Susceptibility testing of anaerobic bacteria: myth, magic, or method?

Abstract

The demand for susceptibility testing of anaerobes has increased, yet consensus as to procedure and interpretation in this area has not been achieved. While routine testing of anaerobic isolates is not needed, certain isolates in specific clinical settings should be tested. Also, laboratories may monitor their local antibiograms by doing periodic surveillance batch testing. The National Committee for Clinical Laboratory Standards has published a protocol of methods approved for susceptibility testing of anaerobic bacteria. Both agar and broth microdilution are included; however, the broth disk elution method is no longer approved by the National Committee for Clinical Laboratory Standards because of method-related interpretive errors. A number of newer methods are undergoing evaluation and seem promising. Clinicians and microbiologists reviewing susceptibility reports should be aware of sources of variability in the test results. Variables in susceptibility testing of anaerobes include the media and methods used, organisms chosen for testing, breakpoints chosen for interpretation, antibiotic, and determination of endpoint. Clustering of MICs around the breakpoint may lead to significant variability in test results. Adherence of testing laboratories to approved methods and careful descriptions of the method and the breakpoints used for interpretation would facilitate interlaboratory comparisons and allow problems of emerging resistance to be noted. A variety of resistance mechanisms occurs in anaerobic bacteria, including the production of beta-lactamase and other drug-inactivating enzymes, alteration of target proteins, and inability of the drug to penetrate the bacterial wall. Antimicrobial resistance patterns in the United States and abroad are described.

Antianaerobic Antimicrobials: Spectrum and Susceptibility Testing

“…A 1991 review by Wexler (35) reported the major reasons for variability in reporting of MIC results. At that time, technical variability among laboratories was a major factor.…”

“…Anaerobes manifest three major mechanisms of resistance to ␤-lactam antibiotics: inactivating enzymes, mainly beta-lactamases (BLAs), which include penicillinases and cephalosporinases; low-affinity penicillin-binding proteins (PBPs); and decreased permeability through alterations in the porin channel (35). The production of BLAs is the most common mechanism of resistance to ␤-lactam antibiotics in anaerobes, especially among the B. fragilis group and Prevotella spp.…”

“…Increased activity of efflux pumps or changes in penicillinbinding proteins (PBPs) have been shown to affect MICs of ␤-lactams for many Bacteroides isolates; systematic surveys of these mechanisms have not been conducted, so the percentage of strains that have or utilize these mechanisms is not known (35,49).…”

Antimicrobial susceptibility testing of Bilophila wadsworthia by using triphenyltetrazolium chloride to facilitate endpoint determination

“…Major differences from previous results were seen with agents that usually are active against anaerobes (e.g., imipenem, cefoxitin, and ticarcillin). Only 70 to 80% of B. wadsworthia strains were reported previously to be susceptible to imipenem, and less than 50% were reported to be susceptible to cefoxitin and ticarcillin (20). In the present study, we reevaluated the susceptibilities by using aerobic TTC MICs correlated with viability counts from agar dilution plates.…”

“…The organism may also be isolated from normal human feces but is a very minor part of the stool flora (105/g, whereas total counts are approximately 1012/g); its high frequency in the clinical specimens suggests its potential importance in the infectious process. Initial susceptibility studies of B. wadsworthia indicated significant resistance to several antimicrobial agents, including imipenem, cefoxitin, penicillin G, and other 3-lactam antibiotics (20). No P-lactamase production was demonstrated (2).…”