Heterogeneity of Streptococcus anginosus ß-hemolysis in relation to CRISPR/Cas

Abstract

Streptococcus anginosus is a commensal of the oral mucosa that can cause severe invasive infections. A considerable proportion of Streptococcus anginosus strains are ß-hemolytic due to the presence of an SLS-like gene cluster. However, the majority of strains do not display ß-hemolysis. To investigate ß-hemolysin heterogeneity in S. anginosus, we determined the presence of sag genes and correlated it with the presence of CRISPR/Cas genes in a collection of ß-hemolytic and non-ß-hemolytic strains. All of the ß-hemolytic strains carried the sag gene cluster. In contrast to other streptococci, clinical S. anginosus strains that do not display ß-hemolysis do not harbor sag genes. Phylogenetic analysis of the ß-hemolytic strains revealed that they belong to two previously defined clusters within S. anginosus. Correlation with CRISPR/Cas genes showed a significant difference for the presence of CRISPR/Cas in ß-hemolytic versus non-ß-hemolytic isolates. The presence of the CRISPR/Cas type IIA or type IIC locus is associated with the absence of sag genes; in 65% of the non-ß-hemolytic strains a CRISPR/Cas locus was found, while only 24% of ß-hemolytic strains carry CRISPR/Cas genes. Further analysis of the spacer content of the CRISPR systems revealed the presence of multiple self-targeting sequences directed against S. anginosus genes. These results support the hypothesis that horizontal gene transfer is involved in the acquisition of ß-hemolysin genes and that CRISPR/Cas may limit DNA uptake in S. anginosus.

Keywords: Streptococcus anginosus; CRISPR; hemolysis; phylogeny.