An update on ampicillin resistance and β-lactamase genes of Bacteroides spp.

Introduction. There are several β-lactamase genes described for Bacteroide s strains, of which cepA and cfiA are specific for Bacteroides fragilis and define two genetic divisions. The expression and phenotypic effects of these genes are usually regulated by insertional activation. Hypotheses/Gap Statement. Information is lacking about how cepA is regulated for most of the B. fragilis strains and whether there could be a genetic element for it. Aim. We aimed to investigate the molecular background of ampicillin (and other β-lactam) resistance among Bacteroides strains as mediated mainly by cepA and also to find a genetic element for it as known for cfiA . Methodology. Various PCR methods were used for β-lactamase-resistance gene and insertion sequence (IS) element detection in 42 Bacteroides strains. β-Lactamase activity measurements and antimicrobial-susceptibility testing using agar dilution were also applied. Further molecular experiments involved sequencing, gene targeting, Southern blotting and bioinformatic analyses. Results. We found that high antibiotic resistance and β-lactamase levels are brought about by insertional activation of the cepA gene or by similar or dissimilar activation of cfxA or cfiA , or by the newly described pbbA genes. Non-activated cepA genes produced low levels of specific β-lactamase activities that did not correlate with ampicillin resistance. We found a genetic element for cepA and another region close to it that are characteristic for division I B. fragilis strains, which are replaced by other sequences in division II B. fragilis strains. Conclusion. cepA usually is not activated by IS elements and usually produces low β-lactamase activities that do not correlate with the ampicillin MICs; therefore, it probably involves some non-β-lactamase-mediated resistance mechanism(s). pbpA is a newly described, effective β-lactamase gene that is located on a plasmid, and cepA resides on a well-defined chromosomal segment that is mutually replaced in division II B. fragilis strains. This latter finding demonstrates the genetic dichotomy of cepA–cfiA in B. fragilis and requires further investigation.