In Vitro Antimicrobial Resistance of Urinary Escherichia coli Isolates
among U.S. Outpatients from 2000 to 2010
Guillermo V. Sanchez,aRonald N. Master,bJames A. Karlowsky,cand Jose M. Bordond
School of Public Health and Health Services, George Washington University, Washington, DC, USAa; Quest Diagnostics Nichols Institute, Chantilly, Virginia, USAb;
Department of Medical Microbiology and Infectious Diseases, University of Manitoba, Winnipeg, Manitoba, Canadac; and Section of Infectious Disease, Providence
Hospital, Washington, DC, USAd
This study examines in vitro antimicrobial resistance data from Escherichia coli isolates obtained from urine samples of U.S.
showed the greatest increases in E. coli resistance from 2000 to 2010 for ciprofloxacin (3% to 17.1%) and trimethoprim-sulfame-
mal change. From 2000 to 2010, the antimicrobial resistance of urinary E. coli isolates to ciprofloxacin and TMP-SMX among
broad-spectrum agents (7). Resistant Escherichia coli isolates are
associated with decreases in clinical cure rates and higher risk of
recurrence (17, 20).
Several studies have described the in vitro susceptibility of E.
coli isolates among outpatients in the United States, and most of
data available for U.S. outpatients were collected from April 2003
urinary E. coli isolates to be 39.3% for ampicillin, 22.6% for trim-
and 1.4% for nitrofurantoin (22). Other, smaller regional studies
suggested a continued trend of rising resistance in the outpatient
setting (8, 13, 16).
Limited data are available to describe long-term trends in an-
timicrobial resistance of E. coli isolates among outpatients in the
antimicrobial resistance of urinary E. coli isolates among outpa-
tients in the United States from 2000 to 2010.
Antimicrobial susceptibility test results were obtained from
The Surveillance Network (TSN) Database—USA (Eurofins
Medinet, Chantilly, VA). This surveillance database collects data
susceptibility testing is performed on-site by each laboratory in
ntimicrobial resistance significantly increases patient mor-
bidity, costs of treatment, rates of hospitalization, and use of
accordance with FDA-approved testing methods and interpreted
using Clinical and Laboratory Standards Institute (CLSI)-recom-
mended breakpoints. TSN data have been used before to evaluate
trends in antimicrobial resistance, and further details of quality
control have been described previously (5, 9, 18, 19).
The present study included antimicrobial susceptibility data
emergency departments, hospital-based outpatient clinics, and
physicians’ offices. E. coli isolates with intermediate susceptibility
were not classified as being resistant. The outcomes of interest in
this study were the changes in and most-recent prevalence of an-
both intravenously administered agents, were selected as narrow-
Received 7 November 2011 Returned for modification 6 December 2011
Accepted 3 January 2012
Published ahead of print 17 January 2012
Address correspondence to Jose M. Bordon, email@example.com.
Copyright © 2012, American Society for Microbiology. All Rights Reserved.
TABLE 1 Annual rates of resistance of urinary Escherichia coli isolates to select antimicrobials among all outpatients from 2000 to 2010a
Antimicrobial resistance rate (%) for indicated yr
2000 200120022003 2004 2005 20062007200820092010
aIsolates demonstrating intermediate susceptibility were not counted as resistant. Amox-Clav, amoxicillin-clavulanate; TMP-SMX, trimethoprim-sulfamethoxazole.
bAll antimicrobial agents demonstrated statistically significant changes from 2000 to 2010.
0066-4804/12/$12.00Antimicrobial Agents and Chemotherapyp. 2181–2183aac.asm.org
and broad-spectrum cephalosporin surrogates, respectively, for
their orally administered formulations.
A chi-square test was performed for each antimicrobial agent
to determine whether a significant difference existed between re-
sistance rates observed in 2000 and those observed in 2010. An
alpha level of 0.05 was used. Analyses were performed using Sta-
tistical Analysis Software (SAS) version 9.1.
Antimicrobial susceptibility results (n ? 12,253,679) for uri-
in resistance among isolates obtained from all outpatients from
17.1% in 2010) and TMP-SMX (17.9% to 24.2%) (Fig. 1). Con-
amoxicillin-clavulanate (5% to 5.3%), and ceftriaxone (0.2% to
2.3%) demonstrated only small changes in resistance. In 2010,
ampicillin, tetracycline, cephalothin, and cefuroxime showed an-
timicrobial resistance rates of 43.4%, 24.9%, 18.1%, and 5.0%,
Emerging antimicrobial resistance of E. coli in the outpatient
setting is well documented (2, 3, 7, 12, 14). In the early 2000s,
quinolones surpassed sulfa drugs as the most common class of
antimicrobials prescribed by clinicians to treat uncomplicated
UTIs (10). This increase in provider use of fluoroquinolones may
account for the rapid rise in antimicrobial resistance of E. coli to
ciprofloxacin, as resistance to this agent has been shown to corre-
to acquire resistance to this agent, use of ciprofloxacin for empir-
only where local antimicrobial resistance rates remain low (4).
Antimicrobial resistance of urinary E. coli isolates to TMP-
SMX continued to increase from 2000 to 2010, a trend that has
continued for decades (1, 6, 11). In the 2010 IDSA guidelines for
ommended as the second-line antimicrobial agent (4). Our data
are consistent with previous reports regarding increases in anti-
microbial resistance of urinary E. coli isolates to TMP-SMX and
potential subsequent decreases in its efficacy as empirical therapy
among U.S. outpatients.
Levels of antimicrobial resistance of E. coli to cephalothin, the
narrow-spectrum oral cephalosporin surrogate, were higher than
those for expanded-spectrum (cefuroxime) and broad-spectrum
absolute change in antimicrobial resistance to ceftriaxone was
in resistance of E. coli to this agent over the study time period.
in our investigation were consistent with those reported previ-
nitrofurantoin demonstrated little change over our study time
period, a finding that is consistent with the resistance prevalence
reported in the NAUTICA study (22).
variety of antimicrobial agents studied, the large number of re-
for which data were reported, and the geographically representa-
include a lack of central laboratory testing, the use of multiple
susceptibility test methods, and an assumed underrepresentation
of isolates from those for whom empirical treatment was success-
ful. These data should be interpreted with caution. Although tra-
ditional in vitro surveillance systems are well designed to provide
insight into overall trends and prevalence of antimicrobial resis-
tance, they are not meant to guide antimicrobial therapy in the
management of individual clinical cases.
In summary, our study shows that from 2000 to 2010, antimi-
crobial resistance of urinary E. coli isolates to ciprofloxacin and
TMP-SMX increased substantially but that resistance to nitrofu-
rantoin and ceftriaxone remained low. Given the frequency with
which UTIs are treated empirically, compounded with the speed
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FIG 1 Cumulative annual change in E. coli antimicrobial resistance in outpatient urinary E. coli isolates from 2001 to 2010. Amox/Clav, amoxicillin-clavu-
Sanchez et al.
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