Function of BriC peptide in the pneumococcal competence and virulence portfolio

<em>Streptococcus pneumoniae</em> (pneumococcus) is an opportunistic pathogen that causes otitis media, sinusitis, pneumonia, meningitis and sepsis. The progression to this pathogenic lifestyle is preceded by asymptomatic colonization of the nasopharynx. This colonization is associated with biofilm formation; the competence pathway influences the structure and stability of biofilms. However, the molecules that link the competence pathway to biofilm formation are unknown. Here, we describe a new competence-induced gene, called <em>briC</em>, and demonstrate that its product promotes biofilm development and stimulates colonization in a murine model. We show that expression of <em>briC</em> is induced by the master regulator of competence, ComE. Whereas <em>briC</em>does not substantially influence early biofilm development on abiotic surfaces, it significantly impacts later stages of biofilm development. Specifically, <em>briC</em> expression leads to increases in biofilm biomass and thickness at 72h. Consistent with the role of biofilms in colonization, <em>briC</em>promotes nasopharyngeal colonization in the murine model. The function of BriC appears to be conserved across pneumococci, as comparative genomics reveal that <em>briC</em> is widespread across isolates. Surprisingly, many isolates, including strains from clinically important PMEN1 and PMEN14 lineages, which are widely associated with colonization, encode a long <em>briC</em>promoter. This long form captures an instance of genomic plasticity and functions as a competence-independent expression enhancer that may serve as a precocious point of entry into this otherwise competence-regulated pathway. Moreover, overexpression of <em>briC</em> by the long promoter fully rescues the <em>comE</em>-deletion induced biofilm defect <em>in vitro</em>, and partially <em>in vivo</em>. These findings indicate that BriC may bypass the influence of competence in biofilm development and that such a pathway may be active in a subset of pneumococcal lineages. In conclusion, BriC is a part of the complex molecular network that connects signaling of the competence pathway to biofilm development and colonization.