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Previous Article | Next Article 
Antimicrobial Agents and Chemotherapy, November 2001, p. 3084-3091, Vol. 45, No. 11
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.11.3084-3091.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Emergence of Reduced Susceptibility and
Resistance to Fluoroquinolones in Escherichia coli
in Taiwan and Contributions of Distinct Selective
Pressures
L. Clifford
McDonald,1,
Feng-Jui
Chen,1
Hsiu-Jung
Lo,1
Hsiao-Chuan
Yin,1
Po-Liang
Lu,2
Cheng-Hua
Huang,3
Pei
Chen,1
Tsai-Ling
Lauderdale,1 and
Monto
Ho1,*
Division of Clinical Research, National
Health Research Institutes,1 and Cathay
General Hospital,3 Taipei, and
Kaohsiung Medical University Hospital,
Kaohsiung,2 Taiwan
Received 20 March 2001/Returned for modification 11 June
2001/Accepted 9 August 2001
 |
ABSTRACT |
A survey of 1,203 Escherichia coli isolates from 44 hospitals in Taiwan revealed that 136 (11.3%) isolates were resistant to fluoroquinolones and that another 261 (21.7%) isolates had reduced
susceptibility. Resistance was more common in isolates responsible for
hospital-acquired (mostly in intensive care units) infections (17.5%)
than in other adult inpatient (11.4%; P = 0.08) and outpatient isolates (11.9%; P > 0.1).
Similarly, reduced susceptibility was more common in isolates
responsible for hospital-acquired infections (30.9%) than in other
adult inpatient (21.0%; P = 0.04) and
outpatient (21.4%; P = 0.06) isolates. Isolates
from pediatric patients were less likely to be resistant (1.3 versus
12.0%; P < 0.01) but were nearly as likely to
have reduced susceptibility (17.7 versus 21.9%;
P > 0.1) as nonpediatric isolates. There was an
inverse relationship in the proportion of isolates that were resistant versus the proportion that had reduced susceptibility among
isolates from individual hospitals (R = 0.031;
P < 0.05). In an analysis of isolates from two hospitals,
all 9 resistant strains possessed double point mutations in
gyrA and all 19 strains with reduced susceptibility strains
had single point mutations; no mutations were found among fully
susceptible strains. Risk factors for resistance included underlying
cancer (odds ratio [OR], 83; 95% confidence interval
[CI95], 7.3 to 2,241; P < 0.001), exposure to a quinolone (OR, undefined; P = 0.02), and exposure to a nonquinolone antibiotic (OR, 20;
CI95, 2.2 to 482; P < 0.001); underlying cancer was the only independent risk factor (OR, 83; CI95, 8.6 to 807; P < 0.001). There
were no significant associations between any of these factors and
reduced susceptibility. Whereas acute and chronic quinolone use in
cancer patients is a major selective pressure for resistance, other
undetermined but distinct selective pressures appear to be more
responsible for reduced susceptibility to fluoroquinolones in E. coli.
| |
INTRODUCTION |
The fluoroquinolones are an
important class of antibiotics for the treatment of urinary tract
infections and other common infections caused by gram-negative bacteria
(11, 13, 19). This is largely due to their excellent
activities against Escherichia coli, one of the most
frequently encountered human bacterial pathogens (27). It is because of their value in human medicine that
resistance to fluoroquinolones in E. coli has been viewed
with great concern wherever it has begun to emerge, including in parts
of Spain (4, 8), other European countries (10, 16,
21, 22), and various countries of East Asia (23, 28,
32). Although there are a few reports of resistance in North
America (3), overall rates remain low (1,
26). Recognized clinical risk factors for fluoroquinolone
resistance in E. coli include the therapeutic and
prophylactic use of fluorquinolones in cancer patients (4, 24,
32) as well as fluoroquinolone use in patients with chronic indwelling urinary catheters or other urinary tract abnormalities (2, 3, 6, 8).
Mutations at the target site appear to be the major mechanism for
fluoroquinolone resistance in E. coli (25). In
addition, recent work suggests that highly resistant strains of
E. coli may also have mutations affecting expression of
efflux pumps such as AcrAB (15, 30). Fluoroquinolones
possess bacteriocidal activity by the formation of enzyme-DNA complexes
involving DNA gyrase and topoisomerase enzymes responsible for
unwinding of the bacterial chromosomal during replication; point
mutations conferring resistance are localized to particular portions of gyrA, which is a gyrase subunit gene, and parC,
which encodes a topoisomerase subunit. In the case of E. coli, mutations in gyrA are predictive of major
differences in the level of resistance, irrespective of mutations in
parC (12, 23, 31). Whereas high-level
resistance in other members of the family Enterobacteriaceae may result from single mutations in gyrA, possibly all
E. coli strains with high-level resistance possess double
mutations (31).
In contrast, E. coli strains with single mutations in
gyrA remain susceptible to the fluoroquinolones.
However, the MICs of the fluoroquinolones for these strains are
markedly increased (i.e., they have reduced susceptibility) and, like
isolates with double mutations, demonstrate resistance to
nonfluorinated quinolones such as nalidixic acid. According to studies
of mutant E. coli strains selected in vitro, mutations in
gyrA result first in a substitution of serine 83 and then in
a substitution of aspartate 87 (12). Results of surveys of
mutant E. coli found in vivo appear to be consistent with
the stepwise occurrence of mutations observed in vitro; nearly all
mutants with a single-site mutation have substitutions at serine 83, and most isolates with substitutions at aspartate 87 also have
substitutions at serine 83 (7, 23).
Although several reports have described the epidemiology of E. coli with full resistance (i.e., double mutants) (2-4, 6, 8, 24, 32) and a substantial prevalence of E. coli
isolates with reduced susceptibility has been reported in Latin America (7), no reports have detailed the epidemiology of reduced
susceptibility to fluoroquinolones (i.e., mutants with single-site
mutations). Whether risk factors for reduced susceptibility differ from
risk factors for resistance and how rates of reduced susceptibility relate to rates of resistance in a community is unknown. Based upon
preliminary results of a nationwide survey of antibiotic resistance
performed in 1998, we found that over 10% of E. coli isolates in Taiwan were resistant to the fluoroquinolones
(14). Moreover, we found another large proportion of
E. coli isolates that, although susceptible, possessed
reduced susceptibility to fluoroquinolones. These findings prompted us
to investigate the epidemiology of emerging fluoroquinolone resistance
in Taiwan. Our objectives were to determine the relationship between
reduced susceptibility and resistance and, if related, to identify the antibiotic pressure most responsible for each level of resistance. We
sought to achieve these objectives through analysis of molecular resistance mechanisms, molecular epidemiology, and clinical risk factors.
| |
MATERIALS AND METHODS |
Collection and testing of clinical isolates.
Clinical
isolates of E. coli were collected between August and
December 1998 from 44 hospitals including 10 medical centers and 34 regional hospitals participating in the Taiwan Surveillance of
Antimicrobial Resistance program sponsored by the National Health
Research Institutes in Taipei, Taiwan. The medical center status of
hospitals is determined and updated regularly by the Taiwan Department
of Health on the basis of a larger bed number, an increased proportion
of intensive care unit beds, and a medical school affiliation. The
hospitals were distributed throughout four geographic regions: 20 in
the north, 10 in the south, 10 in the west, and 4 in the sparsely
populated east. Each hospital was asked to provide approximately 150 sequential isolates (i.e., isolates were not limited to E. coli) from individual patients (i.e., 1 isolate per patient) in
each of four specified categories: 50 from adult inpatients (including
at least 25 recovered from blood cultures), 25 from adult outpatients,
25 from adults with documented hospital-acquired infections, and 25 from pediatric patients, if available. Hospital-acquired infections
were identified by infection control personnel at each hospital as part
of their routine surveillance for nosocomial infections. Surveillance
in most participating hospitals was concentrated in the intensive care
unit and used definitions based upon the surveillance case definitions
for nosocomial infections of the U.S. Centers for Disease Control and
Prevention (Atlanta, Ga.) (9).
All isolates were stored frozen (
70°C) in a central laboratory,
where the species identification was confirmed by standardized methods.
Identification of E. coli was based on Gram staining, colony
morphology, spot indole, oxidase, and 
-glucoronidase test results
and/or results from the Vitek Gram Negative automated identification
system (bioMérieux, St. Louis, Mo.). Antimicrobial susceptibility tests were performed with all isolates by the disk diffusion method according to standards developed by the National Committee for Clinical Laboratory Standards (20). In
addition to ciprofloxacin, the antimicrobials tested included amikacin, ampicillin, aztreonam, amoxicillin-clavulanate, ceftazidime,
cefazolin, cefuroxime, gentamicin, piperacillin, and
trimethoprim-sulfamethoxazole. Zones of inhibition were measured with a
BIOMIC video reader (Giles Scientific Inc., Santa Barbara, Calif.).
Additional antimicrobial susceptibility testing was performed with a
subset of isolates from two hospitals. One hospital, Cathay General
Hospital, is a regional hospital located in Taipei, in the northern
region of Taiwan. The other, Kaoshiung Medical University
Hospital, is a medical center located in the southern city of
Kaoshiung. Disk diffusion testing with ofloxacin and nalidixic acid was
performed with these isolates, in addition to determination of the MICs
of ciprofloxacin and ofloxacin. MIC determinations were performed by
gradient agar diffusion by Etests (AB Biodisk, Solna, Sweden).
Molecular assessment of resistance mutations and strain
typing.
All resistant isolates, all isolates with reduced
susceptibility (see definitions below), and a sample of fully
susceptible E. coli isolates from Cathay General and
Kaoshiung Medical University Hospitals were assessed for point
mutations in the quinolone resistance-determining regions of
gyrA and parC. The oligonucleotide primers used
for amplification of the gyrA fragment were those described
by Weigel et al. (31) Primer gyrA6
(5'-CGACCTTGCGAGAGAAAT-3') corresponded to nucleotides 6 to
23, and primer gyrA631R (5'-GTTCCATCAGCCCTTCAA-3') was
complementary to nucleotides 631 to 614 of the E. coli gyrA sequence. Primer HJL3 (5'-AATGAGCGATATGGCAGAGC-3')
corresponded to nucleotides 1 to 19 of the E. coli
parC sequence, and primer HJL4
(5'-CTGGTCGATTAATGCGATTG-3') was complementary to
nucleotides 594 to 575 of the E. coli parC sequence.
The gyrA and parC gene fragments were amplified
from the chromosomal DNA present in crude cell lysates, prepared as
follows. A single colony was suspended in 60 µl of distilled water,
and the suspension was boiled for 10 min to prepare templates for PCR.
The crude cell lysate was diluted 10-fold in distilled water and was
either used immediately or stored at 20°C until needed.
Amplifications were carried out with a Peltier Thermal Cycler PTC-200
PCR System (MJ Research Inc., Waltham, Mass.) in 50-µl volumes
containing 50 pmol of each primer, 200 µM deoxynucleotide triphosphate, 1× reaction buffer (10 mM Tris-HCl [pH 9], 50 mM KCl, 1.5 mM MgCl2), 1 U of Taq polymerase
(Amersham Pharmacia Biotech Inc., Uppsala, Sweden), and 10 µl of
diluted cell lysate. An initial 5-min period of denaturation at 95°C
was followed by 35 cycles of denaturation (1 min at 94°C), annealing
(1 min at 60°C), and extension (1 min at 72°C), followed by a final
cycle of 72°C for 15 min. Agarose gel electrophoresis and ethidium
bromide staining to confirm the sizes of the gene fragments were used to visualize the amplification products. PCR products were purified on
QIA quick spin columns (QIAGEN, Chatsworth, Calif.) according to the
manufacturer's instructions. The DNA sequences were determined with a
Dye Terminator cycle sequencing kit with a PE Applied Biosystems 377 DNA sequencer (Applera Corporation, Norwalk, Conn.). To eliminate errors caused by amplification artifacts, the forward and reverse sequences of each quinolone resistance-determining region were determined.
Molecular typing of genomic DNA was performed by pulsed-field gel
electrophoresis (PFGE) with a temperature-controlled CHEF MAPPER System
(Bio-Rad Laboratories, Hercules, Calif.). Extraction and digestion of
genomic DNA were done according to the manufacturer's instructions for
the GenePath Group 6 Reagent kit (Bio-Rad). The agarose plugs were
digested with 50 U of XbaI, and PFGE was performed in 1%
Pulsed Field Certified Agarose (Bio-Rad). Gels were
electrophoresed for 28 h at 14°C at a constant voltage of 6 V/cm, with pulse times of 6.75 to 44.69 s with linear ramping.
Bacteriophage lambda ladders (Bio-Rad) were used as markers.
After staining with ethidium bromide, restriction fragments were imaged
with an IS-1000 Digital Imaging System (Alpha Innotech Corporation, San
Leandro, Calif.). PFGE pattern analysis was performed with CHEF Mapper
XA interactive software (version 1.2; Bio-Rad). Cluster analysis was
performed by the unweighted pair group method with arithmetic averages
with the Jaccard coefficient; dendrograms were prepared.
Clinical assessment of potential risk factors.
Available
medical records of all patients infected or colonized with E. coli submitted from Cathay General and Kaoshiung Medical University Hospitals were reviewed. This review included inpatient and
outpatient records from at least 8 weeks before collection of the
culture positive for E. coli through 2 weeks after
collection. Data collected included patient demographics,
hospitalizations, underlying disease, invasive device use, and prior
exposure to antimicrobials.
Data analysis and definitions.
All antimicrobial
susceptibility results, including zone diameters, were downloaded from
the BIOMIC software, converted in file format, and analyzed with Epi
Info software (version 6.04; Centers for Disease Control and
Prevention). A histogram of ciprofloxacin zone diameters was produced
(see Results) and demonstrated the existence of two distinct
populations of ciprofloxacin-susceptible isolates (zone diameters, >15
mm). On the basis of the zone size distributions of the isolates, we
defined full susceptibility as a zone diameter of 32 mm and reduced
susceptibility as a zone diameter of 16 to 31 mm.
Clinical data were entered into a relational database designed in
Access 97 software (Microsoft, Redland, Wash.), converted into a file
format, and analyzed with Epi Info software (version 6.04).
Associations between various potential risk factors (e.g., underlying
diseases, invasive devices, and prior antimicrobials) and outcomes of
full or reduced susceptibility or resistance were determined. For the
analysis of clinical risk factors, full susceptibility was redefined on
the basis of nalidixic acid susceptibility (zone diameter, >16 mm) and
the absence of resistance mutations in the case in which an isolate
with a ciprofloxacin zone diameter of 31 mm was misclassified as having
reduced susceptibility (see Results and Table 2).
The statistical significance of the association between categorical
variables was assessed by the chi-square test; continuous variables
were assessed by analysis of variance, or in cases in which the
variance within populations differed significantly, the Kruskal-Wallis
one-way analysis of variance test was used. Linear regression of the
proportion of resistant isolates and isolates with reduced
susceptibility at each hospital was performed by the method of least
squares. We used SPSS for Windows (release 10; SPSS, Chicago, Ill.) to
perform multivariate analysis of clinical data using binary logistic
regression in a stepwise fashion.
| |
RESULTS |
Antimicrobial susceptibility tests.
A total of 1,203 E. coli isolates were collected from the 44 hospitals. Overall, 1,067 (88.7%) were susceptible to ciprofloxacin and 136 (11.3%) were
resistant to ciprofloxacin. The distribution of ciprofloxacin zone
diameters demonstrated three distinct populations (Fig.
1). In addition to resistant isolates
(zone diameters, 
15 mm), there were populations of 261 (21.7%)
isolates with reduced susceptibility (zone diameters, 16 to 31 mm) and
806 (67.0%) fully susceptible isolates (zone diameters, 
32 mm).
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FIG. 1.
Distribution of ciprofloxacin inhibition zone diameters
for 1,203 E. coli isolates collected throughout Taiwan.
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There were no significant differences in the proportion of isolates
that had reduced susceptibility or that were resistant by regional
hospital versus medical center status, region, geographic location, or
site of the body from which the organism was isolated (data not shown).
Among isolates from individual hospitals, there was an inverse
relationship between the proportions of isolates that were resistant
and the proportions of isolates with reduced susceptibility (Fig.
2).
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FIG. 2.
Proportion of E. coli isolates with reduced
susceptibility to ciprofloxacin versus proportion of E. coli
isolates resistant to ciprofloxacin from 40 hospitals that submitted
more than 10 isolates.
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|
The rate of resistance tended to be greater among isolates from
patients with hospital-acquired (mostly in intensive care units)
infections (17 of 97 [17.5%]) than among isolates from other adult
inpatients (88 of 775 [11.4%]; P = 0.08) and,
although not significantly so, from outpatients (30 of 252 [11.9%]; P > 0.1). Similarly, the rate of reduced
susceptibility was greater among isolates from patients with
hospital-acquired infections (30.9%) than adult inpatient (163 of 775 [21.0%]; P = 0.04) and outpatient (54 of 252 [21.4%]; P = 0.06) isolates. Isolates from pediatric
patients were significantly less likely to be resistant (1 of 79 [1.3%] versus 135 of 1,124 [12.0%]; P < 0.01)
but were not significantly less likely to have reduced susceptibility
(14 of 79 [17.7%] versus 247 of 1124 [21.9%]; P > 0.1) than isolates from nonpediatric sources.
Other forms of antimicrobial drug resistance and their association with
reduced susceptibility or resistance to ciprofloxacin are depicted in
Table 1. For each drug tested, resistance
was most common in ciprofloxacin-resistant isolates, least common in
susceptible isolates, and of intermediate frequency in isolates with
reduced susceptibility.
Molecular assessment of resistance mutations and strain
typing.
A total of 36 isolates from Cathay General and
Kaoshiung Medical University Hospitals were analyzed in depth
(Table 2). These included all 9 isolates
from the hospitals that were fully resistant to ciprofloxacin (zone
diameters, 15 mm), all 19 strains that had reduced susceptibility
(zone diameters, 16 to 31 mm), and 7 randomly chosen fully susceptible
strains. Nalidixic acid resistance, as determined by disk diffusion,
was strongly predictive of reduced susceptibility and resistance to
ciprofloxacin on the basis of either the disk diffusion method or MIC
determination. Similarly, mutations in gyrA were found in
all isolates with reduced susceptibility and resistance to
ciprofloxacin; additional mutations in parC were found in
some isolates with reduced susceptibility and all resistant isolates.
No parC mutations were found in the absence of
gyrA mutations. One isolate (isolate 29) had a ciprofloxacin zone diameter of 31 mm (defined as reduced susceptibility on the basis
of the population distribution of all E. coli isolates) but
had no mutations in gyrA and parC and was
susceptible to nalidixic acid. The MICs of both ciprofloxacin (0.047 µg/ml) and ofloxacin (0.125 µg/ml) for this isolate lay between the
usual MICs for other isolates with reduced susceptibility and full
susceptibility. For the purpose of further clinical epidemiologic
assessment, this isolate was recategorized as fully susceptible.
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TABLE 2.
Quinolone susceptibility and mutations in the
gyrA and parC genes of 36 E. coli
isolates from two hospitals in Taiwan
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All 9 resistant isolates and 15 (79%) isolates with reduced
susceptibility had codon mutations in the genes encoding the 83rd amino
acid of gyrA, leading to a substitution of a leucine (Leu) for the wild-type serine (Ser) at that position. In addition, all
resistant isolates had a substitution of asparagine (Asn) for aspartate
(Asp) at the 87th amino acid position of gyrA. The nine
resistant isolates also had substitutions in the parC
subunit of topoisomerase IV. All but one isolate had a substitution of isoleucine (Ile) for serine (Ser) at the 80th amino acid position of
parC, and two isolates had substitutions for glutamate at
the 84th position (one had a glycine substitution and the other had a
lysine substitution). Of the 19 strains with reduced susceptibility, 3 (16%) had mutations that resulted in a single amino acid change of
parC, in addition to the 83rd gyrA substitution.
Four isolates with reduced susceptibility did not have mutations at the
83rd amino acid position of gyrA; instead, they all had
mutations at the 87th amino acid position.
PFGE of the 36 isolates demonstrated three pairs of isolates with
reduced susceptibility that were greater than 80% related by dendogram
analysis. The first of these pairs (Table 2, isolates 19 and 20) were
approximately 90% related, had identical antibiograms, were from two
different patients at the same hospital, and were collected 6 days
apart. The second pair of isolates (isolates 16 and 23) were just over
80% related, had identical antibiograms, were from different patients
at the same hospital at which isolates 19 and 20 were recovered, and
were collected 16 days apart. The third isolate pair (isolates 27 and
28) were approximately 95% related and had different antibiograms
(one was ampicillin and gentamicin resistant, whereas the other was
susceptible to both drugs) and were collected from patients at two
different hospitals 21 days apart. Whereas the first two pairs of
isolates with greater than 80% homology had codon mutations that
resulted in identical amino acid substitutions, the third pair
(isolates 27 and 28) had codon mutations that resulted in different
amino acid substitutions (Table 2).
Clinical assessment of potential risk factors for reduced
susceptibility and resistance.
There were no significant
differences in demographics, underlying disease, or potential risk
factors for resistance between patients infected with fully susceptible
isolates versus patients infected with isolates with reduced
susceptibility (Table 3). There were,
however, several important differences between patients infected with
resistant isolates and patients infected with fully susceptible
isolates (Table 4). There was no
significant difference between patients infected with resistant
isolates or isolates with reduced susceptibility versus patients
infected with fully susceptible isolates with regard to the body site
of isolation (data not shown). Patients infected with resistant
isolates were more likely to have underlying cancer, to have received
any antibiotic, or to have received a nonquinolone antibiotic than
patients infected with fully susceptible isolates. On the basis of
multivariate analysis, only underlying cancer was confirmed to be an
independent risk factor for resistance (odds ratio, 83; 95% confidence
interval, 8.6 to 807; P < 0.001).
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TABLE 3.
Comparison of demographic and clinical factors for
patients infected with E. coli isolates with reduced
susceptibility to ciprofloxacin versus those infected with fully
susceptible E. coli isolates
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TABLE 4.
Comparison of demographic and clinical factors for
patients infected with ciprofloxacin-resistant versus fully
susceptible E. coli isolates
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There was a nonsignificant trend in both nonquinolone antibiotic and
overall antibiotic exposure (Tables 3 and 4), with rates of exposure
being the lowest in patients infected with fully susceptible isolates
(28%), intermediate in patients infected with isolates with reduced
susceptibility (35 to 40%), and highest in patients infected with
resistant isolates (89%). The one patient who was infected with a
strain with reduced susceptibility and who was exposed to a quinolone
was exposed only to nalidixic acid, whereas both patients infected with
resistant strains were exposed to ciprofloxacin.
| |
DISCUSSION |
We found that 11.3% of E. coli isolates in Taiwan were
resistant to fluoroquinolones and that another 21.7% had reduced
susceptibility. Isolates submitted from two hospitals demonstrated a
predictable association between single and double mutations in
gyrA and the MICs and zone diameters for fluoroquinolones
(Table 2). All isolates with reduced susceptibility from these
hospitals possessed single point mutations, a necessary prerequisite
for resistance. Therefore, the finding of such a large proportion of
E. coli (21.7%) isolates throughout Taiwan with reduced
susceptibility suggests that the rate of resistance may rapidly
increase further. In addition to increasing the likelihood of evolution
toward resistance, this large number of strains with reduced
susceptibility may portend an increased risk of clinical failure when
infections are treated with fluoroquinolones. For example, it has been
demonstrated that infections caused by Salmonella spp. with
reduced susceptibility have a higher clinical failure rate than
infections caused by fully susceptible strains when treatment is with a
fluoroquinolone (18, 29).
The finding that cancer patients were predisposed to infection with
fluoroquinolone-resistant E. coli strains has been reported previously (4, 24, 32) and is likely related to the
frequent therapeutic and prophylactic use of fluoroquinolones in cancer patients. The fact that the underlying cancer rather than quinolone exposure was an independent risk factor for resistance may be explained
by the fact that we recorded only those antibiotic exposures that
occurred over the 2 months before the culture date. It is likely that
cancer acted as a surrogate marker for previous quinolone exposures
that occurred at a higher frequency in cancer patients over an extended
time period, even several years.
E. coli strains with reduced susceptibility to
fluoroquinolones appeared to be intermediate to fully susceptible and
resistant isolates with regard to certain characteristics. Among these
were the rates of other forms of resistance in isolates with reduced susceptibility; for each drug tested, the rate of resistance in isolates with reduced susceptibility to fluoroquinolones lay
between the rates observed in isolates fully susceptible and
resistant to fluoroquinolones (Table 1). In addition, the rates of
exposure to both quinolones (5%) and other antibiotics (35%) among
patients infected with isolates with reduced susceptibility
fluoroquinolones were intermediate to the rates of exposure in patients
infected with fully susceptible (0 and 28%, respectively) and
resistant (22 and 89%, respectively) isolates (Tables 3 and 4). These findings are consistent with the observations of other investigators suggesting that most resistant strains are selected from a preexisting pool of strains with reduced susceptibility (5, 7, 23).
We found an inverse relationship between the proportions of isolates
with reduced susceptibility and the proportions of resistant isolates
at different hospitals (Fig. 1). Given the likelihood that resistant
strains are selected from strains with preexisting reduced
susceptibility (5, 7, 23), this relationship may reflect
the presence of distinct selective pressures responsible for each
population. Otherwise, if the same selective pressure was responsible
for both reduced susceptibility and resistance, one would expect the
highest rates of reduced susceptibility to occur among hospitals with
the highest rates of resistance.
Other findings support the concept that distinct selective pressures
are responsible for reduced susceptibility and resistance. One of these
is the proportion of pediatric isolates that had reduced susceptibility
(17.7%) and the fact that although fluoroquinolones are not approved
for pediatric use in Taiwan, this proportion was similar to the overall
proportion of isolates with reduced susceptibility (21.7%). Yet,
despite this similar proportion of isolates with reduced susceptibility
among pediatric and adult isolates, only 1 of 79 (1.3%) pediatric
isolates were resistant, confirming that human fluoroquinolone use is
an important selective pressure for the development of full resistance.
In addition, we failed to identify the same patient risk factors for
reduced susceptibility as for resistance, suggesting that there may be
other distinct but unidentified patient risk factors for reduced
susceptibility. However, because our study of patient risk factors
involved a small sample size, some of the same risk factors for
resistance found by univariate analysis (i.e., underlying cancer,
quinolone exposure, and other antibiotic exposure) may in fact be risk
factors for reduced susceptibility that were not detected by an
underpowered study. Yet, if this is true, these are likely to be much
weaker risk factors for reduced susceptibility than for resistance, as
we used a slightly larger sample size to study reduced susceptibility
versus resistance (Tables 3 and 4).
Because we studied patient quinolone use only over the 2 months prior
to isolation of E. coli, one possible risk factor for reduced susceptibility is previous human use of nonfluorinated quinolones in the community. Although the precise amounts used are
unknown, claims data submitted to the National Health Insurance Bureau
in Taiwan indicate that nalidixic acid and pipemidic acid, both
of which are nonfluorinated quinolones, are still used in some practice
settings (L. C. McDonald, H. T. Yu, H. C. Yin, C. A. Hsiung, and M. Ho, Proc. 1st Int. Congr. Asia Pacific Soc. Infect. Control, 1999). It is conceivable that nonfluorinated quinolones are dispensed predominantly in small clinics or pharmacies and that this antibiotic pressure selects for reduced susceptibility in
the community apart from hospital inpatient and outpatient care areas.
Another possible explanation for such a large proportion of E. coli isolates with reduced susceptibility to fluoroquinolones could be the frequent exposure of humans to bacteria with reduced susceptibility in the food supply. This has been suggested as a
possible explanation for the fluoroquinolone-resistant E. coli that have been found to colonize healthy humans in the
community in Spain, where fluoroquinolones are used in commercial
poultry production and where a large number of flocks are colonized
with resistant E. coli strains (8). Although
the actual amounts used are unknown, both fluorinated and
nonfluorinated quinolones are used in commercial poultry production in
Taiwan (17). Moreover, we have found that a large
proportion of retail chicken carcasses purchased in both traditional
and modern markets around Taipei, Taiwan, are contaminated with
E. coli strains with reduced susceptibilities to
fluoroquinolones (L. C. McDonald, T. L. Lauderdale, and
M. Ho, Abstr. 100th Gen. Meet. Am. Soc. Microbiol. 2000, abstr.
Y-7, p. 682, 2000). In addition, we have found that a
surprisingly similar proportion of human non-serovar Typhi
Salmonella spp. in Taiwan possess reduced susceptibilities
to fluoroquinolones (14) and that Salmonella
spp. with reduced susceptibilities can also be recovered from retail
chicken carcasses (McDonald et al., Abstr. 100th Gen. Meet. Am. Soc.
Microbiol. 2000).
There is an urgent need to control the use of fluoroquinolones in
certain patient populations, such as cancer patients, in whom
resistance is most widespread. However, there is also an urgent need to
better understand the relationship between the various steps along the
path to resistance, the selective factors involved in these steps, and
the best strategies for the prevention and control of resistance. For
instance, in settings such as Taiwan, where a very large proportion of
isolates with single point mutations already have reduced
susceptibility, the use of even relatively limited amounts of
fluoroquinolones may result in the rapid development of resistance. We
recommend that the prevalence of isolates with reduced susceptibility
be investigated in other settings where fluoroquinolone-resistant
E. coli strains are emerging. Finally, until
there is a better understanding of the precise forms of antibiotic use
and overuse most responsible for fluoroquinolone resistance in E. coli, we recommend that the appropriateness and necessity of all
quinolone use be carefully reconsidered, including the use of
fluorinated and nonfluorinated quinolones in humans and animals.
Failure to do so may result in the early senescence of this important
class of antibiotics that has already proven so useful to human medicine.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Division of
Clinical Research, National Health Research Institutes, 128 Yen-Chiu-Yuan Rd. Sec. 2, Taipei 11529, Taiwan, Republic of China.
Phone: 886-2-2653-4401, ext. 7120. Fax: 886-2-2789-0254. E-mail:
monto{at}nhri.org.tw.
Present address: Division of Infectious Diseases, University of
Louisville, Louisville, Ky.
| |
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Antimicrobial Agents and Chemotherapy, November 2001, p. 3084-3091, Vol. 45, No. 11
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.11.3084-3091.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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Top
Abstract
Introduction
