Inducible Metronidazole Resistance and nim Genes in Clinical Bacteroides fragilis Group Isolates

Abstract

Nitroimidazole resistance (nim) genes were detected in 2% of 1,502 clinical Bacteroides fragilis group strains isolated from 19 European countries, and a novel nim gene was identified. High metronidazole resistance could be induced in nim-positive strains, which emphasizes the importance of acknowledging metronidazole resistance in the clinical setting.Anaerobic infections are common and can cause diseases associated with severe morbidity but are easily overlooked in the clinical setting. Bacteroides fragilis group species are opportunistic pathogens that constitute a dominating bacterial group in the normal intestinal microflora. In particular, B. fragilis is among the most clinically relevant anaerobic species. Interspecies lateral transfer of resistance genes among anaerobes has been shown for several clinically important antimicrobial agents such as beta-lactam agents, clindamycin, tetracycline, and metronidazole (3,17,19,22,24). Metronidazole, a 5-nitroimidazole agent, is particularly useful against Bacteroides spp. that tend to be resistant to a wide range of antimicrobial agents (1, 2). Although the rate of resistance remains low, at Ͻ1%, metronidazole-resistant organisms are beginning to emerge (8,11,15,16,21). Five nitroimidazole resistance genes, nimA to -E, have been identified that confer reduced susceptibility to 5-nitroimidazole antibiotics on species of the B. fragilis group (10,23,26). The proposed resistance mechanism conferred by the nim genes is that they encode a 5-nitroimidazole reductase (4). Four of the nim genes have been shown to be associated with different mobile insertion sequence (IS) elements flanked by inverted repeats (10, 25). There is strong evidence that these IS elements carry regulatory signals for expression of certain resistance genes, including the nim genes (10,13,15,21,25).A total number of 1,502 clinical B. fragilis group isolates were surveyed for the prevalence and distribution of various nim genes. The strains, originating from a previous surveillance study of antimicrobial susceptibility including 19 European countries (11), were collected during 1999 to 2001. The different Bacteroides species had previously been identified by conventional biochemical tests, and the MIC of metronidazole was determined by the agar dilution method as recommended by the National Committee for Clinical Laboratory Standards (NCCLS) (14).The strains were screened for nim genes by PCR with specific primers (26) and cleaved with the restriction enzyme Hsp92II for determination of the nim type. In addition, HpaII and TacI were used for selected strains. One strain (HUN19) was sequenced after it was found to have unique and novel restriction fragment length polymorphism (RFLP) patterns. All nim-positive strains were further investigated by PCR for associated IS elements and the locations of the elements with respect to the nim gene (17) with specific primers newly designed or previously described (10).Twelve nim-positive clinical strains for which the initial MICs were relatively low (0.25 to...

Bacteroides: the Good, the Bad, and the Nitty-Gritty

“…Resistance genes nimA to -F, which have been implicated in sporadic cases of metronidazole resistance in several hospitals worldwide, have been identified on transferable plasmids (56,148,279). Transferable plasmid-linked chloramphenicol acetyltransferase conferring high-level resistance was observed in a clinical isolate of Bacteroides uniformis (152).…”

“…The nim gene can be "silent" unless activated by IS elements (111). Four nim genes (nimA to -D) can occur in all Bacteroides species; each of these genes is associated with a distinct mobile genetic element, and each also has a specific activating IS (111,148,211,278). The presence of any one of these genes can be detected by PCR (279).…”

Mobile genetic elements in the genus Bacteroides, and their mechanism(s) of dissemination

“…Resistance genes nimA-nimF, encoding metronidazole resistance, have alos been identified on transferrable plasmids and recovered worldwide. 36 The cfiA gene, conferring resistance to carbapenems, has also been found in a plasmid in clinical isolates. 37 Conjugative and mobilizable plasmids.…”

Gut microbiome diversity in acute infective and chronic inflammatory gastrointestinal diseases in North India

“…Metronidazole resistance-associated genes (nim genes) have been associated with anaerobes including Bacteroides and Peptostreptococcus [40,41]. We determined the presence of nim genes [42,43] in all fecal and pus samples by PCR amplification with nim-specific primers.…”