Auto-aggregation in Streptococcus intermedius is driven by the Pel polysaccharide

Abstract

The Streptococcus milleri group (SMG), comprising Streptococcus intermedius, anginosus, and constellatus, can asymptomatically colonize various mucosal sites of healthy individuals. These bacteria are opportunistic pathogens that cause differentdifferentdifferenttypes of infections across various anatomical sites. Although the pathogenic mechanisms leading to infections are not well defined in the SMG, auto-aggregation is a key driver of biofilm adhesion and cohesion in many Streptococci and Staphylococci. Here, we identify a S. intermedius gene cluster with significant homology to the Bacillus cereus and Pseudomonas aeruginosa pel operons, which are required for Pel exopolysaccharide production and biofilm formation in these species. This cluster contains five genes that are homologous to pelDEADAFG in other gram-positive species and four additional genes of unknown function. Characterization of a panel of clinical S. intermedius isolates identified a range of adherent biofilm and aggregation phenotypes, and aggregation in strain C1365 was dependent on each of the pelDEADAFG genes. Deletion of two of the additional genes, SIR_1592 and SIR_1594, reduced but did not abolish the aggregation phenotype. Furthermore, we demonstrate that SIR_1591 is a glycoside hydrolase and that C1365 produces a GalNAc-rich polymer as aggregates were disrupted by theα−1,4-N-acetylgalactosaminidases PelA and Sph3, but not the α−1,4-N-galactosaminidase Ega3. Using an abscess model of mouse infection, we show that loss of Pel production in a C1365 ΔpelF mutant allows for more effective bacterial clearance. The polymer also affects how S. intermedius interacts with the host immune system. Collectively, our data suggest that Pel biosynthesis contributes to S. intermedius pathogenicity.