Resistance mechanisms of anaerobic bacteria against the 5-nitroimidazole derivative widely used in the treatment of anaerobic bacterial infections, metronidazole are varied and complex.

It has been suggested that the nitroimidazole reductase enzyme found in anaerobic bacteria plays an important role in resistance to metronidazole. This enzyme is under the control of nim genes defined within the genomic structure of the bacteria. The nim genes have been identified in 11 different structures (nimA-nimK) so far. It has been reported that the nim genes, which are mostly defined in the Bacteroides fragilis group, are not sufficient in the formation of metronidazole resistance alone, there are also different resistance mechanisms

Mechanisms such as pyruvate ferrodoxine oxidoreductase (PFOR) enzyme activated during glucose metabolism of bacteria, efflux pump system under the control of bmeRABC5 gene, excessive release of DNA repair proteins controlled by the RecA gene were determined to play an important role in the resistance of anaerobic bacteria to metronidazole.

In this article, different resistance mechanisms created by anaerobic bacteria against 5-nitroimidazoles and debatable role of the nim genes role in these resistance mechanisms were indicated.