Antibiotic resistance pattern of Bacteroides fragilis isolated from clinical and colorectal specimens.

Background: Bacteroides fragilis is a part of the normal gastrointestinal flora, but it is also the most common anaerobic bacteria causing the infection. It is highly resistant to antibiotics and contains abundant antibiotic resistance mechanisms.
Methods: The antibiotic resistance pattern of 78 isolates of B. fragilis (22 strains from clinical samples and 56 strains from the colorectal tissue) was investigated using agar dilution method. The gene encoding Bacteroides fargilis toxin bft, and antibiotic resistance genes were targeted by PCR assay.
Results: The highest rate of resistance was observed for penicillin G (100%) followed by tetracycline (74.4%), clindamycin (41%) and cefoxitin (38.5%). Only a single isolate showed resistance to imipenem which contained cfiA and IS1186 genes. All isolates were susceptible to metronidazole. Accordingly, tetQ (87.2%), cepA (73.1%) and ermF (64.1%) were the most abundant antibiotic-resistant genes identified in this study. MIC values for penicillin, cefoxitin and clindamycin were significantly different among isolates with the cepA, cfxA and ermF in compare with those lacking such genes. In addition, 22.7 and 17.8% of clinical and GIT isolates had the bft gene, respectively.
Conclusions: The finding of this study shows that metronidazole is highly in vitro active agent against all of B. fragilis isolates and remain the first-line antimicrobial for empirical therapy.