The in-vitro activity of metronidazole against strains of Escherichia coli with impaired DNA repair systems.

The minimal inhibitory concentration (MIC) of metronidazole was lower for strains of Escherichia coli with lesions in their DNA repair systems than for their parent strains. The viable numbers of all strains of E. coli tested were considerably reduced by metronidazole, even at sub-MIC levels, and the effect was greater when DNA repair lesions were more severe. In addition, metronidazole caused cell elongation of E. coli and slowly disappeared from the medium upon incubation with the bacterium. The drug-induced decrease in viable numbers of E. coli was followed by an increase. Most cells in this 'recovery' population were found to be anaerogenic and were relatively unaffected by subsequent exposure to metronidazole. The results show that E. coli consumes and is susceptible to metronidazole, under anaerobic conditions. DNA damage is involved in the antibacterial action of the drug, and E. coli resistant to the drug can be obtained readily in in-vitro incubations.