Long-range mapping of the Streptococcus agalactiae phylogenetic lineage restriction digest pattern type III-3 reveals clustering of virulence genes

Abstract

Human isolates of serotype III Streptococcus agalactiae (group B streptococcus [GBS]) can be divided into three separate phylogenetic lineages based on analysis of the restriction digest patterns (RDPs) of chromosomal DNA. Nine DNA sequences that are present in all isolates of the RDP III-3 phylogenetic lineage, but not in the other lineages, were identified by genomic subtractive hybridization. A complete physical map of a III-3 chromosome was constructed. Six of the nine III-3-specific sequences mapped to a 340-kb Sse8387I fragment which contains or is located close to known GBS virulence genes. One of the III-3-specific probes, AW-10, encodes part of GBSi1, a group II intron that is inserted at two sites within the GBS genome. The second chromosomal site for GBSi1 was isolated, sequenced, and mapped to a location near the locus responsible for hemolysin production. These findings suggest that the genetic variation that distinguishes the RDP type III-3 strains from other serotype III strains occurs largely within localized areas of the genome containing known or putative virulence genes.

FIG. 1.

Physical and genetic map of the RDP type III-3 GBS isolate 874391. The sizes of the restriction fragments are shown. The hatched areas represent areas of the genome that contain additional restriction fragments not definitively mapped, including the area containing the six 16S rRNA operons. See text for details.

FIG. 2.

Map of the AW-10 genomic clone. The arrowheads indicate the positions of HindIII restriction sites. The putative insertion site for the GBSi1 group II intron, consisting of an inverted repeat and poly(T) tail, is shown.