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99
Identifying and Controlling a Multiresistant Pseudomonas aeruginosa Outbreak in a Latin-American
Cancer Centre and Its Associated Risk Factors
Jorge Alberto Cortes1, Sonia Isabel Cuervo1,2, Ana María Urdaneta3, Guillermo Potdevin3, Patricia Arroyo2, Diana Bermúdez2,
Adriana Correa4 and María Virginia Villegas4
1Department of Medicine, School of Medicine, Universidad Nacional de Colombia; 2Instituto Nacional de Cancerología; 3Universidad de la
Sabana; Bogotá, Colombia; 4Centro Internacional de Investigaciones Médicas, CIDEIM; Cali, Colombia
Pseudomonas aeruginosa is an important and frightening microorganism for patients suffering from cancer.
Multiresistant P. aeruginosa (MRPA) may appear as a consequence of exposure to multiple antibiotics or from a
breakdown in infection control practices. This article reports an MRPA outbreak in a cancer treatment centre and
the consequent case control study. Mechanical ventilation was identified as being the main risk factor for developing
MRPA colonisation or infection; molecular analysis confirmed the outbreak. A multifaceted strategy was adopted,
involving reinforcing hand-washing practices, contact isolation, antibiotic restriction and suction devices for
mechanically-ventilated patients. MRPA was controlled and the outbreak ended. Such strategy may be effective in
controlling MRPS in low-resource environments amongst high risk cancer patients.
Key-Words: Pseudomonas aeruginosa, epidemiology, multiple drug resistance, bacteria, risk factor.
Received on 12 October 2008; revised 22 February 2009.
Address for correspondence: Dr. Jorge Alberto Cortes, MD. Medicine
Department, Oficina 317, Facultad de Medicina, Ciudad Universitaria,
Bogotá, Colombia. Telephone: (57 1) 3165000 Ext. 15011. Fax: (57
1) 3165000 Ext. 15012. E-mail: jorgecortes@yahoo.com.
The Brazilian Journal of Infectious Diseases 2009;13(2):99-103.
© 2009 by The Brazilian Journal of Infectious Diseases and Contexto
Publishing. All rights reserved.
Pseudomonas aeruginosa is a frequent cause of
nosocomial infection, such as nosocomial pneumonia (21%
of the cases), urinary tract infection (10% of cases) and surgical
site infections [1]. The most vulnerable patients are those
suffering from cancer and those submitted to mechanical
ventilation in intensive care units (ICU) [2]. P. aeruginosa is
the second Gram-negative bacillus more commonly identified
in our institution’s ICU (Instituto Nacional de Cancerología,
Bogotá, Colombia, devoted to caring for cancer in a developing
country) and the third most commonly found in nosocomial
infections in hospital wards [3].
P. aeruginosa has intrinsic resistance to several antibiotics
expressed in variable degrees by different isolates and
measured by several mechanisms [4]. When treating infections
caused by this microorganism it has been found that 10% of
patients develop resistance to at least one antibiotic [5,6].
Increased frequency of isolates having resistance to several
groups of medicaments has been observed during the last
few years [7], having great variation in observed prevalence
according to time and geographical region [8].
Increased frequency of P. aeruginosa isolates resistant to
several groups of antibiotics was observed in our institution’s
microbiology laboratory at the end of 2001 and start of 2002.
This article shows the risk-factors, molecular findings and
control strategies used during this particular multiresistant P.
aeruginosa (MRPA) epidemic in an institution for treating
patients suffering from cancer.
Materials and Methods
P. aeruginosa isolates resistant to amikacin, ciprofloxacin,
piperacillin, ceftazidime and imipenem (MRPA) were taken,
having minimum inhibitory concentration (MIC) cut-off points,
selected according to the following norms: ceftazidime, ≥ 32
mg/L, ciprofloxacin, ≥ 4 mg/L, amikacin ≥ 64 mg/L, pipercillin,
≥ 32 mg/L and imipenem, ≥ 16 mg/L. The MicrosScan (Dade
Behring, California, USA) automated system was used
according to NCCLS standards (2000). The isolates were
taken from patients diagnosed as suffering from cancer,
admitted to the Instituto Nacional de Cancerologia, (INC)
in Bogotá, Colombia, between May 2001 and May 2002.
This was intended to demonstrate the risk-factors
associated with MRPA in 26 isolates found during this
period by a retrospective case and control study. The
controls were randomly chosen in 1:3 (cases:controls) ratio
in the sense that they had been hospitalised on the same
day as the cases. The medical records of all the patients
identified as being cases and all the controls were reviewed
and possible risk-factors reported in the literature
associated with MRPA infection or colonisation were
evaluated: age, gender, type of tumour, antecedents of
having undergone chemotherapy during the month prior
to the isolate or hospitalisation, antecedents of co-
morbidity, the presence of neutropenia, having undergone
prior surgery, stay in the ICU, mechanical ventilation and
death.
Antecedents of being exposed to aminoglycosides,
ciprofloxacin, pipercillin-tazobactam, ceftazidime,
cefoperazon-sulbactam and/or imipenem during the month
prior to the isolate or during hospitalisation were evaluated.
Attributable mortality was defined as being that which
occurred during the 7 days following the isolate being taken.
A diagnosis of nosocomial infection was sought in the cases
according to CDC criteria [9]. Cases in which the isolate was
considered as being contaminant were also identified.
Pulsed-Field Electrophoresis (PFGE)
Chromosomal DNA from the strains was analysed
according to recommendations made by Gautom et al., some
modifications having been made. Briefly, a cell suspension
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was made in CSB buffer (100 mM Tris, 100 mM EDTA).
Minidisks were prepared using 200 uL cell suspension and
200uL LPM agarose; these were then placed in lysis buffer
(50mM Tris; 50 mM EDTA, pH 8.0, 1% N-laurylsarcosine, 1
mg/mL proteinase K) and incubated at 55°C in a water-bath
with shaking at 170 rpm. The minidisks were washed twice for
15 min with preheated (50°C) sterile ultra-pure water and then
with TE buffer (10 mM Tris, pH 8.0; 1 mM EDTA). The DNA
was digested with Xba I (Promega, Madison, WI) according
to the manufacturer’s instructions and electrophoresis was
carried out on 1% agarose gel using a Chef Mapper (BioRad,
Fremont, CA). The gels were visualised with ethidium bromide
and the results were analysed using Diversity software (Bio-
Rad) for determining similarity between the different isolates.
The strains whose percentage of similarity characterised them
as being indistinguishable and closely related were considered
to be clones.
Statistical Analysis
Univariate analysis was used for identifying the potential
factors for MRPA infection or colonisation. The χ squared test
was used for finding differences between categorical variables.
A multivariate logistical-regression model was constructed
using variables having greater clinical relevance or those having
a significant value in univariate analysis. The SPSS statistical
package (version 11.5) was used for statistical analysis.
Results
Description of the Epidemic
The epidemic was identified during March 2002 from 3 P.
aeruginosa isolates, obtained on the same day from 3 different
patients, having an identical antibiotic susceptibility pattern.
These patients were found in the ICU. A retrospective study
was carried out in which all P. aeruginosa isolates having the
same antibiotic resistance pattern were identified. It was
observed that the first patient having the multiresistant strain
had been admitted to the hospital on the 20th May 2001 and
the first isolate having this susceptibility pattern was identified
on the 20th June 2001. No isolate having this type of
multiresistance pattern had been reported by the laboratory
before this date. At least one multiresistant P. aeruginosa
isolate was detected in 24 patients between this date and
May 2002. An unsuccessful search for common sources was
carried out in the ICU during the epidemic’s initial detection.
A total of 78 isolates having the same susceptibility pattern
were obtained during this period. Figure 1 shows the
epidemic’s progression.
Average case age was 51 years; Table 1 gives the clinical
background. Two-thirds of the cases (n=16) corresponded to
nosocomial infections, 31% to operation site infection (n=5),
31% were patients having pneumonia associated with
ventilator (n=5) and 50% of the patients had bacteraemia (n=8);
some patients presented nosocomial infection and
bacteraemia. The P. aeruginosa isolates were considered as
being colonisation in the 8 remaining cases.
General mortality was 50% for the cases; however, mortality
attributable to nosocomial infection was 25%. This was
distributed as follows: 3 out of the 5 patients having
pneumonia associated with ventilator died (60%), 2 out of the
8 patients having bacteraemia died (25%) and one out of the 5
cases having operation site infection died (20%). None of the
patients treated to control the infection received antibiotics
having in vitro susceptibility to the isolates (most with
quinoline and beta-lactam combinations).
Case and Control Study
Table 1 shows the frequency of the different antecedents
and risk-factors recognised in the literature evaluated during
the case and control study. The multivariable regression model
results can also be observed with the variables having clinical
interest or greater statistical significance in the univariate
model. Having been submitted to mechanical ventilation was
considered to be the most important independent risk-factor
for developing MRPA infection or colonisation.
Even though previous exposure to antibiotics to which
the isolates presented resistance were not individually
considered to be risk-factors, the average number of antibiotics
received was significantly different between cases and
controls. The cases received an average of 1.3 antibiotics
before the first MRPA isolate was recognised, whilst the
controls received 0.33 antibiotics during their stay in hospital
(p<0.001). Additionally, stay in hospital was longer for cases
(41 days, on average) compared to controls (13 days, p <0.001).
Pulsed-Field Electrophoresis
Thirteen strains were kept for PFGE analysis; 2 of them
became degraded during the procedure and were not included
in the analysis. Indistinguishable patterns were observed in 8
out of the remaining 11 strains (72%); Figure 2 shows these
strains in lanes 4, 5, 6, 8, 9, 10, 11 and 12 from the haematological,
surgical hospitalisation and ICU rooms. The samples were
taken at different times during the epidemic, beginning in
October 2001. The strains shown in lanes 2, 3 and 7 in Figure
2 were classified as being closely related; each of these strains
only had one band of difference which might have been related
to a single genetic change (detailed mutation). The strains in
lanes 2 and 3 came from the oncology room, but were taken
during different months (October and November 2001) whilst
the strain in lane 7 came from the oncology hospitalisation
room and was taken during December 2001.
Control Strategies
Samples of possible fomites for P. aeruginosa were taken
in March 2002 without any positive result being obtained in
the ICU. A multidisciplinary strategy was then implemented
consisting of the following aspects. The active search for
colonised or infected patients and potential fomites was
continued. The policy of prescribing antimicrobial drugs was
changed to reduce selective pressure on the epidemic clone
and the prescribing of imipenem was suspended throughout
Multiresistant Pseudomonas Outbreak
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BJID 2009; 13 (April) 101
Figure 1. Time-line for the appearance of cases due to multiresistant P. aeruginosa epidemic in patients suffering from cancer.
Figure 2. Pulsed-field electrophoresis of isolates recovered
during the outbreak. Lane 1 shows molecular weight markers
and lanes 2 to 12 show the isolates obtained.
the whole institution. Patients identified as being cases were
submitted to strict isolation from contact in rooms apart and
were given independent nursing. The existing hand-washing
system was replaced by an automatic laser flushing device.
Education about hand-washing was reinforced amongst
health personnel. The method of secretion suction in patients
on mechanical ventilation in the ICU was changed from an
open to a closed system, so that there was no contact with
the environment.
Only one more patient was detected following this
intervention. The patients were kept in isolation until being
discharged from hospital. No P. aeruginosa isolates were
observed during 2003 and 2004 having the described
susceptibility pattern.
Discussion
A multiresistant P. aeruginosa epidemic clone was identified
which had been propagated for at least 10 months before being
detected in either the ICU or the hospitalisation rooms without
having been able to determine the transmission vector.
The means by which multiresistant P. aeruginosa clones
are disseminated can usually be identified as, for example,
aerosol dissemination [10], transrectal ultrasound [11] or
endoscopies [12]. MRPA clones have been isolated in vectors
such as bath toys [13] and plumbing circuits [14] in patients
suffering from cancer. No vector was identified in our case;
however, the epidemic was effectively eradicated once the
control measures had been implemented.
Several effective control strategies have been described
in the literature such as washing hands with alcohol-based
solutions, using microbiological surveillance for having up-
to-date information about multiresistant microorganism
hospital epidemiology, monitoring and early removal of
invasive devices and programmes for the rational use of
antimicrobial drugs [15].
Multidisciplinary intervention was developed in our case
which included effective strategies directed towards
previously-known modifiable risk-factors identified by
epidemiological study, and with which it was possible to
eradicate the MRPA outbreak.
According to multivariate analysis results, the variables
significantly associated with the MRPA isolate in our epidemic
were mechanical ventilation, antibiotics’ use and length of
hospital stay (in days). Other risk-factors identified in the
literature (such as advanced age, stay in ICU, presence of co-
morbidity or having received fluoroquinolines or imipenem/
meropenem during the 15 days prior to the isolate being taken)
[16] revealed no statistically significant difference between
cases and controls in our study.
Paramythiotu et al. have shown that using certain anti-
pseudomonas antibiotics plays a role in the appearance of
multiresistant strains [17]. Bearing in mind that the probability
of developing resistance during treatment is close to 10% [5],
it is possible that a patient having a longer stay in hospital
and greater exposure to different antibiotics [18], sequencially
[7], may develop resistance, especially in the context of cross
transmission in a small unit [16]. Bertrand et al., have described
a large-scale epidemic in a surgical ICU which lasted 55 months
[19]. No environmental reservoir was identified, but
colonisation of health personnel’s hands was documented.
Crossed P. aeruginosa transmission seems to be a frequent
cause of nosocomial acquisition. Close to half the isolates in
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Table 1. Univariate and multivariate analysis of the risk-factors studied during the multiresistant Pseudomonas aeruginosa
epidemic in the Instituto National de Cancerología, Bogotá, Colombia from May 2001 to June 2002.
Variable No. (%) of patients P analysis P analysis OR 95 %
Group MRPA Controls univariate mutltivariate CI
(n = 24) (n=72)
Age (average years) 51* 41* 0.62
Gender
Mal e 15 (62.5) 34(47.2) 0.39
Type of tumour
Solid 17 (70.8) 48 (66.7) 0.71
Co-morbidity 6 (25) 7 (9.7) 0.03
Chemotherapy 8 (33.3) 32 (44.4) 0.34
Neutropenia 5 (20.8) 11 (15.3) 0.53
Surgery 20 (83.3) 30(41.7) <0.001 0.14 2.8 0.7-11.7
Stay in ICU 19 (79.2) 14 (19.4) <0.001 0.91 1.2 0.6-12.7
Mechanical ventilation 18 (75) 8 (11.1) <0.001 0.014 12.2 0.1-12.6
Antibiotic use 21 (87.5) 38 (60.3) 0.002 0.19 2.7 1.2-125
Aminoglucosides 11 (45.8) 8 (12.7)
Cefoperazon/sulbactam 6 (26.1) 11 (17.5)
Ceftazidime 1 (4.3) 1 (1.6)
Ciprofloxacin 4 (16.7) 3 (4.8)
Imipenem 6 (26.1) 4 (6.3)
Pipercillin/tazobactam 5 (20.8) 1 (1.6)
Number of antibiotics used (average) 1,33 0,38 0,09
Death 12 (50) 9 (12.7) <0.001
MRPA, multiresistant Pseudomonas aeruginosa; *media. 1Institutional support by Instituto Nacional de Cancerología and Universidad Nacional de
Colombia.
that epidemic presented related electrophoretic patterns
suggesting that this had been the mode of adquisition [20]. In
an 18-bed ICU in France, 26% of the admitted patients acquired
P. aeruginosa carriage or colonization. Of those with P.
aeruginosa carriage, 23% ended with infection, that is almost
6% of all admitted patients during the study time [21].
Molecular methods have a role in identifying the clonal
spread of multiresistant isolates in a hospital. PFGE, randomly
amplified polymorphic DNA analysis, and ribotyping have been
used to evaluate circulation of P. aeruginosa in clinical areas
[22,23]. The first two are better able to discriminate genotypic
heterogeneity. The P. aeruginosa O:12 serotype has been more
frequently identified as being involved in miltidrug-resistant
epidemics [24]. The relationship between clonal dissemination
and antibiotic use has been shown in a recet paper by Jonas et
al. [25], in a prospective surveillance study in Germany; they
were able to establish a relationship between clonal spread of
resistant strains and antibiotic pressure. Although resistance
pressure is present by use of antibiotics, even in ICU setting
with high genodiversity (better infection control practices), the
increase in resistance is higher with the same antibiotic use in
low genodiversity units.
The outcome for patients from whom MRPA was isolated
showed a tendency towards greater mortality, this being
confirmed by total deaths amongst cases (12 = 50%), compared
to controls (9 = 14.5%).
Patients Suffering from Cancer and Pseudomonas
Infection in cancer patients since the 1970s has been mainly
caused by Gram-negative bacilli [26]. The frequency of
Pseudomonas spp. infections can be stressed within this
group, especially in neutropenic patients where they emerged
as a challenge due to the high mortality with which they were
associated, reaching rates of around 90% in patients suffering
from persistent neutropenia [27].
A clear tendency has been found towards reduced
Pseudomonas detection in patients suffering from
cancer. The frequency has been reduced from 4.7 cases
per 1,000 admissions to 2.8 cases during a 5 year-period
in a US cancer centre [28], without concomitant changes
having been noted in acute leukaemia prevalence.
Bacteraemia due to Pseudomonas is twice more frequent
in neutropenic patients than in non-neutropenic patients
and more common in patients diagnosed as having acute
leukaemia [27].
Regarding pneumonia, it has been found that it is mainly
caused by Pseudomonas in this group of patients,
representing 42% of the total of pneumonias and 68% of those
caused by Gram-negative bacilli [29]. MRPA dissemination in
a cancer-management institution has serious implications due
to the high rate of mortality associated with it and the
opportunity of becoming disseminated to patients at greater
risk of morbidity-mortality.
Multiresistant Pseudomonas Outbreak
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This panorama becomes complicated by the difficulty in
gaining access to alternative therapies (polymyxin B and
colistin) for MRPA in developing countries. The final result is
that a patient infected by this type of strain has a catastrophic
outcome, as shown by our study having 50% mortality in the
cases. It has also been shown that up to 20% of the patients
die during the first 24 hours after the onset of infection [30],
confirming the great impact of early diagnosis and treatment
on survival.
Identifying modifiable risk-factors, together with
implementing traditional multidisciplinary strategies (i.e.
isolation, hand-washing, rational use of antibiotics), leads
(even in areas with few resources) to effective control of
outbreaks having high mortality risk and possibilities of
dissemination amongst a susceptible population such as the
patients suffering from cancer attended in our institution.
Acknowledgements
We would like to thank Dr. Andrés Cardona and Dr. Carlos
Molina for their collaboration while investigating the outbreak.
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