Evidence for rare capsular switching in Streptococcus agalactiae

Abstract

The polysaccharide capsule is a major antigenic factor in Streptococcus agalactiae (Lancefield group B streptococcus [GBS]). Previous observations suggest that exchange of capsular loci is likely to occur rather frequently in GBS, even though GBS is not known to be naturally transformable. We sought to identify and characterize putative capsular switching events, by means of a combination of phenotypic and genotypic methods, including pulsed-field gel electrophoretic profiling, multilocus sequence typing, and surface protein and pilus gene profiling. We show that capsular switching by horizontal gene transfer is not as frequent as previously suggested. Serotyping errors may be the main reason behind the overestimation of capsule switching, since phenotypic techniques are prone to errors of interpretation. The identified putative capsular transformants involved the acquisition of the entire capsular locus and were not restricted to the serotype-specific central genes, the previously suggested main mechanism underlying capsular switching. Our data, while questioning the frequency of capsular switching, provide clear evidence for in vivo capsular transformation in S. agalactiae, which may be of critical importance in planning future vaccination strategies against this pathogen.

FIG. 1.

Dendrogram analysis of the PFGE profiles of 463 GBS isolates. UPGMA and the Dice coefficient (indicated as percentages in the scale above the dendrogram) were used to construct the dendrogram. Each PFGE cluster (defined as a group of ≥5 isolates with a Dice coefficient of ≥80% in the dendrogram) is represented by a triangle with size proportional to the number of isolates included in the cluster. The clusters are identified by capital letters outside each triangle. Putative capsular transformants were selected based on exhibiting a serotype different from the dominant serotype of the PFGE cluster. M₁ and M₂ are two sublineages of the same cluster, defined at >80% similarity in the dendrogram.

FIG. 2.

Genome map of S. agalactiae indicating loci of interest. The relative positions of housekeeping genes characterized in the MLST scheme (pheS, atr, tkt, glnA, sdhA, glcK, and adhP), genes identified for surface protein gene profiling (same relative positions for bca, alp2, alp3, alp4, rib, or eps), bac, regions characterized for pilus island profiling PI-1 and PI-2 (same relative positions for PI-2a or PI-2b), and the capsular locus region are depicted. Numbers inside the circle represent the scale in megabases.

FIG. 3.

Diagram representing the DNA sequence of the cpsG-cpsH region of the capsular locus of S. agalactiae serotypes Ia and Ib compared to that of strain 312754. The cpsG and cpsH genes are represented by dashed and dotted arrows, respectively. The cpsG-cpsH fusion in strain 312754 is predicted to generate a fusion protein since the reading frame is maintained (see the text). The fragments with high DNA identity are represented by black boxes, whereas lower identity fragments are represented by lighter shades of gray. Similar fragments are connected by lines.