Susceptibility testing of anaerobic bacteria--the state of the art

Demand for susceptibility testing of anaerobes has increased, but no consensus on procedure and interpretation has been achieved. The need for reliable methods for testing anaerobic bacteria extends from small hospital laboratories to large research centers. Agar dilution testing is too costly and labor intensive for many clinical laboratories. Microbroth dilution is more convenient; however, some fastidious anaerobes do not grow well enough in this system, and the choice of antimicrobial agents may not reflect the hospital formulary. Disapproval of the broth disk elution system leaves fewer options open to clinical laboratories and emphasizes the need for more convenient and reliable techniques. Some newer methods are undergoing evaluation. Variables in susceptibility testing of anaerobes include the media and methods used, organisms chosen, breakpoints chosen, and endpoint determination. This latter variable is probably the most problematic since no endpoint based on interaction of organism and antimicrobial agent rather than on subjective observation has been defined. Also, clustering of MICs around the breakpoint may lead to significant variability in reported methods. A more accurate MIC measurement is needed. Adherence of laboratories to approved methods and careful reporting of methods and the interpretive breakpoints would facilitate interlaboratory comparisons and the identification of emerging resistance.