Mechanisms of antimicrobial resistance in anaerobic bacteria: the predictive approach.

Antibiotic resistance in anaerobic bacteria probably evolves from broadly undetected mechanisms of resistance, as the generally accepted breakpoints in susceptibility testing are mainly based on pharmacological grounds. Some of the strains considered susceptible by conventional techniques may harbour mechanisms of resistance. These resistant organisms should be screened using the antibiotic concentrations immediately above those inhibiting the fully susceptible population of a given species as breakpoints. The following break-points for bacteroides strains are suggested as definitions of the entirely susceptible population: piperacillin 8 mg/l; ceftizoxime 8 mg/l: cefoxitin 8-16 mg/l: imipenem 1 mg/l: ticarcillin-clavulanate 2 mg/l; clindamycin 1 mg/l: chloramphenicol 8 mg/l: tetracycline 4 mg/l: metronidazole 1 mg/l. Detection of an increased frequency of organisms presenting low-level resistance may be useful to predict and possibly control the appearance and spread of fully resistant anaerobic organisms.