Molecular characteristics of Streptococcus agalactiae strains deficient in alpha-like protein encoding genes

Streptococcus agalactiae (group B streptococci, GBS) are important human and animal pathogens, which can be subdivided based on different capsular polysaccharides and surface-anchored alpha-like proteins (Alps), as well as other proteins. Nearly all GBS strains possess an Alp (Alp GBS), although Alp-negative GBS (non-Alp GBS) do occur. In this study, 10 (1.1 %) of 932 clinical human GBS tested lacked an Alp encoding gene. All 10 strains were from patients with bloodstream infection, confirming that non-Alp GBS can be highly virulent. All non-Alp GBS expressed one or more of the surface-anchored proteins R3, Z1 and Z2, while less than 10 % of unselected clinical strains express any of these proteins. In contrast to Alp GBS, all non-Alp strains tested were PCR negative for the upstream sequence of the insertion site of the Alp encoding gene of Alp GBS. Genome sequencing showed that all but one of the 10 clinical non-Alp strains and the non-Alp reference strain CNCTC 10/84 lacked a region surrounding the Alp gene commonly present in Alp GBS strains. These strains instead harboured an 849 bp region not present in the Cα prototype strain A909. We have shown that non-Alp GBS differ from Alp GBS in the region surrounding the insertion site of Alp genes of Alp GBS as well as in their content of other surface proteins and that PCR for the upstream flanking region of the Alp gene may be useful for differentiation between Alp and non-Alp GBS.