Metronidazole resistance among clinical isolates belonging to the Bacteroides fragilis group: time to be concerned?

J Antimicrob Chemother 1999; 44: 580–581 Sir, Despite metronidazole (5-nitroimidazole) now being in its fortieth year, this remarkable drug remains extensively and effectively used for the prevention and treatment of anaerobic infections, as well as for the treatment of patients with infections caused by Trichomonas vaginalisand Helicobacter pylori. There is, however, a paucity of data on the incidence of resistance to metronidazole among clinical isolates of anaerobes, although there has been a recent report of metronidazole treatment failure in a patient with an appendicectomy wound infection caused by isolates of Bacteroides fragilis and Bacteroides ovatus that were resistant to metronidazole (MICs > 32 mg/L).1 The Anaerobe Reference Unit (ARU) provides a service for the identification of anaerobic bacteria isolated from clinical material and routinely determines the susceptibilities of these isolates to a range of antibiotics, including metronidazole. This surveillance has demonstrated a possible increase in the incidence of metronidazole resistance among Bacteroides spp. During 1998, 93 isolates referred to the ARU were identified as belonging to the B. fragilis group; 7.5% of these were resistant to metronidazole (MICs > 16 mg/L),2 compared with 1.9% of the isolates evaluated in 1995 and 3.8% in 1997. This trend may be the result of a selection bias, as some isolates were referred solely to confirm putative resistance to a 5 µg metronidazole disc, or it may reflect a true increase in the incidence of resistance, an observation that would have major implications for clinical microbiology laboratories, as well as for the prophylactic and treatment regimens administered to surgical patients.