Auto-aggregation in is driven by the Pel polysaccharide.

The Streptococcus milleri group (SMG), comprising , and , can asymptomatically colonize various mucosal sites of healthy individuals. These bacteria are opportunistic pathogens that cause different types 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 gene cluster with significant homology to the and operons, which are required for Pel exopolysaccharide production and biofilm formation in these species. This cluster contains five genes that are homologous to in other gram-positive species and four additional genes of unknown function. Characterization of a panel of clinical isolates identified a range of adherent biofilm and aggregation phenotypes , and aggregation in strain C1365 was dependent on each of the genes. Deletion of two of the additional genes and , 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-acetylgalactosaminidases PelA and Sph3, but not the α-1,4-galactosaminidase Ega3. Using an abscess model of mouse infection, we show that loss of Pel production in a C1365 Δ mutant allows for more effective bacterial clearance. The polymer also affects how interacts with the host immune system. Collectively, our data suggest that Pel biosynthesis contributes to pathogenicity.IMPORTANCESMG species are increasingly being recognized as pathogens. Despite their clinical relevance, little is known about how SMG members transition between asymptomatic colonization and infection. Herein, we show that clinical isolates of can be classified into four groups based on their aggregation and adherent biofilm phenotypes. We demonstrate that aggregation is dependent on the Pel polysaccharide and that Pel production allows bacteria not only to persist longer during infection but also modulates the immune responses of the host. Pel production requires the canonical genes. We also identified four additional genes in the cluster and found that under the conditions tested, two of these genes play a role in aggregation and Pel production. Functional homologs of the additional genes play major roles in host-pathogen interactions and stress responses in other bacteria, suggesting that these additional genes could play a role in Pel-related infections.