Streptococcus agalactiae Cβ protein gene (bac) sequence types, based on the repeated region of the cell-wall-spanning domain: relationship to virulence and a proposed standardized nomenclature

Abstract

The Cβprotein (Bac) ofStreptococcus agalactiae(group B streptococcus; GBS) is an IgA binding protein encoded bybac, of which at least 39 sequence types have been described, based on polymorphisms in the repeated region of the cell-wall-spanning domain (‘bacsequence types’). Cβis usually found in serotype Ib, less commonly in serotype II, and rarely in other serotypes. The aim of this study was to examine the prevalence, variety and distribution, among GBS serotypes and between invasive and superficial isolates, ofbacsequence types. A total of 1101 GBS isolates were tested, from 10 countries, with abac-specific PCR, and amplicons from all 255 (23 %) with positive results were sequenced. Ninety-seven percent (184/190) of serotype Ib and 37 % of serotype II isolates werebacpositive. The Cαprotein gene (bca) was present in 98 % (251/255), and insertion sequences IS1381and IS861in 94 % (239/255), ofbac-positive isolates. The authors identified 59bacsequence types belonging to 19 groups, based on length, from 496 to 946 bp, with up to six sequence variants (a–f) in each group. The medianbacsequence length of invasive isolates was significantly shorter than that of superficial isolates overall (640 versus 586 bp;P<0.001) and specifically for serotype Ib (541 versus 676 bp;P<0.001), and invasive isolates were significantly (P<0.001) more likely to have one or more 18 bp deletions relative to the original publishedbacsequence (X59771).bacsequence typing is a useful addition to the previously described genotyping system, and will help to predict relative virulence amongS. agalactiaeserotype Ib strains.