Controversies in susceptibility testing of anaerobes.

Antimicrobial susceptibility testing of anaerobic bacteria is controversial and confusing. A number of laboratory and clinical variables affect the performance and interpretation of the susceptibility data. We know that the various methods used in susceptibility testing have inherent differences that produce significantly contrasting results for certain antibiotics. We also know that some antimicrobials are affected to a greater degree than others, such as I demonstrated with ceftizoxime and cefoxitin (Table IV). We need to study the laboratory variables that lead to differences, such as the effect of inoculum size, various test media, osmolarity, pH, incubation time and environment, and antibiotic resistant endpoints for anaerobes. When we know which variables achieve the most reliable results, we can formulate a standard method by which antimicrobial susceptibility data can be compared. Studies to establish a method that correlates well with the clinical outcome of antimicrobial therapy in patients with infections involving anaerobes is important. These studies may take a long time but are necessary to ensure that the laboratory data are useful to the physician in the management of patients with anaerobic infections. Finally, it is important to determine if time-kill kinetic studies of anaerobes and antimicrobials can be useful in predicting clinical outcome in patients with anaerobic infections. The establishment of the bactericidal activity of antimicrobials at dosing intervals may or may not prove to be more useful than traditional MIC data.