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

^1,2Medicine and Animal Health Department¹ and Biochemistry, Molecular Biology and Genetics Department², University of Extremadura, 10071 Cáceres, Spain ³`Severo Ochoa’ Molecular Biology Centre, CSIC-UAM, 28049 Cantoblanco (Madrid), Spain

Correspondence Juan Ayala jayala{at}cbm.uam.es

Received July 30, 2003
Accepted December 9, 2003

This study examines the role of the penicillin-binding proteins (PBPs) of Bacteroides fragilis in the mechanism of resistance to different ß-lactam antibiotics. Six of the eight strains used were ß-lactamase-positive by the nitrocefin assay. These strains displayed reduced susceptibility to imipenem (MIC, 2–16 mg l^-1) 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-ß-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 ß-lactamases indicate that these differences are not involved in the mechanism of resistance to imipenem and/or cephalexin.

The GenBank/EMBL/DDBJ accession numbers for sequences of the pbpA gene (for PBP2) of Bacteroides fragilis strains NCTC 9344, AK-2, AK-4, R212, 119 and 7160 are respectively AJ544243, AJ544246, AJ575646, AJ575647, AJ544245 and AJ544244.

Deduced amino acid sequences of the pbpABfr gene of strains used in this study are available as supplementary material in JMM Online.

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