Relationship between penicillin-binding protein patterns and β-lactamases in clinical isolates of Bacteroides fragilis with different susceptibility to β-lactam antibiotics

Abstract

This study examines the role of the penicillin-binding proteins (PBPs) of Bacteroides fragilis in the mechanism of resistance to different â-lactam antibiotics. Six of the eight strains used were â-lactamase-positive by the nitrocefin assay. These strains displayed reduced susceptibility to imipenem (MIC, 2-16 mg l À1 ) and some of them were resistant to the actions of ampicillin, cefuroxime, cephalexin, cefoxitin and piperacillin. When studying specific enzymic activity, the capacity to degrade cefuroxime was only detected in strains AK-4, R212 and 0423 and the capacity to degrade cephalexin was only detected in strains R212 and 2013E; no specific activity was detected on imipenem. Metallo-â-lactamase activity was only detected in strains AK-2 and 119, despite the fact that the cfiA gene was identified in four strains (AK-2, 2013E, 119 and 7160). The cepA gene was detected in six of the eight strains studied. Three high-molecular-mass PBPs were detected in all strains; however, in some cases, PBP2Bfr and/or PBP3Bfr appeared as a faint band. PBP4Bfr and PBP5Bfr were detected in six strains. PBP6Bfr only was detected in B. fragilis strains AK-2, 0423, 119 and 7160. By analysis of the sequence of B. fragilis chromosomal DNA and comparison with genes that are known to encode PBPs in Escherichia coli, six genes that encode PBP-like proteins were detected in the former organism. The gene that encodes the PBP2 orthologue of E. coli (pbpABfr, PBP3Bfr) was sequenced in six of the eight strains and its implications for resistance were examined. Differences in the PBP3Bfr amino acid sequences of strains AK-2 and 119 and their production of â-lactamases indicate that these differences are not involved in the mechanism of resistance to imipenem and/or cephalexin.

Screening of isolates from faeces for carbapenem-resistant Bacteroides strains; existence of strains with novel types of resistance mechanisms

“…However, mixed resistance mechanisms of amino acid change and alterations in OMP profile could also be presumed for these latter strains, as in the cases of some carbapenem-resistant klebsiellae [19]. In a recent study, however, the imipenem resistance of some B. fragilis strains was mediated mainly by ␤-lactamases, and penicillin-binding protein alterations in such resistance were not involved [20].…”

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

“…The affinities of the PBP1 complex (86 kDa) of B. thetaiotaomicron 238m for cefoxitin and piperacillin were Ͼ100-fold and ϳ70-fold reduced, respectively, compared with those of its parent strain (85). The ortholog of the E. coli PBP3 gene (pbpBBfr, which encodes the protein PBP2Bfr) was implicated in binding to imipenem (6,188).…”

“…The completion of the genome sequence of B. fragilis allowed a more complete analysis, and seven putative PBP genes were identified. The gene sequences for the closest homologs to the E. coli PBPs in the B. fragilis genome (pbp1abBfr, pbp1cBfr, pbpABfr, pbpBBfr, pbp4Bfr, and pbp7Bfr) were deduced as the orthologs for the E. coli genes ponBEco (PBP1bEco), pbpCEco (PBP1cEco), pbpAEco (PBP2Eco), pbpBEco (PBP3Eco), dacBEco (PBP4Eco), and pbpGEco (PBP7Eco), respectively (188). In any case, as the authors finally conclude, it is very difficult to correlate the 50% inhibitory concentration affinity of a particular PBP for an antibiotic with the MIC, since this is not generally the only mechanism responsible for the resistance.…”

Proteomic analysis of ampicillin‐resistant oral Fusobacterium nucleatum

“…Resistance to ampicillin and other β‐lactam antibiotics can be mediated by β‐lactamase and/or other resistance mechanisms. However, the levels of β‐lactamase production and resistance, expressed as the minimum inhibitory concentration (MIC), are not always in a linear relationship and other resistance determinants could exist simultaneously (18, 24). In general, proteins that are responsible for bacterial resistance to ampicillin are either β‐lactamase enzymes, mainly in gram‐negative bacteria, or altered penicillin‐binding proteins (PBP), primarily in gram‐positive bacteria, or mutant porins.…”