BACTEROIDES-FRAGILIS WITH REDUCED SENSITIVITY TO METRONIDAZOLE - AN UNUSUAL PHENOTYPIC-EXPRESSION

Transferable resistance to 5-nitroimidazoles has been previously demonstrated in Bacteroides fragilis group strains. Such a character is often expressed by strains which are characterized by low levels of resistance to metronidazole (2 to 8 mg/l). Following, zones of inhibition around the metronidazole disc are slightly smaller than that observed with any full susceptible strain. Thus, the detection of these strains using the disc assay on agar is not obvious after 24 or 48 hours of incubation. The phenomenon is more evident by using a disc of tinidazole or MIC determination, but those tests are not performed routinely in many laboratories. This study was undertaken to propose an alternative way, using the metronidazole disc. We have studied the phenotypic expression of 100 strains by the disc susceptibility test after an extended incubation (up to 96 hours) in an anaerobic work station. The average of the zone diameters around a 16 mcg metronidazole tablet (Rosco laboratories) was 43 mm with 30 fully susceptible strains and 21 mm with 60 reduced sensitivity strains. Moreover, all the latter strains exhibited squater colonies within the inhibition area. These colonies, when retested, proved an unmodified sensitivity to 5-nitroimidazoles. Although this resistance still appears to be uncommon, it should be looked if this relative resistance is clinically relevant.