Genetic determinants for cfxA expression in Bacteroides strains isolated from human infections

Objectives: To identify genetic determinants that determine beta-lactamase expression in Bacteroides strains isolated from human infections. Methods: beta-Lactam susceptibility and beta-lactamase enzyme expression were characterized in selected strains. beta-Lactamase genes and surrounding regions were analysed by PCR, inverse PCR and Southern hybridization. Results: High resistance to penicillins and cephalosporins was found among most isolated strains, in which all known beta-lactamase genes from Bacteroides are represented, but differences were found in their expression of enzyme activity. In contrast to the cepA gene, ubiquitously found but frequently inactive, or cfiA, which only confers carbapenem resistance in two strains, the detection of high beta-lactamase expression correlates closely with the presence of cfxA genes. This genetic determinant shares variability of upstream regulatory elements, including sequence tags from Tn4555, Tn4351 and IS614B, and polymorphisms of encoded amino acid sequences at positions G(57) C and Y(259)C, which might determine enzyme expression characteristics. Conclusions: The main determinant for beta-lactamase expression in Bacteroides strains is the cfxA gene, in which IS614B integration upstream of the coding sequence represents a molecular marker for higher levels of enzyme activity.