Journal of Chemotherapy
Vol. 22 - n. 5 (345-355) - 2010
REVIEW Do Different Susceptibility breakpoints Affect the Selection of Antimicrobials for treatment of Uncomplicated Cystitis? G.C. sCHiTO 1 - L. GUALCO 1 - K.G. nABER 2 - H. BOTTO 3 - J. PALOU 4 - T. MAZZEi 5 - A. MARCHEsE 1 1
institute of Microbiology, University of Genoa, Genoa, italy. 2 Technical University of Munich, Munich, Germany. 3 Department of Urology, Hôpital Foch, suresnes, France. 4 Fundació Puigvert, Autonomous University of Barcelona, Barcelona, spain. 5 Department of Preclinical and Clinical Pharmacology, University of Florence, Florence, italy. Corresponding author: Anna Marchese, Di.s.C.M.i.T., Microbiology Unit, University of Genoa, Largo R. Benzi, 10, 16132 Genoa, italy. Tel.: +39-010-3537502; Fax: +39-010-3537651; e-mail:
[email protected]
Summary because of increasing antibiotic resistance in Escherichia coli, the main uropathogen of uncomplicated urinary tract infections (Utis), updated susceptibility data are vital in guiding the selection of first-line treatment agents. interpretation of these data depends on the breakpoints adopted, that may vary among different guidelines. in this study we report the minimum inhibitory concentrations (miCs) of eight antibiotics and compare antimicrobial susceptibility results obtained in 2315 E. coli strains recently collected during the AReSC survey using eUCASt and ClSi breakpoints. we have also evaluated the clinical impact of breakpoint discrepancies on the overall susceptibility patterns fosfomycin, nitrofurantoin and mecillinam showed the highest susceptibility rates in all countries (>92%) according to both ClSi and eUCASt criteria. minor category
shifts were observed for ciprofloxacin, amoxicillin-clavulanic acid, ampicillin and trimethoprim/sulfamethoxazole. A large number of strains classified as intermediate resistant to cefuroxime according to ClSi are included by the eUCASt in the susceptible category. in conclusion, fosfomycin, mecillinam, and nitrofurantoin have preserved their in vitro activity in all countries investigated, regardless of the criteria adopted. they continue to represent effective options for the empiric therapy of female patients with uncomplicated cystitis. the use of different interpretative criteria for E. coli responsible for Utis therefore has no influence on the decision to be taken by the physicians managing the patients. Key words: ClSi, eUCASt, breakpoints , urinary tract infections, miC.
inTRODUCTiOn
uropathogens circulating in the community of nine European nations and Brazil 4. The aim of the present study was to compare antimicrobial susceptibility results obtained on a large number of e. coli strains recently collected during the AREsC survey using both EUCAsT and CLsi breakpoints and to evaluate the impact of breakpoint discrepancies on the overall susceptibility patterns.
Uncomplicated urinary tract infections (UTis) are among the most frequent infections affecting women. Between one-quarter and one-half of all women experience a UTi during their lifetime 1. The overall etiology of UTis has not changed in recent years and escherichia coli remains by far the most common uropathogen, accounting for 75-95% of all positive cultures 2. in the community the management of these conditions is generally empirical but recommendations of the most appropriate first-line agents should also be based on updated local epidemiological data derived from antimicrobial susceptibility testing results 3,4. international and national committees are responsible for issuing guidelines for the performance and interpretation of antimicrobial susceptibility testing 5,6. These committees also establish interpretive criteria or ‘breakpoints’ for each antimicrobial agent tested. Breakpoints are minimum inhibitory concentration (MiC) cut-off values used to divide a bacterial population into the susceptible, intermediate and resistant categories. Because different agencies such as CLsi (Clinical and Laboratory standards institute) and EUCAsT (European Committee on Antimicrobial susceptibility Testing) may suggest different breakpoints, the assigning of a pathogen to a defined category (susceptible, intermediate and resistant) depends on the specific guideline adopted. As a consequence, breakpoint discrepancies could have an important impact, possibly leading to divergent conclusions impinging on the selection of the drug to be selected by the physician. Recently, an international survey, the AREsC study (Antimicrobial Resistance Epidemiological survey on Cystitis) was conducted to determine the susceptibility of the major © E.s.i.F.T. srl - Firenze
MATERiALs AnD METHODs
Bacteria A total of 2315 non-repetitive e. coli strains, isolated from the urine of female patients suffering from uncomplicated lower UTi and enrolled in the AREsC study over a 3-year period (2003-2006) in nine European countries (Austria, France, Germany, Hungary, italy, Poland, Russia, spain and The netherlands) and Brazil, were analyzed. Detailed patient inclusion/exclusion criteria and the microbiological protocol for classifying urine samples as positive have been reported previously 4. Each local site forwarded the strains isolated to the central laboratory (DisCMiT, Microbiology section, University of Genoa, italy) by courier, where the pathogens were re-identified using the APi system (bioMérieux, Milan, italy) and stored at ć70°C until susceptibility tests were performed.
Susceptibility testing All e. coli strains were assayed against the following antimicrobial agents: ampicillin, amoxicillin/clavulanic acid, cefuroxime, ciprofloxacin, trimethoprim/sulfamethoxazole, fosfomycin, mecillinam and nitrofurantoin. The compounds
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G.C. sCHiTO - L. GUALCO - K.G. nABER - H. BOTTO - J. PALOU - T. MAZZEi - A. MARCHEsE
were obtained from their respective manufacturers or from commercial sources. The MiCs of the antimicrobials were determined by the broth microdilution method in cation-adjusted Mueller-Hinton broth as suggested by the CLsi 7. The MiCs of mecillinam and fosfomycin were determined by the agar dilution method in Mueller-Hinton agar, supplemented with glucose-6-phosphate at a concentration of 25 mg/L when fosfomycin was studied following the suggestions of the CLsi 7. e coli ATCC 25922 was included in each run as a control. strains were defined as susceptible, intermediate-resistant or resistant according to the most recent CLsi and EUCAsT recommendations 5,6 (table 1).
TABLE 1 - CLSi and eUCASt MiC breakpoints of antimicrobial agents assayed against e. coli isolated from Uti. Antimicrobial agent
CLSI
EUCAST
MIC breakpoint (mg/L)
MIC breakpoint (mg/L)
S Ampicillin
≤8
I
R
16 ≥32
S
I
R
/
/
≥16
Amoxicillin/clavulanic acid
≤8
16 ≥32
/
/
≥16
Mecillinam
≤8
16 ≥32
≤8
/
≥16
Cefuroxime
≤4 8-16 ≥32
≤8
/
≥16
Ciprofloxacin
≤1
2
≥4
Trimethoprim/sulfamethoxazole ≤2
/
≥4
≤0.5 1
≥2 ≥8
≤2
4
nitrofurantoin
≤32 64 ≥128
≤64
/ ≥128
Fosfomycin
≤64 128 ≥256
≤32
/
≥64
s: susceptible; i: intermediate; R: resistant. /: Minimal inhibitory concentration (MiC) value does not exist.
REsULTs tables 2-12 show the MiC distributions for all antibiotics tested on the whole collection of e. coli strains studied and for each country. The following MiC patterns were observed, ampicillin: >128 mg/l, amoxicillin/clavulanic acid: 8 mg/l, mecillinam: 0.12 mg/l, cefuroxime: 4 mg/l, nalidixic acid: 4 mg/l, ciprofloxacin: ≤0.015 mg/l, trimethoprim/sulfamethoxazole: >16 mg/l, nitrofurantoin: 16 mg/l and fosfomycin: 1 mg/l. The highest MiC of fosfomycin was 512 mg/L (one strain isolated in spain), while the highest MiC value of nitrofurantoin was 256 mg/L a value observed for strains isolated in Brazil, Germany, Russia and spain. table 13 reports the antimicrobial susceptibilities of the 2315 uropathogenic e. coli to the eight antimicrobial agents tested, according to CLsi and EUCAsT interpretive breakpoints. Globally, the susceptibility rates calculated following the two guidelines appear to be similar. in particular, for trimethoprim/sulfamethoxazole and mecillinam the percentages of susceptible strains were identical. For ampicillin and amoxicillin/clavulanic acid EUCAsT does not suggest breakpoints for the intermediate and susceptible categories, while the rates for resistant strains were higher when adopting their interpretive criteria. The proportion of e. coli strains classified as ampicillin and amoxicillin/clavulanic acid intermediate by CLsi are shifted to the resistant category when
using EUCAsT criteria. Using the CLsi breakpoint, the percentage of cefuroxime susceptible isolates was lower than that obtained when using EUCAsT (82.4% vs 95.1.6% respectively) since a large number of strains classified as cefuroxime-intermediate according to CLsi are deemed susceptible according to EUCAsT. similar percentages of susceptibility were observed for ciprofloxacin according both to CLsi (91.7%) and EUCAsT (90%). EUCAsT breakpoints for the susceptible, intermediate and resistant categories are one dilution lower than those suggested by CLsi for this antimicrobial agent. The rates of nitrofurantoin-resistant e. coli calculated by CLsi and EUCAsT breakpoints were identical (1.6%). However, the EUCAsT breakpoint is less restrictive for nitrofurantoin and strains categorized as intermediate by CLsi are included in the EUCAsT susceptible category. Conversely, the EUCAsT breakpoints are more restrictive for fosfomycin in comparison to CLsi. in fact, strains inhibited by 64 mg/L of fosfomycin are categorized as susceptible by the CLsi and as resistant according to EUCAsT. Despite this major shift in category the overall percentage of susceptible strains did not vary significantly (98.1% vs 96.5%). These general observations hold true when considering the susceptibility results for each of the countries included in the study. The most remarkable discrepancies were observed with cefuroxime in all countries (table 14). DisCUssiOn During the last decade the emergence and spread of e. coli strains which are resistant to several antimicrobial agents has been observed worldwide 3,8. Classification of susceptibility/resistance depends on the breakpoints that are used routinely in the clinical laboratory and these obviously influence clinical decision-making. However, these breakpoints vary in different guidelines. CLsi, the most widely used guide, has based preliminary breakpoints on MiC distributions, pharmacokinetic-pharmacodynamic (PK-PD) parameters and mechanisms of antimicrobial resistance. These suggestions are later confirmed in clinical trials. Additional methodologies are now utilized to evaluate the PK-PD of antimicrobials 9. The European Committee on Antimicrobial susceptibility Testing (EUCAsT) uses PK-PD simulations as a chief component of its breakpoint-setting process for old and new antimicrobials 10,11. Because of these differences CLsi and EUCAsT breakpoints are widely divergent in several instances and the adoption of CLsi or EUCAsT interpretive criteria may therefore lead to different results and conclusions. in this study we have compared the susceptibility data calculated both with EUCAsT and CLsi breakpoints for a large number of e. coli strains collected during the AREsC study, a survey established to update epidemiological resistance data in several European countries and Brazil and to discuss the implications, if any, for empiric therapy of patients to be treated for uncomplicated lower UTi (cystitis). Using both CLsi and EUCAsT breakpoints, ampicillin and amoxicillin/clavulanic acid showed a wide range of resistant e. coli strains, depending on the country analyzed. The proportion of e. coli strains classified as ampicillin and amoxicillin/clavulanic acid intermediate by the CLsi are included by EUCAsT in the resistant category and for this reason the rates of resistance were higher when adopting this last interpretive criterion. However, the sum of the incidence of intermediate and resistant strains defined by CLsi guidelines was identical to the rate of resistant strains following EUCAsT parameters. This was true for all countries included in the study. This minor category shift has therefore no influence on physi-
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n=total % n=total % n=total % n=total % n=total % n=total % n=total % n=total %
1563 67.6 42 1.9
16
Minimal inhibitory concentrations (MIC) (mg/l)
350 15.1 33 1.4
54 2.3 66 2.8 20 0.9 19 0.8
32
72 3.2 39 1.7
33 1.4 11 0.5 19 0.8 14 0.6
64
27 1.1 29 1.2
39 1.7 10 0.4 8 0.3 8 0.4
128 991 42.9 2 0.1 17 0.7 14 0.6
>128
11 0.5 12 0.5
256
1 0.1
512
128 23 37.0
256
512
0.25
0.06->16
