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 Cβ protein (Bac) of Streptococcus agalactiae (group B streptococcus; GBS) is an IgA binding protein encoded by bac, of which at least 39 sequence types have been described, based on polymorphisms in the repeated region of the cell-wall-spanning domain (‘bac sequence types’). Cβ is 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, of bac sequence types. A total of 1101 GBS isolates were tested, from 10 countries, with a bac-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 were bac positive. The Cα protein gene (bca) was present in 98 % (251/255), and insertion sequences IS1381 and IS861 in 94 % (239/255), of bac-positive isolates. The authors identified 59 bac sequence 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 median bac sequence 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 published bac sequence (X59771). bac sequence typing is a useful addition to the previously described genotyping system, and will help to predict relative virulence among S. agalactiae serotype Ib strains.

Inactivation of DNA-Binding Response Regulator Sak189 Abrogates β-Antigen Expression and Affects Virulence of Streptococcus agalactiae

“…Thus, we suggest that β-antigen located on the bacterial surface can interfere with the disease process and play a role of anti-virulent factor (at least, in mice) similarly to hyaluronate lyase of S. pyogenes [41]. It is also in agreement with results of Kong et al who demonstrated that presence of shorter variants of β-antigen is associated with increased virulence [32].…”

“…One of the strains, 168/00, which demonstrated the large diameter of growth inhibition zone, was selected for further study. In this strain, the bac gene has unique allele with the closest similarity with 676a and 676b alleles according to classification of Kong et al [32].…”

“…Variety of publications are currently available regarding the occurrence of bac gene among S. agalactiae strains as well as genetic polymorphism of bac gene and use of encoded β-antigen as a vaccine component [32], [34]–[38]. In some strains, the mutations in bac gene resulted in expression of truncated non-functional forms of β-antigen [39], [40].…”

Recombinant Group BStreptococcusAlpha-Like Protein 3 Is an Effective Immunogen and Carrier Protein

“…The ability of rAlp3 antisera to opsonize invasive GBS type III strains bearing the cross-reactive proteins, presumably Rib, is of particular importance given the correlation of low maternal levels of Rib antibody with neonatal GBS disease (28). BCP, encoded by the bac gene, is found on nearly all strains of GBS serotype Ib as well as on some isolates of types Ia, II, and V (31), and approximately 23% of GBS strains contained the bac gene according to bac-specific PCR analysis (21). Our recently developed rBCP ⌬IgA demonstrated a higher degree of efficacy against a BCP-positive GBS type Ia strain when coupled to type III CPS and rAlp3 than as a component of an uncoupled mixture, despite evoking similar levels of maternal IgG.…”

Serotype IX, a Proposed New Streptococcus agalactiae Serotype

“…They were part of a large international collection of GBS isolates which have been studied previously (24,47,48,50). After retesting, many were successfully serotyped and most discrepancies between conventional and molecular serotypes were resolved, but 135 isolates remained nontypeable.…”