Antimicrobial activity of metronidazole in anaerobic bacteria.

The antimicrobial activity of metronidazole was investigated in anaerobic bacteria by use of time-viability studies. This antimicrobial agent has a rapid onset of bactericidal activity under proper reducing conditions. The bactericidal rates were not affected by inoculum size or nutritional requirements, nor by inhibition of growth and protein synthesis by chloramphenicol. Using supernatant fractions of actively growing cultures of susceptible organisms, we observed a disappearance of metronidazole and a loss of biological activity, but there was no significant change in preparations from resistant bacteria. The decrease in drug concentration with susceptible cells occurred during the time that its bactericidal action was being exerted. Extracts from susceptible organisms rapidly reduced the concentration of metronidazole, confirming previous observations which suggest that the drug acts as a terminal electron acceptor. Radioisotope experiments with [14C]metronidazole revealed that the compound was taken up by both resistant and susceptible bacteria, although there was a difference in rate and extent of accumulation. These studies demonstrate that metronidazole's antimicrobial activity against anaerobic bacteria is bactericidal and independent of growth rate, and that it involves the uptake and metabolism of the compound.