Resistance Trends in Antimicrobial Susceptibility of Anaerobic Bacteria, Part I

Abstract

Despite the suggestions of both the Clinical and Laboratory Standards Institute (CLSI) and the American Society for Microbiology (ASM) Manual of Clinical Microbiology that hospitals should test individual patient isolates to assist in their care, periodically establish patterns of resistance for certain anaerobes, and include these data in the hospital antibiogram, anaerobic susceptibility studies are performed in only a minority of clinical laboratories and their patterns of susceptibility are obtainable mostly from published surveys conducted by a small number of research centers scattered worldwide. The Bacteroides fragilis group species, the most frequently studied anaerobes, have been reported to vary in their frequencies of resistance to all antimicrobial agents worldwide. Limited data exist about the resistance patterns of other genera and species. Part I of this two-part article reviews the methods used for anaerobic susceptibility testing and the correlation of in vitro susceptibility test results with clinical data and antimicrobial resistance that has been reported for gram-negative and gram-positive anaerobes. Part II of this article will review mechanisms of resistance among anaerobes to commonly used antimicrobial agents.

Introduction

While anaerobic bacteria are important pathogens in a variety of clinical infections, many clinical laboratories are either unable or unwilling to culture anaerobes except from very specific sites; even fewer laboratories perform antimicrobial susceptibility testing (AST) (1). Consequently, the patterns of anaerobes' susceptibility and resistance remain virtually inaccessible to laboratorians and clinicians, except for those published surveys conducted by a small cadre of research centers scattered worldwide or contained in individual case reports. Resistance trends in anaerobes are therefore biased and sometimes anecdotal.

Both the Clinical and Laboratory Standards Institute (CLSI) (2) and the American Society for Microbiology's Manual of Clinical Microbiology (3) suggest that hospitals test individual patient isolates to assist in their care, periodically establish patterns of resistance for certain anaerobes, and include these data in the hospital antibiogram. AST data from at least 10 members of the Bacteroides fragilis group, Prevotella spp., Fusobacterium spp., and Clostridium spp. should be included. Because B. fragilis is a more common pathogen, more isolates, up to 30 for each species, should be included in the antibiogram. The antimicrobial agents tested should be those available on the hospital's formulary to help clinicians in their selection of empirical therapy. CLSI provides recommendations for testing of various antibiotics based upon the partial identification of the anaerobic isolate. Susceptibility testing on individual isolates from specific patients should be performed to assist patient care when (i) selecting an active agent is critical for disease management; (ii) the patient is likely to receive long-term therapy; (iii) isolates are recovered from the blood, brain, bone, or joint; (iv) therapy fails; or (v) organisms are known to have unpredictable susceptibility patterns.

Of note is the variation of breakpoints that may occur for selected antimicrobial agents between the U.S. Food and Drug Administration (FDA), CLSI, and the European Committee on Antimicrobial Susceptibility Testing (EUCAST). In addition, breakpoint changes have recently been proposed for carbapenems, cephalosporins, and β-lactam–β-lactamase inhibitor combinations for Enterobacteriaceae, which may ultimately impact breakpoints for anaerobes, as well.