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MAJOR ARTICLE
Extended-Spectrum b-Lactamase–Producing
Escherichia coli and Klebsiella pneumoniae:
Risk Factors for Infection and Impact
of Resistance on Outcomes
Ebbing Lautenbach,1,2 Jean Baldus Patel,3 Warren B. Bilker,2 Paul H. Edelstein,3 and Neil O. Fishman1
1
Division of Infectious Diseases, Department of Medicine, the 2Department of Biostatistics and Clinical Epidemiology, and the 3Department
of Pathology and Laboratory Medicine, University of Pennsylvania Medical Center, Philadelphia
The prevalence of antibiotic resistance among extended-spectrum b-lactamase (ESBL)–producing Escherichia
coli and Klebsiella pneumoniae has increased markedly in recent years. Thirty-three patients with infection
due to ESBL-producing E. coli or K. pneumoniae (case patients) were compared with 66 matched controls.
Total prior antibiotic use was the only independent risk factor for ESBL-producing E. coli or K. pneumoniae
infection (odds ratio, 1.10; 95% confidence interval, 1.03–1.18; P p .006 ). Case patients were treated with an
effective antibiotic a median of 72 hours after infection was suspected, compared with a median of 11.5 hours
after infection was suspected for controls (P ! .001 ). ESBL-producing E. coli or K. pneumoniae infection was
associated with a significantly longer duration of hospital stay and greater hospital charges (P p .01 and
P ! .001, respectively). Finally, many ESBL-producing E. coli and K. pneumoniae isolates were closely related.
ESBL-producing E. coli and K. pneumoniae infections have a significant impact on several important clinical
outcomes, and efforts to control outbreaks of infection with ESBL-producing E. coli and K. pneumoniae should
emphasize judicious use of all antibiotics as well as barrier precautions to reduce spread.
Initial enthusiasm surrounding the introduction of oxyimino b-lactam agents in 1981 was quickly tempered by
the emergence of pathogens that are resistant to these
agents. Extended-spectrum b-lactamase (ESBL)–producing organisms were first isolated in Germany in 1983
[1], and outbreaks of infection due to these organisms
soon occurred in several European centers [2, 3]. The
Received 26 June 2000; revised 21 August 2000; electronically published 26
March 2001.
Presented in part: 36th annual meeting of the Infectious Diseases Society of
America, Denver, 12–15 November 1998.
This study was reviewed and approved by the Committee on Studies Involving
Human Beings of the University of Pennsylvania School of Medicine.
Reprints or correspondence: Dr. Ebbing Lautenbach, Presbyterian Medical Center,
University of Pennsylvania, Wright-Saunders Building, Suite W-250, 39th and
Market Streets, Philadelphia, PA 19104-6021 ([email protected]).
Clinical Infectious Diseases 2001; 32:1162–71
2001 by the Infectious Diseases Society of America. All rights reserved.
1058-4838/2001/3208-0006$03.00
1162 • CID 2001:32 (15 April) • Lautenbach et al.
first ESBL-producing isolates in the United States were
reported in 1989 [4, 5], and shortly thereafter, a number
of outbreaks of infection due to such organisms were
reported [6, 7].
The incidence of infections due to organisms resistant to b-lactam agents has increased sharply in recent
years. At 144 hospitals in the United States, the prevalence of ceftazidime-resistant Klebsiella pneumoniae
increased from 1.5% in 1989 to 3.6% in 1991 [8]. The
prevalence of such organisms in intensive care units
(ICUs) in the United States increased from 3.6% in
1990 to 14.4% in 1993, and it was as high as 21.8% in
large teaching hospitals [9].
It is imperative that risk factors for infections due
to ESBL-producing organisms be clearly identified so
that effective strategies to limit outbreaks of these infections may be developed. Several studies have attempted to elucidate risk factors for infections due to
ESBL-producing organisms, but the results have been
widely disparate. Although this disparity may be due in part
to true differences in the epidemiology of different outbreaks,
other potential reasons for this lack of consensus include failure
to distinguish colonization from true infection [6, 10–12], lack
of a comparison group [6, 7, 10, 13], small numbers of patients
[12–14], inclusion of only specific patient populations [7, 13],
and the limiting of the investigation to patients in ICUs [11,
12]. In addition, little is known regarding what role, if any, that
resistance plays in the prediction of negative outcomes.
Beginning in June 1997, we noted a marked increase in the
number of ESBL-producing isolates at our institution. We conducted the current study to identify risk factors for infection
with ESBL-producing Escherichia coli or K. pneumoniae and to
determine whether clinical outcomes differed between patients
with infections caused by resistant organisms and those with
infections caused by susceptible organisms.
METHODS
Risk factors for infection due to ESBL-producing organisms. This investigation was conducted at the University of
Pennsylvania Medical Center, a 725-bed academic tertiary care
hospital located in Philadelphia. To assess risk factors for infection due to ESBL-producing E. coli or K. pneumoniae, a
matched case-control study was conducted. All case patients
and control patients were identified through records of the
Clinical Microbiology Laboratory, which processes and cultures
all specimens obtained at this institution. All patients for whom
culture results were positive for E. coli or K. pneumoniae from
1 June 1997 through 31 May 1998 were eligible for inclusion
in the study. Designation as a case patient or a control patient
was based solely on whether the infecting organism was found
to demonstrate ESBL resistance.
Each patient was included as a case patient only once. If
ESBL-producing E. coli or K. pneumoniae was isolated on multiple occasions, only the first episode of infection was reviewed.
Potential controls were identified among hospitalized patients
who were infected with non–ESBL-producing E. coli or K. pneumoniae during the same period. Controls were matched in a
2:1 ratio to case patients according to the following 3 variables:
species of infecting organism, anatomic site of infection, and
date of isolation. Only patients who met the Centers for Disease
Control and Prevention’s criteria for infection were included
[15]. Nosocomial acquisition of infection was defined as follows: infection that occurred 148 h after admission to the hospital; infection that occurred !48 h after admission to the hospital, for patients who had been hospitalized within the 2 weeks
prior to admission; and infection that occurred !48 h after
admission to the hospital, for patients who had transferred from
an outside hospital or nursing home.
Potential risk factors for ESBL-producing E. coli or K. pneu-
moniae infection were ascertained by means of a review of
medical records. Data obtained included age, sex, race, hospital
location, number of hospital days prior to infection, and severity of illness, as calculated by means of the Acute Physiological and Chronic Health Evaluation (APACHE) II score [16].
The presence of a central venous catheter, urinary catheter, or
mechanical ventilation was also assessed. Finally, all antimicrobial therapy that was administered in the 30 days prior to
admission was documented.
The presence of the following comorbid conditions was documented: hepatic dysfunction, malignancy, diabetes mellitus,
renal insufficiency (indicated by a creatinine level of 12.0 mg/
dL or the requirement of dialysis [17]), HIV infection, neutropenia, corticosteroid use, prior organ transplantation, use
of an immunosuppressive agent in the 30 days prior to admission to the hospital, and surgical procedure or trauma in
the 30 days prior to admission.
Role of ESBL-resistance in outcomes. To evaluate the effect
of ESBL-producing E. coli or K. pneumoniae infection on clinical outcome, a retrospective cohort study including the same
case- and control patients was conducted. The following outcomes were assessed: clinical outcome, microbiological outcome, mortality attributable to infection, duration of hospital
stay after infection, and hospital charges accrued after infection.
Clinical outcome was classified as follows: “complete response,” for patients who had resolution of fever, leukocytosis,
and all signs of infection; “partial response,” for patients who
had abatement of abnormalities in the above parameters without complete resolution; “failure,” for patients who had absence
of abatement or deterioration in any clinical parameters; and
“uncertain,” for patients who had intermittent or recurrent
signs and symptoms that were not clearly attributable to infection [18]. Microbiological outcome was classified as follows:
“definite response,” for patients whose cultures were sterile after
a course of antimicrobial therapy; “probable response,” for patients whose cultures were sterile during a course of antimicrobial therapy; “failure,” for patients who had persistent
isolation of the organism after at least 3 days of antimicrobial
therapy; and “uncertain,” for patients who had intermittent
isolation of the pathogen with no clear temporal association
with antimicrobial therapy or absence of subsequent cultures
to assess microbiological response [18].
“Mortality directly attributable to bacteremia” was defined
as death in the setting of clinical evidence of active infection
and a positive culture result. “Mortality indirectly attributable
to bacteremia” was defined as infection that caused failure or
further compromise of an organ system and death that occurred
as a result of organ failure. The proportion of deaths directly
and indirectly attributable to infection defined the attributable
mortality rate [18, 19].
Microbiological methods. Susceptibilities to all antimicro-
Infections Due to ESBL-Producing Organisms • CID 2001:32 (15 April) • 1163
bial agents were determined according to criteria of the National Committee for Clinical Laboratory Standards [20] by
means of either a semiautomated system (MicroScan WalkAway
System, NC16 panel; Dade Behring) or disk diffusion susceptibility testing. K. pneumoniae and E. coli isolates for which the
MIC of ceftazidime was 12 mg/mL were suspected of producing
an ESBL or AmpC-type b-lactamase. Such isolates were subjected to the double-disk diffusion test, as described by Thomson and Sanders [21], with the exception that the ceftazidime
and amoxicillin/clavulanic acid disks were placed 15 mm apart.
AmpC-type b-lactamase production was suspected in isolates
for which the MIC of ceftazidime was elevated (12 mg/mL) that
were resistant to both amoxicillin/clavulanic acid and cefoxitin.
Isolates that demonstrated AmpC-type b-lactamase production
were also classified as ESBL-producing organisms.
Selected isolates were evaluated for relatedness by means of
genomic fingerprinting by using the typing procedure described
by Maslow et al. [22]. Fingerprinting data were interpreted according to the established guidelines [23]. To identify possible
plasmid-mediated resistance, bacterial conjugations were performed according to the broth mating procedure described by
Rice et al. [7]. For K. pneumoniae donor cells, the recipient strain
was E. coli MB4800 (strA derivative of E. coli SK2267 [24], which
was a gift of L. Silver, Merck Research Laboratories, Rahway, NJ).
Transconjugants were selected by plating the conjugation mixture
(donor and recipient cells) on laked blood media that contained
ceftazidime (3 mg/mL) and streptomycin A (1000 mg/mL). For
E. coli donor cells, the recipient strain was E. coli SM10lpir [25],
and the transconjugants were selected by plating the conjugation
mixture on laked blood media that contained ceftazidime (3 mg/
mL) and kanamycin (200 mg/mL).
Statistical analysis. Categorical and continuous variables
were compared by use of the Mantel-Haenzel test and conditional
logistic regression, respectively [26]. Multivariable analysis was
performed by means of conditional logistic regression [26]. All
variables for which a P value of ⭐.15 was determined by means
of bivariable analysis were considered for inclusion in an explanatory multivariable model. Building of the model began with
inclusion of certain key variables based on a priori hypotheses
(i.e., duration of antibiotic therapy and total number antibiotics
administered) as well as variables (e.g., age and sex) believed
likely to influence the association between the key variables and
the outcome of interest. The effect of inclusion of the matching
variables in the final multivariable model was also evaluated.
Conditional logistic regression was used to evaluate possible
associations between ESBL-producing E. coli or K. pneumoniae
infection and categorical outcomes. The association between
ESBL-producing E. coli or K. pneumoniae infection and continuous outcomes was evaluated by use of generalized estimating
equation regression for clustered data [27], following log transformation of the continuous outcomes (e.g., length of stay) to
1164 • CID 2001:32 (15 April) • Lautenbach et al.
approximate more closely a normal distribution. In evaluating
the association between ESBL-producing E. coli or K. pneumoniae
infection and various outcomes, we controlled for certain variables (i.e., APACHE II score and duration of hospitalization prior
to infection), which, on the basis of a priori hypotheses, were
believed likely to influence the association between ESBL-producing E. coli or K. pneumoniae infection and outcomes of interest. A 2-tailed P value of !.05 was considered significant. All
statistical calculations were performed by use of standard programs in STATA, version 5.0 (Stata).
RESULTS
During the study period, ESBL-producing E. coli or K. pneumoniae isolates were identified in 44 case patients, of whom 38
met criteria for infection [15]. Of these 38 case patients, 33
(86.8%) had medical records available for review.
Of the 33 patients with ESBL-producing E. coli or K. pneumoniae infection, 25 (75.8%) of them had infections due to K.
pneumoniae and 8 (24.2%) had infections due to E. coli. The
sites of infection were as follows: urinary, in 17 patients
(51.5%); wound, in 5 (15.2%); central venous catheter, in 4
(12.1%); blood, in 3 (9.1%); respiratory, in 3 (9.1%); and abdominal, in 1 (3.0%).
Case patients were significantly younger than were control
patients; also, they were more frequently male and had higher
APACHE II scores (table 1). In addition, case patients were
more likely to have nosocomial infection, to have had longer
hospitalizations prior to infection, and to have a central venous
catheter or urinary catheter in place than were controls (table
1). No significant differences were noted when hospital locations of case patients and control patients were compared. Nine
case patients (27.3%) and 13 controls (19.7%) were located on
a medical floor (OR, 1.62; 95% CI, 0.57–4.69; P p .37), whereas
6 case patients (18.2%) and 6 control patients (9.1%) were
located on a surgical floor (OR, 0.57; 95% CI, 0.19–1.69;
P p .31). Thirteen case patients (39.4%) and 17 control patients (25.8%) were located in an ICU (OR, 1.87; 95% CI,
0.78–4.52; P p .16).
When the comorbid conditions of the 2 groups were compared, case patients were significantly less likely than were control patients to have malignant disease (table 2). Renal insufficiency and diabetes mellitus were more common among case
patients than they were among control patients, although the
differences were not statistically significant. Finally, case patients
had significantly greater prior cumulative antibiotic exposure
(in terms of both total number of antibiotics and total duration
of antibiotic treatment) as well as greater exposure to extendedspectrum cephalosporins, fluoroquinolones, aminoglycosides,
cotrimoxazole, vancomycin, and metronidazole than did control patients (table 3).
Table 1. General characteristics of patients with infection due to extended spectrum b-lactamase (ESBL)–producing Escherichia coli or Klebsiella pneumoniae (case patients) and patients
with infection due to non–ESBL-producing E. coli or K. pneumoniae (control patients) in a study
of the risk factors for infection and the role of resistance in negative therapeutic outcomes in a
setting of an outbreak of infection.
Characteristic
Case patients
(n p 33)
Control patients
(n p 66)
OR (95% CI)
P
Age, median y (range)
54.0 (17–82)
65.5 (20–97)
0.97 (0.95–0.99)
.01
Male
17 (51.5)
18 (27.3)
3.27 (1.22–8.72)
.02
White
18 (54.6)
45 (68.2)
2.10 (0.78–5.66)
.14
Nosocomial infection
32 (97.0)
35 (53.0)
0.04 (0.01–0.32)
.002b
6 (18.2)
17 (25.8)
0.59 (0.19–1.84)
.37b
12.0 (1–27)
9.0 (0–30)
1.10 (1.02–1.18)
.02a
Polymicrobial infection
Median APACHE II score (range)
a
b
b
b
Nursing home residence
3 (9.1)
2 (3.0)
3.0 (0.50–17.95)
.23
Transferred from outside hospital
8 (24.2)
7 (10.6)
3.18 (0.92–11.00)
.07
Diarrhea
Central venous catheter
Urinary catheter
Mechanical ventilation
Duration of hospital stay,
median d (range)
9 (27.3)
18 (27.3)
1.0 (0.41–2.43)
19 (57.6)
16 (24.2)
11.56 (2.59–51.67)
b
b
1.0
b
.001
b
18 (54.6)
19 (28.8)
2.95 (1.22–7.11)
.02
8 (24.2)
10 (15.2)
2.38 (0.65–8.74)
.19b
2.0 (0–66)
1.05 (1.02–1.09)
.003
11.0 (0–167)
a
NOTE. Data are no. (%) of patients, unless otherwise indicated. APACHE, Acute Physiological and Chronic Health
Evaluation.
a
b
Conditional logistic regression.
Mantel-Haenzel test.
The only variable that remained an independent risk factor
for ESBL-producing E. coli or K. pneumoniae infection after
multivariable analysis was duration of antibiotic therapy (OR
for each additional day of antibiotic therapy, 1.10; 95% CI,
1.03–1.18; P p .006). In addition, there was a borderline significant association between ESBL-producing E. coli or K. pneumoniae infection and presence of a central venous catheter (OR,
9.85; 95% CI, 0.87–111.34; P p .06) and diabetes (OR, 5.10;
95% CI, 0.87–30.00; P p .07). The variables “age” and “sex”
were also included in the final model, but they were not associated with ESBL-producing E. coli or K. pneumoniae infection. Inclusion of the variables “duration of hospital stay prior
to infection” and “nosocomial versus community acquisition
of infection” in the model did not significantly alter the effect
sizes of the primary model variables and, therefore, they were
not included in the final explanatory model.
Results of antimicrobial susceptibility testing for the 33 ESBLproducing E. coli or K. pneumoniae isolates are shown in figure
1. Imipenem was the only agent to which these isolates did not
demonstrate resistance. As treatment (determined on the basis
of final results of antimicrobial susceptibility testing), 14 case
patients (42.4%) received levofloxacin, 7 (21.2%) received an
aminoglycoside, 5 (15.2%) received cotrimoxazole, 5 (15.2%)
received imipenem, and 2 (6.1%) received doxycycline.
Although patients with infections due to susceptible organisms were treated with an effective antibiotic (i.e., an agent to
which the infecting organism was susceptible) a median of 11.5
h after a specimen was sent for culture, patients with infections
due to resistant organisms were treated with an effective antibiotic a median of 72 h after the specimen was sent for culture
(OR for each additional hour of delay in effective therapy, 1.05;
95% CI, 1.02–1.07; P ! .001).
Of the case patients, 25 (75.8%) had complete or partial
clinical response to therapy compared with 55 (83.3%) of control patients (OR, 1.43; 95% CI, 0.95–2.14; P p .08; table 4).
When microbiological outcomes were compared, 19 case patients (57.6%) and 22 control patients (33.3%) had complete
or partial response to therapy (OR, 0.61; 95% CI, 0.39–0.93;
P p .02; table 4). Of note, microbiological outcomes for case
patients for whom repeated cultures were not performed were
considered to be nonresponses, and control patients were much
less likely than case patients to have repeated cultures performed (42.4% vs. 63.6%, respectively).
Infection with ESBL-producing E. coli or K. pneumoniae was
significantly associated with a greater median hospital charge
accrued subsequent to infection than was non–ESBL-producing
E. coli or K. pneumoniae infection ($66,590 vs. $22,231, respectively; charges were 2.90 times higher for case patients than
for control patients; 95% CI, 1.76–4.78; P ! .001). This difference remained significant after multivariable analysis was performed that controlled for APACHE II score and duration of
hospitalization prior to infection (charges were 1.71 times
Infections Due to ESBL-Producing Organisms • CID 2001:32 (15 April) • 1165
Table 2. Comorbid conditions in patients with infection due to extended-spectrum b-lactamase (ESBL)–producing Escherichia coli or Klebsiella pneumoniae
(case patients) and patients with infection due to non–ESBL-producing E. coli or
K. pneumoniae (control patients).
a
a
Comorbid condition
Case patients
(n p 33)
Hepatic dysfunction
3 (9.1)
Malignancy
4 (12.1)
12 (36.4)
13 (19.7)
Diabetes
Neutropenia
HIV infection
0
0
Control patients
(n p 66)
b
OR (95% CI)
P
7 (10.6)
0.86 (0.22–3.31)
.82
26 (39.4)
0.19 (0.05–0.68)
.01
2.36 (0.90–6.24)
.08
1 (1.5)
0
Renal insufficiency
12 (36.4)
14 (21.2)
2.46 (0.95–6.42)
.06
Surgery
16 (48.5)
25 (37.9)
1.66 (0.66–4.22)
.28
Trauma
2 (6.1)
3 (4.6)
1.33 (0.22–7.98)
.75
Steroid use
8 (24.2)
0
Transplant
5 (15.2)
6 (9.1)
1.67 (0.51–5.46)
.40
Immunosuppressive
agent treatment
6 (18.2)
6 (9.1)
2.16 (0.65–7.19)
.21
a
b
Data are no. (%).
Mantel-Haenzel test.
higher for case patients than for control patients; 95% CI,
1.01–2.88; P p .04).
Infection with ESBL-producing E. coli or K. pneumoniae was
also associated with a longer median duration of hospital stay
subsequent to infection than was non–ESBL-producing E. coli
or K. pneumoniae infection (11 vs. 7 days, respectively; median
duration of hospital stay for case patients was 1.76 times greater
than that for control patients; 95% CI, 1.17–2.64; P p .01).
This association remained significant after multivariable analysis was performed that controlled for APACHE II score at the
time of infection (median duration of hospital stay for case
patients was 1.73 times greater than that for control patients;
95% CI, 1.14–2.65; P p .01) but not after controlling for both
APACHE II score and duration of hospitalization prior to infection (median duration of hospital stay for case patients was
1.23 times greater than that for control patients; 95% CI,
0.81–1.87; P p .34).
Finally, 5 case patients (15.2%) had mortality attributable to
infection, compared with 6 control patients (9.1%; OR, 1.91;
95% CI, 0.49–7.42; P p .35). Of the 5 case patients whose
mortality was attributable to ESBL-producing E. coli or K. pneumoniae infection, 1 (20%) received appropriate therapy within
72 h of admission to the hospital. In comparison, 15 (53.6%)
of 28 case patients who survived received appropriate therapy
within 72 h of the time that the specimen was sent for culture.
Of 8 case patients whose infections involved the bloodstream,
3 died. Only 1 (33.3%) of these 3 patients received appropriate
therapy within 72 h of admission to the hospital, while 4 (80%)
of 5 patients who survived received appropriate therapy during
this period.
1166 • CID 2001:32 (15 April) • Lautenbach et al.
Molecular analysis of resistant E. coli and K. pneumoniae
isolates provided evidence of nosocomial transmission. Restriction patterns of chromosomal DNA from 11 (KP4–KP9,
KP11–KP13, KP15, and KP16) of the 13 K. pneumoniae isolates
analyzed were identical or differed by ⭐4 bands, which indicated either a clonal relationship or a close relationship, respectively (figure 2). Likewise, the restriction patterns of chromosomal DNA from E. coli isolates EC2 and EC6 indicated
that these isolates were closely related, and the restriction patterns of isolates EC3 and EC4 demonstrated a clonal relationship with each other. These data suggest that many of the
resistant isolates were acquired from an external source rather
than from the patient’s endogenous flora.
A conjugative plasmid conferring increased resistance to ceftazidime was detected in 7 of 16 K. pneumoniae isolates and 1
of 7 E. coli isolates (table 5). The inability to detect a conjugative
plasmid in all isolates was not unexpected, because other researchers have reported similar results [28]. Besides conferring
ceftazidime resistance, all of the plasmids conferred resistance
to gentamicin and tobramycin and, in the case of 2 plasmids
from isolates KP3 and KP16, amikacin resistance. Finally, 5 of
the plasmids transmitted resistance to cotrimoxazole (data not
shown). Colocalization of aminoglycoside and cotrimoxazole
resistance with ceftazidime resistance on a conjugative plasmid
is consistent with the increased OR associated with prior aminoglycoside and cotrimoxazole therapy.
DISCUSSION
Past attempts to identify risk factors for infection due to
ESBL-producing organisms have come to very different conclusions. This fact may be partly because most previous studies
failed to distinguish colonization with such pathogens from
true infection [6, 10–12]. It has been suggested that risk factors
for infection and colonization may indeed be different [29].
For example, although patients with severe underlying diseases
may have altered host defenses (and, therefore, greater risk of
infection), host defenses are of little importance in the determination of whether a patient is colonized by an organism that
is a natural inhabitant of the gastrointestinal tract [29]. To
identify specific patients with an increased likelihood of developing a clinical infection due to ESBL-producing E. coli or
K. pneumoniae, we have emphasized the elucidation of those
factors that predict true infection.
Although early correlational studies suggested an association
between antimicrobial use and the emergence of ESBL-producing E. coli or K. pneumoniae infections [6, 10, 13], these
studies failed to control for potential confounding factors and
lacked comparison groups. Subsequent case-control studies either noted no association between antimicrobial use and infection due to ESBL-producing organisms [11] or an association that did not remain after the researchers controlled for
other variables [14, 29, 30]. In fact, even 5 years after ESBLs
were first described in the United States, it was noted that the
true nature of the selective effect of antibiotics in fostering these
epidemics was not clear [31]. We found total exposure antimicrobial agents to be the only independent predictor of infection with ESBL-producing E. coli or K. pneumoniae; to our
Figure 1. Results of antimicrobial susceptibility testing for 33 extended-spectrum b-lactamase–producing Escherichia coli and Klebsiella
pneumoniae isolates recovered in an outbreak. n, Number of isolates
tested.
knowledge, this is the first time that such an independent association has been reported.
We found that the use of certain antibiotics was associated
with infection with ESBL-producing E. coli or K. pneumoniae.
In addition to greater exposure to extended-spectrum cephalosporins, we also noted that a significantly greater number of
case patients than control patients had been exposed to cotrimoxazole and aminoglycosides. Conjugation experiments demonstrated that resistance to these drugs is associated with the
presence of a conjugative plasmid that confers resistance to
aminoglycosides and, in some cases, cotrimoxazole in addition
Table 3. Prior antibiotic exposure of patients with infection due to extended spectrum blactamase (ESBL)–producing Escherichia coli or Klebsiella pneumoniae (case patients) and
patients with infection due to non–ESBL-producing E.coli or K. pneumoniae (control patients).
Case patients
(n p 33)
Control patients
(n p 66)
OR (95% CI)
P
Total no. of antibiotics, mean
no. (range)
2.58 (0–7)
0.65 (0–4)
2.47 (1.56–3.93)
!.001
Duration of antibiotic treatment, mean d (range)
21.7 (0–61)
3.41 (0–49)
1.12 (1.06–1.19)
!.001
16.0 (2.00–127.92)
.01b
Finding
a
a
No. (%) of patients treated
with specific antibiotic
Ceftazidime/ceftriaxone
8 (24.2)
1 (1.5)
Fluoroquinolone
7 (21.2)
0
Aminoglycoside
9 (27.3)
3 (4.6)
6.0 (1.62–22.16)
.01
Cotrimoxazole
13 (39.4)
6 (9.1)
20.68 (2.68–159.64)
.004
Vancomycin
15 (45.5)
5 (7.6)
7.15 (2.36–21.63)
!.001
4.82 (1.51–15.35)
b
Metronidazole
a
b
11 (33.3)
6 (9.1)
b
b
b
.01
Conditional logistic regression.
Mantel-Haenzel test.
Infections Due to ESBL-Producing Organisms • CID 2001:32 (15 April) • 1167
Figure 2. Pulsed-field gel electrophoresis patterns for chromosomal DNA from extended-spectrum b-lactamase–producing isolates recovered in an
outbreak. Top, Klebsiella pneumoniae isolates; bottom, Escherichia coli isolates. The number of each lane corresponds to the number of the isolate
(e.g., the isolates in lane 1 of top and lane 1 of bottom are KP1 and EC1, respectively). The lanes labeled l correspond to a pulsed-field l ladder
size standard (Bio-Rad). kb, Kilobase.
to the ESBL phenotype. Colocalization of multiple resistance
genes to a plasmid carrying ESBL-mediating genes has been
reported elsewhere [32]. These data suggest that the spread of
ESBLs may be partly due to the selective effect of other
antibiotics.
Although we found no association between hospital location
and infection with ESBL-producing E. coli or K. pneumoniae,
molecular epidemiological studies of these isolates clearly indicated the nosocomial spread of ESBL-producing E. coli or K.
pneumoniae isolates. On the basis of these results, as well as
the strong association between infection with ESBL-producing
E. coli or K. pneumoniae and antibiotic use, we hypothesize
that multiple events may be required for infection with ESBLproducing E. coli or K. pneumoniae to develop. First, the patient
must acquire ESBL-producing E. coli or K. pneumoniae through
contact with a colonized health care worker or contaminated
fomite. Second, the isolate must emerge as a result of the selective effect of antibiotic use.
These findings have important implications with regard to
possible interventions aimed at curbing such outbreaks. Efforts
should emphasize limiting contact transmission of resistant isolates as well as controlling antibiotic use. Most interventions
aimed at limiting antibiotic use have focused on restricting the
use of extended-spectrum cephalosporins and have met with only
1168 • CID 2001:32 (15 April) • Lautenbach et al.
modest success [6, 7, 10]. As demonstrated in our study, infection
with ESBL-producing E. coli or K. pneumoniae may be related
to the use of several different antibiotics, but it is most closely
associated with total antibiotic use. Elimination of one class of
Table 4. Clinical and microbiological outcomes for
patients with infection due to extended-spectrum b-lactamase (ESBL)–producing Escherichia coli or Klebsiella
pneumoniae (case patients) and patients with infection
due to non–ESBL-producing E. coli or K. pneumoniae
(control patients).
Outcome
Case patients
(n p 33)
Control patients
(n p 66)
Clinical
Complete response
16 (48.5)
49 (74.2)
Partial
9 (27.3)
6 (9.1)
Failure
1 (3.0)
2 (3.0)
Uncertain
7 (21.2)
9 (13.6)
Microbiological
Definite response
4 (12.1)
3 (4.5)
Probable response
15 (45.5)
19 (28.8)
Failure
0
Uncertain
NOTE.
14 (42.4)
2 (3.0)
42 (63.6)
See Methods for definitions of outcomes.
Table 5. Summary of characterized isolates from patients with infection due
to extended-spectrum b-lactamase–producing Escherichia coli or Klebsiella
pneumoniae.
Isolate
Location or
department
Date
PFGE
a
pattern
Detection of
conjugative
plasmid
K. pneumoniae
KP1
2 June 1997
Surgical ICU
KP-B
Yes
KP2
6 July 1997
Medicine
KP-C
Yes
KP3
24 August 1997
CCU
KP-D
Yes
KP4
16 September 1997
Surgery
KP-A
Yes
KP5
13 September 1997
CCU
KP-A1
No
KP6
21 September 1997
Medicine
KP-A1
No
KP7
17 October 1997
Surgery
KP-A
Yes
KP8
18 November 1997
Surgery
KP-A2
No
KP9
16 December 1997
Surgery
KP-A3
No
KP10
16 December 1997
Surgical ICU
KP-Eb
Yes
KP11
29 December 1997
Medicine
KP-A4
No
KP12
6 January 1998
Surgery
KP-A5
No
KP13
25 February 1998
CCU
KP-A3
No
KP14
24 February 1998
Medical ICU
KP-Fb
No
KP15
23 April 1998
Medicine
KP-A1
No
KP16
12 May 98
Medicine
KP-A1
Yes
EC1
1 June 1997
Surgical ICU
ND
Yes
EC2
3 November 1997
Surgical ICU
EC-A
No
EC3
4 January 1998
Medical ICU
EC-B
No
EC4
4 March 1998
Rehabilitation
EC-C
No
EC5
4 March 1998
Rehabilitation
EC-C
No
EC6
18 March 1998
Neurology
EC-D
No
EC7
21 August 1998
Medicine
EC-B
No
E. coli
NOTE. CCU, critical care unit; ICU, intensive care unit; ND, not determined; PFGE, pulsedfield gel electrophoresis.
a
Isolates with identical patterns have the same letter or letter and number designation.
Isolates that are closely related have the same letter designation as the related isolate but were
given a unique number.
b
These isolates are possibly related to KP-A.
antibiotics, or the substitution of one class for another, without
fully addressing the widespread problem of inappropriate antibiotic use will likely result in continued inadequate control of
such outbreaks of infection. Emphasis must be placed on the
rational and judicious use of all antimicrobial agents.
Despite the fact that patients with infections due to ESBLproducing organisms did not receive appropriate antimicrobial
therapy until a median of 72 h after infection was first suspected, these delays in treatment did not result in worse clinical
outcomes. One explanation for this lack of association may be
that treatment efficacy is affected by the site of infection. Empirical antimicrobial therapy for urinary tract infection with
agents to which the organism is resistant may be successful
because of the high antibiotic levels achieved in the urine and/
or local defense mechanisms. Of note, several studies have
found that the conditions of patients with urinary tract infections due to ESBL-producing organisms improve despite the
patients having received treatment with agents to which the
organisms are resistant [3, 6, 13]. The potential impact of inappropriate antimicrobial therapy might become clearer if one
focuses on more serious infections (e.g., bacteremia). Indeed,
a 1997 study of patients with bloodstream infection due to
ESBL-producing K. pneumoniae found a 75% crude mortality
rate among patients who received ineffective initial empirical
therapy, compared with 28% among patients whose initial therapy was active against the organism [33].
There are additional outcome measures that are of importance in assessing the impact of infections due to resistant
Infections Due to ESBL-Producing Organisms • CID 2001:32 (15 April) • 1169
organisms. We found that infection with ESBL-producing E.
coli or K. pneumoniae was associated with significantly longer
durations of hospital stay and increased hospital charges than
were infections due to susceptible organisms. These results suggest that, even if infections due to ESBL-producing organisms
have little impact on overall mortality, their impact on cost
may still be of great significance.
There are several potential limitations to our study. Although
the possibility of selection bias is normally of concern in a casecontrol study, all case patients and control patients were identified through the same microbiology laboratory. Because control patients were selected from the same hospital-based
population that provided the case patients and were matched
according to clearly defined criteria, the potential of selection
bias should be small, with the exception of what was introduced
through lost charts. Although misclassification bias is likewise
often of concern, both case- and control patients were identified
solely on the basis of data from antimicrobial susceptibility
testing. Because these tests were conducted without prior
knowledge of the status of the patients with regard to possible
exposures or outcomes of interest, there is not likely to be any
differential misclassification bias.
Although any retrospective chart review study may be limited
by the availability and completeness of medical records, we
found that 185% of charts were available for review. Even
though information concerning in-hospital antibiotic use was
available from the medical records, the possibility exists for
inaccuracies in data concerning antibiotic use at an outside
medical facility or as an outpatient. However, data regarding
recent antibiotic use would have been assessed at the time of
the patient’s admission, before the doctor knew whether the
patient had developed infection due to a resistant or susceptible
organism. Therefore, the nature of the bias due to missing
information, if present, is likely not differential, which would
bias the results toward the null hypothesis. Despite this potential bias, we found a highly significant difference in antibiotic
use in these 2 groups.
In conclusion, we found that total antimicrobial exposure
was the only independent predictor of ESBL-producing E. coli
or K. pneumoniae infection. Furthermore, molecular epidemiological analysis revealed that many of the ESBL-producing
E. coli or K. pneumoniae isolates were closely related. These
results suggest that if attempts to control outbreaks of infections
due to these organisms are to be successful, such efforts must
emphasize the rational and judicious use of all antimicrobial
agents. Careful attention to barrier precautions to prevent the
nosocomial spread of ESBL-producing E. coli or K. pneumoniae
infections must also be stressed. Finally, ESBL-producing E. coli
or K. pneumoniae infection was associated with a significantly
longer duration of hospital stay and greater hospital charges,
1170 • CID 2001:32 (15 April) • Lautenbach et al.
thereby demonstrating that these infections have an important
impact on clinical outcomes.
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