Analysis of Point Mutations in the pbp2x, pbp2b, and pbp1a Genes of Streptococcus agalactiae and Their Relation with a Reduced Susceptibility to Cephalosporins.

ABSTRACT

The emergence of group B Streptococcus (GBS) isolates with reduced penicillin susceptibility (PRGBS) and their tendency to be nonsusceptible to fluoroquinolones prompted us to analyze the possible presence of amino acid mutations in penicillin-binding proteins (PBPs) (PBP2X, PBP1A, and PBP2B) from a collection of fluoroquinolone-resistant GBS isolates. We analyzed 21 GBS isolates resistant to levofloxacin. Sequence analysis of genes for PBPs was performed. The minimal inhibitory concentrations (MICs) for penicillin, ceftibuten, cefaclor, cefixime, cefotaxime, and ceftizoxime were performed by the Etest method and by broth microdilution method. The isolates were furthermore characterized by PCR-based capsular typing and analysis of surface protein genes. Genetic relatedness among the isolates was examined by multilocus sequence typing. Phylogenetic analysis of PBPs sequences was performed by Molecular Evolutionary Genetics Analysis software (MEGA7). All isolates were susceptible to penicillin, even if different mutations were detected in all PBPs in most of the isolates (12/21, 57%). However, we observed a reduced susceptibility to cefixime in seven isolates and to cefaclor in six isolates. These PSGBS isolates shared an I377V mutation in PBP2X and a T145A mutation in PBP1A. Most of the isolates belongs to the clonal complex 1, has serotype III and rib as surface protein. The results of phylogenetic comparative analysis show that several genetic lineages of our isolates with reduced susceptibility to cefixime/cefaclor have been independently emerging through the accumulation of mutations in their pbp genes, especially in pbp1a. If the MICs of penicillins and cephalosporins for GBS increase, careful epidemiological surveillance on this issue is recommended.