Acquisition of insertion sequences and the GBSi1 intron by Streptococcus agalactiae isolates correlates with the evolution of the species

Abstract

The prevalence of insertion sequences IS1548, IS861, IS1381, and ISSa4 and of the group II intron GBSi1 within Streptococcus agalactiae human isolates strongly correlates with the genetic lineages obtained by multilocus sequence typing. Our results yielded an evolutionary scheme for the acquisition of these genetic elements linked to the ecosystems from which the isolates were obtained.

FIG. 1.

UPGMA dendrogram showing the genetic relationships between the 52 S. agalactiae isolates. The dendrogram was constructed with the MEGA 2.1 software by using the nucleotide sequences of the seven housekeeping genes (pheS, atr, tkt, glcK, sdhA, glnA, and adhP) concatenated into a single “supergene.” Bootstrap values are shown at the major nodes. Isolates can be divided into two divisions (A and B) and five subdivisions (A1α, A1β, A2, B1, and B2). For each isolate, the serotype, ST, presence or absence of the four ISs and of the group II intron GBSi1, and genetic variants (deduced from the combination of the four ISs and GBSi1 and named V1 to V9) are indicated. L and LJ refer to meningitis isolates, H to endocarditis isolates, G to gastric fluid isolates, and V and VV to vaginal isolates.

FIG. 2.

Evolutionary scheme showing the chronology of acquisition of the four ISs and of the group II intron GBSi1 during S. agalactiae evolution. This scheme was deduced from the prevalence of each mobile genetic element within the divisions and subdivisions obtained by MLST as indicated in Fig. 1. IS1381 may have been acquired (acquisition of ISs or GBSi1, ▾) by division A, IS1548 by subdivision A1α, ISSa4 by subdivision B2, and GBSi1 by subdivision B2. IS861 may have been acquired by subdivisions A1α, A2, and B2 (Fig. 2a), or it may have been acquired by the common ancestor and then lost (loss of ISs or GBSi1, ▿) by subdivisions A1β and B1 (Fig. 2b). Branch values are not representative of genetic distances.