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Penicillin-binding proteins of Bacteroides fragilis and their role in the resistance to imipenem of clinical isolates

¹`‘Severo Ochoa’’ Molecular Biology Centre, CSIC-UAM, 28049 Cantoblanco, Madrid, Spain ^2,3Biochemistry, Molecular Biology and Genetics Department², and Medicine and Animal Health Department³, University of Extremadura, 10071 Cáceres, Spain

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

Received October 15, 2004
Accepted July 12, 2005

In this study penicillin-binding proteins (PBPs) of Bacteroides fragilis and the resistance mechanisms of this micro-organism to 11 ß-lactam antibiotics were analysed. The study focused on the role of PBP2Bfr and metallo-ß-lactamase in the mechanism of resistance to imipenem. The mechanism of ß-lactam resistance in B. fragilis was strain dependent. The gene encoding the orthologue of Escherichia coli PBP3 gene (pbpBBfr, which encodes the protein PBP2Bfr) was sequenced in five of the eight strains studied, along with the ccrA (cfiA) gene in strain 119, and their implications for resistance were examined. Differences were found in the amino-acid sequence of PBP2Bfr in strains AK-2 and 119, and the production of ß-lactamases indicated that these differences may be involved in the mechanism of resistance to imipenem. In vitro binding competition assays with membrane extracts using imipenem indicated that the PBP that bound imipenem with the highest affinity was PBP2Bfr, and that increased affinity in strain 7160 may be responsible for the moderate susceptibility of this strain to imipenem. In the same way, the importance of the chromosomal class A ß-lactamase CepA in the resistance mechanism of the B. fragilis strains NCTC 9344, 7160, 2013E, AK-4, 0423 and R-212 was studied. In these strains this is the principal resistance mechanism to antimicrobial agents studied other than imipenem.

The GenBank/EMBL/DDBJ accession numbers for the Bacteroides fragilis pbpBBfr gene sequences for penicillin-binding protein 3 of strains NCTC 9344, 119, 7160, AK-2 and R212 are AJ831484–AJ831488, respectively, and for the cfiA gene of strain 119 is AJ831823.

An amino acid sequence alignment of PBP2Bfr from several B. fragilis strains is available as supplementary material in JMM Online.

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