Molecular epidemiology of sil locus in clinical Streptococcus pyogenes strains

Abstract

Streptococcus pyogenes (group A Streptococcus [GAS]) causes a wide variety of diseases, ranging from mild noninvasive to severe invasive infections. Mutations in regulatory components have been implicated in the switch from colonization to invasive phenotypes. The inactivation of the sil locus, composed of six genes encoding a quorum-sensing complex, gives rise to a highly invasive strain. However, studies conducted on limited collections of GAS strains suggested that sil prevalence is around 15%; furthermore, whereas a correlation between the presence of sil and the genetic background was suggested, no link between the presence of a functional sil locus and the invasive status was assessed. We established a collection of 637 nonredundant strains covering all emm genotypes present in France and of known clinical history; 68%, 22%, and 10% were from invasive infections, noninvasive infections, and asymptomatic carriage, respectively. Among the 637 strains, 206 were sil positive. The prevalence of the sil locus varied according to the emm genotype, being present in >85% of the emm4, emm18, emm32, emm60, emm87, and emm90 strains and absent from all emm1, emm28, and emm89 strains. A random selection based on 2009 French epidemiological data indicated that 16% of GAS strains are sil positive. Moreover, due to mutations leading to truncated proteins, only 9% of GAS strains harbor a predicted functional sil system. No correlation was observed between the presence or absence of a functional sil locus and the strain invasiveness status.

FIG 1

sil locus organization and PCR assays used to detect its presence and variability. (A) Schematic representation of the sil locus and PCR products obtained with the primers listed in Table 2. (B) Gel electrophoresis of the 4 PCR products obtained on a wild-type (WT) sil locus strain (CCH20080458) (PCR1, -2, -3, -4) and on a silD-deleted mutant strain (CCH20080441) (PCR1, -2, -3, -4). The PCR product deleted in the mutant strain is indicated by an asterisk. MW, molecular weight markers (Bio-Rad).

FIG 2

Mutation occurrences in silCR and silC. Both genes are symbolized by solid lines. Below the sequence of the JS95 gene all of the mutations found during this study are indicated; their occurrences are shown on the left.

FIG 3

Mutation occurrences in SilD. The SilD sequence is symbolized by the solid line. Fragments of the JS95 SilD sequence are shown, separated by double oblique lines. The mutations leading to premature translation arrest or to deletions are indicated below the sequence; their occurrences are shown on the left.