Trends in antimicrobial resistance of the Bacteroides fragilis group: a 20-year study at a medical center in Buenos Aires, Argentina.

Abstract

species belonging to the B. fragilis group is shown in figure Although susceptibility testing for anaerobic bacteria is not rou- 1. Clindamycin resistance was more frequently detected in tinely performed by some laboratories, the increasing resistance B. distasonis, B. thetaiotaomicron, and B. vulgatus isolates of B. fragilis group isolates emphasizes the need to survey the (9%, 6%, and 5%, respectively). Similar results were observed susceptibility patterns of these organisms (2). Our study assessed with regard to cefoxitin. the antimicrobial resistance trends among B. fragilis group isolates Over the past few years, several studies have demonstrated that during a 20-year period. anaerobes have undergone changes in their susceptibility patterns. A total of 497 strains belonging to the B. fragilis group were Indeed, antimicrobial resistance is becoming less predictable and collected between 1975 and 1995 in our hospital, the Centro de may fluctuate from one medical center to another, as well as from EducacionM e dica e Investigaciones Clinicas (Buenos Aires). one geographic region to another (5). Thus, every hospital should They were identified according to the Wadsworth Anaerobic Bacte- periodically search for emerging resistance. In addition, specific riology Manual (3) as B. fragilis (294 strains), B. distasonis (92 instances may require susceptibility testing of individual isolates strains), B. thetaiotaomicron (62 strains), B. vulgatus (35 strains), in order to guide antimicrobial therapy. Bacteroides ovatus (10 strains), and other species of the B. fragilis In this study, species of the B. fragilis group other than B. group (4 strains). fragilis showed patterns of the most resistance, especially The MICs were determined by the agar dilution method in against clindamycin and cefoxitin. These findings are in agree- accordance with guidelines of the National Committee for ment with those reported from Europe and the United States (1, Clinical Laboratory Standards (NCCLS) (4), except that brain- 2). The changing resistance trends observed here emphasize heart infusion agar (Difco Laboratories, Detroit) was used in- the need of routinely surveying the in vitro susceptibility of stead of Wilkins-Chalgren (WC) medium. WC agar was not anaerobic bacteria. available as the reference medium at the beginning of this study, and we did not find significant differences between the two media for B. fragilis group isolates in previous studies (data not shown). The following drugs were supplied as standard antimicrobial powders: ampicillin/sulbactam (Pfizer, Buenos Aires), cefoxitin and imipenem (Merck Sharpe & Dohme, Buenos Aires), chlor- amphenicol (Argentia, Buenos Aires), metronidazole (Rhodia, Buenos Aires), ornidazole (Roche Laboratories, Buenos Aires), meropenem (Zeneca Farma, Buenos Aires), and clindamycin (Upjohn, Buenos Aires). Breakpoints were adopted from NCCLS documents (4). B. fragilis ATCC 25285 was used as the control. Table 1 shows MIC50 and MIC90 values (MICs inhibiting 50% and 90% of strains, respectively), MIC ranges, and percentage of resistance at the resistant breakpoint, in testing of several antibiotics against 497 isolates of the B. fragilis group. No resis- tance to imipenem, meropenem, chloramphenicol, metronida- zole, and ornidazole was found over the 20-year period, and there were no significant changes in MIC50 and MIC90 values.