR bacteria containing specific
resistance mechanisms [19,46,47]. Nevertheless, direct
association between duration of antibiotic administration
and multiresistant A. baumannii acquisition in the
ICU, as shown in the study, was not previously
Medical patients and patients with longer duration of
mechanical ventilation were at higher risk of acquiring
multiresistant A. baumannii during their ICU stay. For
medical patients, this could be attributed to underlying
comorbidities; however, this hypothesis can be neither
adopted nor rejected by the current study. Duration of
mechanical ventilation or CVC catheterization has
already been described as a risk factor for A. baumannii
Molecular analysis revealed one predominant cluster
of A. baumannii isolates among other minor ones,
probably reflecting endemicity and cross-transmission .
The study had limitations. First, compliance with hand
antisepsis and contact precautions was not monitored.
However, as our study was a nonblind prospective one,
the announcement of adherence monitoring could have
influenced the staff attitude and possibly biased the
results. Our aim was to evaluate A. baumannii
acquisitions under the current infection-control practices in
the ICU and not to estimate the level of staff
compliance or the effectiveness of standard and contact
precautions on the control of an outbreak.
Second, the workload for each ICU patient was not
calculated. Instead, we evaluated an important number
of patient-dependent factors, such as comorbidities,
severity scores, duration of invasive procedures, and
antibiotic exposure, which indirectly indicate the degree
of nurse workloads.
An important impact of colonization pressure on
multiresistant A. baumannii acquisition among ICU patients
was demonstrated, and this finding probably adds new
perspectives to the causative approach to sustained
endemic situations. ICUs with similar problems should
apply carriage-surveillance practices and implement
additional infection-control measures immediately after
identification of the first two carriers, rather than expect
manifestation of infections or further spread of the
pathogen. The factor Time, either as hospital days
before ICU admission, or as antibiotic treatment-days
and duration of mechanical ventilation during the ICU
stay, was recognized as an independent risk factor for
imported and acquired carriage, respectively. In settings
in which carriage screening is unattainable because of
increased cost and workload, infection-control measures
should focus on, at least, the timely checking of patients
with these risk factors.
Our study suggests that colonization pressure and
patient-related risk factors should be considered when
multiresistant A. baumannii acquisition remains an
unresolved issue in the ICU setting. Their consideration
may trigger implementation of appropriate preventive
measures such as better hand-hygiene compliance,
reduction of health care worker movement between
carriers and noncarriers, creation of cohorts of either
patients or nursing staff, and limitation of the number
of physicians entering patient rooms during rounds.
Multiresistant A. baumannii acquisition is
correlated to colonization pressure and the total number
of carriers per week. In endemic ICUs, surveillance
practices and additional infection-control measures
should be implemented immediately after
identification of the first two carriers, rather than expecting
further dissemination of the pathogen or infection
An important proportion of multiresistant A.
baumannii carriers are admitted into the ICU from
other wards of the hospital or other hospitals,
creating a substantial reservoir for new acquisitions in
ICUs in which the pathogen is endemic.
Duration of hospitalization before the ICU is an
important predictor of multiresistant A. baumannii
carriage at ICU admission.
Durations of antibiotic administration and
mechanical ventilation in the ICU are independent risk factors
for multiresistant A. baumannii acquisition.
APACHE II: Acute Physiologic and Chronic Health Evaluation; CI: confidence
interval; CVC: central venous catheter; ICU: intensive care unit; IQR:
interquartile range; ODIN: Organ Dysfunctions and/or Infection; OR: odds
ratio; SAPS II: Simplified Acute Physiology Score II.
We thank Theodoros Sakellariou for his technical assistance in developing
figures and tables. We thank Wyeth Hellas for molecular analysis.
KA and DL conceived the study and elaborated the data-analysis plan; KA,
DM, RR, PN, and VK were responsible for data acquisition; KA and DL revised
study data; DL and ABH were responsible for statistical analysis; DL, ABH, KA,
and RR performed data interpretation; KA and DL drafted the final
manuscript; and KA and SM revised all presented data and reevaluated the
manuscript. The corresponding author had full access to all the data in the
study and had the final responsibility for the decision to submit for
publication. All authors critically commented on the draft and approved the
The authors declare that they have no competing interests.
1. Peleg AY , Seifert H , Paterson DL : Acinetobacter baumannii: emergence of a successful pathogen . Clin Microbiol Rev 2008 , 21 : 538 - 582 .
2. Van Looveren M , Goossens H : Antimicrobial resistance of Acinetobacter spp . in Europe. Clin Microbiol Infect 2004 , 10 : 684 - 704 .
3. Garnacho-Montero J , Ortiz-Leyba C , Fernndez-Hinojosa E , Aldab-Palls T , Cayuela A , Marquez-Vcaro JA , Garcia-Curiel A , Jimnez-Jimnez FJ : Acinetobacter baumannii ventilator-associated pneumonia: epidemiological and clinical findings . Intensive Care Med 2005 , 31 : 649 - 655 .
4. Cisneros JM , Rodrguez-Bao J : Nosocomial bacteremia due to Acinetobacter baumannii: epidemiology, clinical features and treatment . Clin Microbiol Infect 2002 , 8 : 687 - 693 .
5. Briggs S , Ellis-Pegler R , Raymond N , Thomas M , Wilkinson L : Gram-negative bacillary meningitis after cranial surgery or trauma in adults . Scand J Infect Dis 2004 , 36 : 165 - 173 .
6. Al Shirawi N , Memish ZA , Cherfan A , Al Shimemeri A : Post-neurosurgical meningitis due to multidrug-resistant Acinetobacter baumanii treated with intrathecal colistin: case report and review of the literature . J Chemother 2006 , 18 : 554 - 558 .
7. Perez F , Hujer AM , Hujer KM , Decker BK , Rather PN , Bonomo RA : Global challenge of multidrug-resistant Acinetobacter baumannii . Antimicrob Agents Chemother 2007 , 51 : 3471 - 3484 .
8. Abbo A , Navon-Venezia S , Hammer-Muntz O , Krichali T , Siegman-Igra Y , Carmeli Y : Multidrug-resistant Acinetobacter baumannii . Emerg Infect Dis 2005 , 11 : 22 - 29 .
9. Villegas MV , Hartstein AI : Acinetobacter outbreaks, 1977 - 2000 . Infect Control Hosp Epidemiol 2003 , 24 : 284 - 295 .
10. Ansaldi F , Canepa P , Bassetti M , Zancolli M , Molinari MP , Talamini A , Ginocchio F , Durando P , Mussap M , Orengo G , Viscoli C , Icardi G : Sequential outbreaks of multidrug-resistant Acinetobacter baumannii in intensive care units of a tertiary referral hospital in Italy: combined molecular approach for epidemiological investigation . J Hosp Infect 2011 , 79 : 134 - 140 .
11. Cristina ML , Spagnolo AM , Ottria G , Sartini M , Orlando P , Perdelli F , Galliera Hospital Group: Spread of multidrug carbapenem-resistant Acinetobacter baumannii in different wards of an Italian hospital . Am J Infect Control 2011 , 39 : 790 - 794 .
12. Markogiannakis A , Fildisis G , Tsiplakou S , Ikonomidis A , Koutsoukou A , Pournaras S , Manolis EN , Baltopoulos G , Tsakris A : Cross-transmission of multidrug-resistant Acinetobacter baumannii clonal strains causing episodes of sepsis in a trauma intensive care unit . Infect Control Hosp Epidemiol 2008 , 29 : 410 - 417 .
13. Pittet D , Hugonnet S , Harbarth S , Mourouga P , Sauvan V , Touveneau S : Effectiveness of a hospital-wide programme to improve compliance with hand hygiene . Lancet 2000 , 356 : 1307 - 1312 .
14. Larson EL , Early E , Cloonan P , Sugrue S , Parides M : An organizational climate intervention associated with increased handwashing and decreased nosocomial infections . Behav Med 2000 , 26 : 14 - 22 .
15. Vicca AF : Nursing staff workload as a determinant of methicillin-resistant Staphylococcus aureus spread in an adult intensive therapy unit . J Hosp Infect 1999 , 43 : 109 - 113 .
16. Nseir S , Grailles G , Soury-Lavergne A , Minacori F , Alves I , Durocher A : Accuracy of American Thoracic Society/Infectious Diseases Society of America criteria in predicting infection or colonization with multidrugresistant bacteria at intensive-care unit admission . Clin Microbiol Infect 2010 , 16 : 902 - 908 .
17. 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 - 211 .
18. Koeleman JG , Parlevliet GA , Dijkshoorn L , Savelkoul PH , VandenbrouckeGrauls CM : Nosocomial outbreak of multi-resistant Acinetobacter baumannii on a surgical ward: epidemiology and risk factors for acquisition . J Hosp Infect 1997 , 37 : 113 - 123 .
19. Bonten MJ , Slaughter S , Ambergen AW , Hayden MK , van Voorhis J , Nathan C , Weinstein RA : The role of colonization pressure in the spread of vancomycin-resistant enterococci: an important infection control variable . Arch Intern Med 1998 , 158 : 1127 - 1132 .
20. Merrer J , Santoli F , Appere 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 - 723 .
21. D'Agata EM , Thayer V , Schaffner W : An outbreak of Acinetobacter baumannii: the importance of cross-transmission . Infect Control Hosp Epidemiol 2000 , 21 : 588 - 591 .
22. Garner JS : Guideline for isolation precautions in hospitals: The Hospital Infection Control Practices Advisory Committee . Infect Control Hosp Epidemiol 1996 , 17 : 53 - 80 .
23. Siegel JD , Rhinehart E , Jackson M , Chiarello L , Health Care Infection Control Practices Advisory Committee: 2007 Guideline for Isolation Precautions: preventing transmission of infectious agents in health care settings . Am J Infect Control 2007 , 35 : S65 - S164 .
24. Garner JS , Jarvis WR , Emori TG , Horan TC , Hughes JM : CDC definitions for nosocomial infections . Am J Infect Control 1988 , 16 : 128 - 140 .
25. 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 - 332 .
26. Rosenthal VD , Bijie H , Maki DG , Mehta Y , Apisarnthanarak A , Medeiros EA , Leblebicioglu H , Fisher D , Alvarez-Moreno C , Khader IA , Del Roco Gonzlez Martnez M , Cuellar LE , Navoa-Ng JA , Abouqal R , Garcell HG , Mitrev Z , Pirez Garca MC , Hamdi A , Dueas L , Cancel E , Gurskis V , Rasslan O , Ahmed A , Kanj SS , Ugalde OC , Mapp T , Raka L , Meng CY , Thu LT , Ghazal S , Gikas A , Narvez LP , Meja N , Hadjieva N , Gamar Elanbya MO , Guzmn Siritt ME , Jayatilleke K , INICC members: International Nosocomial Infection Control Consortium (INICC) report, data summary of 36 countries, for 2004-2009 . Am J Infect Control 2011 .
27. Peleg AY , Hooper DC : Hospital-acquired infections due to gram-negative bacteria . N Engl J Med 2010 , 362 : 1804 - 1813 .
28. Liu C , Bayer A , Cosgrove SE , Daum RS , Fridkin SK , Gorwitz RJ , Kaplan SL , Karchmer AW , Levine DP , Murray BE , J Rybak M, Talan DA , Chambers HF : Clinical practice guidelines by the infectious diseases society of america for the treatment of methicillin-resistant Staphylococcus aureus infections in adults and children: executive summary . Clin Infect Dis 2011 , 52 : 285 - 292 .
29. Rello J , Ulldemolins M , Lisboa T , Koulenti D , Maez R , Martin-Loeches I , De Waele JJ , Putensen C , Guven M , Deja M , Diaz E , EU-VAP/CAP Study Group: Determinants of prescription and choice of empirical therapy for hospital-acquired and ventilator-associated pneumonia . Eur Respir J 2011 , 37 : 1332 - 1339 .
30. Mermel LA , Allon M , Bouza E , Craven DE , Flynn P , O'Grady NP , Raad II , Rijnders BJ , Sherertz RJ , Warren DK : Clinical practice guidelines for the diagnosis and management of intravascular catheter-related infection: 2009 Update by the Infectious Diseases Society of America . Clin Infect Dis 2009 , 49 : 1 - 45 .
31. Ewig S : Nosocomial pneumonia: de-escalation is what matters . Lancet Infect Dis 2011 , 11 : 155 - 157 .
32. Joffe AR , Muscedere J , Marshall JC , Su Y , Heyland DK : The safety of targeted antibiotic therapy for ventilator-associated pneumonia: a multicenter observational study . J Crit Care 2008 , 23 : 82 - 90 .
33. McCabe WR , Jackson GG : Gram negative bacteremia: I. Etiology and ecology . Arch Intern Med 1962 , 110 : 845 - 847 .
34. Knaus WA , Zimmerman JE , Wagner DP , Draper EA , Lawrence DE : APACHEacute physiology and chronic health evaluation: a physiologically based classification system . Crit Care Med 1981 , 9 : 591 - 597 .
35. Le Gall JR , Lemeshow S , Saulnier F : A new Simplified Acute Physiology Score (SAPS II) based on a European/North American multicenter study . JAMA 1993 , 270 : 2957 - 2963 .
36. Fagon JY , Chastre J , Novara A , Medioni P , Gibert C : Characterization of intensive care unit patients using a model based on the presence or absence of organ dysfunctions and/or infection: the ODIN model . Intensive Care Med 1993 , 19 : 137 - 144 .
37. Berlau J , Aucken H , Malnick H , Pitt T : Distribution of Acinetobacter species on skin of healthy humans . Eur J Clin Microbiol Infect Dis 1999 , 18 : 179 - 183 .
38. NCCLS: Performance Standards for Antimicrobial Susceptibility Testing; 15th Informational Supplement . 2005 , 10 , Document M100 -S15.
39. Snelling AM , Gerner-Smidt P , Hawkey PM , Heritage J , Parnell P , Porter C , Bodenham AR , Inglis T : Validation of use of whole-cell repetitive extragenic palindromic sequence-based PCR (REP-PCR) for typing strains belonging to the Acinetobacter calcoaceticus-Acinetobacter baumannii complex and application of the method to the investigation of a hospital outbreak . J Clin Microbiol 1996 , 34 : 1193 - 1202 .
40. Bagley SA , White H , Colomb BA : Logistic regression in the medical literature: standards for use and reporting, with particular attention to one medical domain . J Clin Epidemiol 2001 , 54 : 979 - 985 .
41. Corbella X , Montero A , Pujol M , Dominguez MA , Ayats J , Argerich MJ , Garrigosa F , Ariza J , Gudiol F : Emergence and rapid spread of carbapenem resistance during a large and sustained hospital outbreak of multiresistant Acinetobacter baumannii . J Clin Microbiol 2000 , 38 : 4086 - 4095 .
42. Schwaber MJ , Lev B , Israeli A , Solter E , Smollan G , Rubinovitch B , Shalit I , Carmeli Y , Israel Carbapenem -Resistant Enterobacteriaceae Working Group: Containment of a country-wide outbreak of carbapenem-resistant Klebsiella pneumoniae in Israeli hospitals via a nationally implemented intervention . Clin Infect Dis 2011 , 52 : 1 - 8 .
43. Dancer SJ , Coyne M , Speekenbrink A , Samavedam S , Kennedy J , Wallace PG : MRSA acquisition in an intensive care unit . Am J Infect Control 2006 , 34 : 10 - 17 .
44. Kaier K , Meyer E , Dettenkofer M , Frank U : Epidemiology meets econometrics: using time-series analysis to observe the impact of bed occupancy rates on the spread of multidrug-resistant bacteria . J Hosp Infect 2010 , 76 : 108 - 113 .
45. Azim A , Dwivedi M , Rao PB , Baronia AK , Singh RK , Prasad KN , Poddar B , Mishra A , Gurjar M , Dhole TN : Epidemiology of bacterial colonization at intensive care unit admission with emphasis on extended-spectrum beta-lactamase- and metallo-beta-lactamase-producing Gram-negative bacteria: an Indian experience . J Med Microbiol 2010 , 59 : 955 - 960 .
46. del Mar Tomas M , Cartelle M , Pertega S , Beceiro A , Llinares P , Canle D , Molina F , Villanueva R , Cisneros JM , Bou G : Hospital outbreak caused by a carbapenem-resistant strain of Acinetobacter baumannii: patient prognosis and risk-factors for colonisation and infection . Clin Microbiol Infect 2005 , 11 : 540 - 546 .
47. Tacconelli E , De Angelis G , Cataldo MA , Pozzi E , Cauda R : Does antibiotic exposure increase the risk of methicillin-resistant Staphylococcus aureus (MRSA) isolation? A systematic review and meta-analysis . J Antimicrob Chemother 2008 , 61 : 26 - 38 .
48. Dent LL , Marshall DR , Pratap S , Hulette RB : Multidrug resistant Acinetobacter baumannii: a descriptive study in a city hospital . BMC Infect Dis 2010 , 10 : 196 .
49. Nseir S , Blazejewski C , Lubret R , Wallet F , Courcol R , Durocher A : Risk of acquiring multidrug-resistant Gram-negative bacilli from prior room occupants in the intensive care unit . Clin Microbiol Infect 2011 , 17 : 1201 - 1208 .
50. Fillaux J , Dubouix A , Conil JM , Laguerre J , Marty N : Retrospective analysis of multidrug-resistant Acinetobacter baumannii strains isolated during a 4-year period in a university hospital . Infect Control Hosp Epidemiol 2006 , 27 : 647 - 653 .