High-level resistance to metronidazole among Bacteroides fragilis isolates from surgical site infections

Background: Bacteroidesfragilisis the most common anaerobic pathogen isolated from surgical site infections (SSIs). Metronidazoleresistance is increasing and the mechanisms of resistance are not clear in some isolates. The aim of the present study was to investigate the metronidazole susceptibility prevalence, and detect nimgenes in B. fragilisisolates from SSIs. Methods: This study included 100 surgery patients with signs and symptoms indicative of SSIs. Syringe aspiration of the infected site was used to collect specimens. All specimens were cultured on BBA (Brucellablood agar), KVLB (kanamycin-vancomycin laked blood), and BBE (Bacteroides bile esculin) agar. The MIC (minimum inhibitory concentration) of metronidazole was determined by the agar dilution method according to the Clinical and Laboratory Standard Institute (CLSI). Then the PCR method was used to determine the presence of the nimgene. Results: In the present study, 26 B. fragiliswere isolated from 100 SSIs specimens. Eight isolates were metronidazole resistant; the metronidazole MIC was 32 µg/mL for 7 isolates and 64 µg/mL for one isolate. All isolates were nimgene negative. Conclusion: The emergence of metronidazole-resistant B. fragilis limits the application of this drug for treatment and prophylaxis of SSIs. Thus, rapid identification of metronidazole -resistant B. fragilisis essential to restrict inappropriate, super fluous administration. In spite of various metronidazole resistance mechanisms other than that depending on the nimgene, detection of nimby PCR is unsuitable for identifying resistant isolates. Therefore, phenotypic methods are better to screen for and identify metronidazole-resistant B. fragilis.

Keywords:

Bacteroidesfragilis , surgical site infections , metronidazole , minimum inhibitory concentration , nim gene