Autoaggregation in Streptococcus intermedius is driven by the Pel polysaccharide

Abstract

The Streptococcus Milleri Group (SMG) comprising of Streptococcus intermedius, anginosus and constellatus are commensal bacteria commonly found in healthy individuals. These bacteria are increasingly being recognized as opportunistic pathogens that can cause purulent infections at sterile body sites and have also been identified in the sputum of individuals with cystic fibrosis. Although the mechanisms of conversion to opportunistic pathogens are not well understood, auto-aggregation is a key driver of biofilm adhesion and cohesion in many Streptococci and Staphylococci. Here, we identify a gene cluster in the S. intermedius genome with significant homology to the pel operons in Bacillus cereus and Pseudomonas aeruginosa, which are required for Pel exopolysaccharide production and biofilm formation in these species. Characterization of a panel of clinical S. intermedius strains identified a range of aggregating phenotypes. Analysis of the pel operon in the hyper-aggregating C1365 strain revealed that each of the canonical pelDEADAFG genes, but not the four additional genes are required for aggregation. Further, we demonstrate that C1365 produces a GalNAc-rich exopolysaccharide and that aggregates can be disrupted by the 1,4 N-acetylgalactosaminidases, PelA and Sph3, but not other glycoside hydrolases, proteinase K or DNase I. Using an abscess model of mouse infection, we show that Pel driven aggregation leads to longer lasting infections, and that lack of Pel allows for the bacteria to be cleared more effectively. The polymer also affects how the bacteria interacts with the host immune system. Collectively, our data suggest that the pel operon has relevancy to S. intermedius pathogenicity.