Mechanism of Resistance in Metronidazole

Metronidazole [1-(2 hydroxyethyl)-2-methyl-5-nitroimidazole] was introduced in the 1960s. Since then it has been drug of choice for human infections caused by various anaerobic and microaerophilic bacteria (Bacteroides, Clostridia, Helicobacter) and parasites (Trichomonas, Giardia, Entamoeba). Other Gram-positive anaerobes (e.g., lactobacilli, propionobacterium acnes, majority of the periodontal pathogens, peptostreptococci) are known to be inherently resistant to metronidazole. Virtually all the anaerobic Gram-negative rods are known to be susceptible to metronidazole. Sensitivity testing for anaerobes is not performed routinely. Therefore, resistance to metronidazole is under-reported. With improvements in molecular detection, increasing resistance rates are being noted. This emerging resistance to metronidazole poses various diagnostic and therapeutic dilemmas. Mechanisms of resistance are being defi ned, and a better understanding is the key for prevention of resistance and improved management of these infections.