Streptolancidin D mediates intra-species competition in Streptococcus pneumoniae through the Rgg144/SHP144 quorum-sensing system

Streptococcus pneumoniae is a major colonizer of the human nasopharynx, where inter- and intra-strain competition plays a critical role in shaping population structure and influencing vaccine outcomes. Bacteriocins are key mediators of intra-species competition, yet many of their functions and regulatory mechanisms remain poorly understood. Here, we identify and characterize streptolancidin D, a previously uncharacterized bacteriocin encoded by the sldA-T locus and demonstrate its contribution to pneumococcal competition. Using a naturally colonizing strain, we show that deletion of sldA-T impairs inhibition of competitor strains in in vitro biofilms and during murine co-colonization. We further show that sldA-T is regulated by the Rgg144/SHP144 quorum sensing system, with promoter activity abolished in a SHP-deficient background and partially restored by exogenous peptide stimulation. Genomic analysis revealed that sldA-T is consistently encoded downstream of the Rgg144/SHP144 system and is present in [~]12% of over 7,500 pneumococcal genomes, with lineage-specific patterns of distribution. This observation suggests a regulated mechanism that provides competitive advantages in strain-strain interactions, which might help explaining the long-term persistence of certain clones even under PCV13 pressure. Our findings uncover a previously unrecognized quorum sensing regulated antimicrobial system in S. pneumoniae that enhances strain competitiveness and may contribute to lineage persistence and replacement dynamics under selective pressures such as vaccination. IMPORTANCEBacteriocins are central to bacterial competition and niche occupation, particularly in structured environments like the human nasopharynx. While several pneumococcal bacteriocins have been characterized, the functions of many of these systems are unknown, limiting our understanding of how these systems shape strain fitness and population dynamics. This study characterizes streptolancidin D, a bacteriocin that enhances intraspecies competitiveness both in vitro and in vivo. We demonstrate that its expression is tightly regulated by the conserved Rgg144/SHP144 quorum sensing system and that the locus is non-randomly distributed across the pneumococcal population. By integrating genomic, functional, and regulatory analyses, our work expands the known diversity of pneumococcal bacteriocins and reveals how quorum sensing-regulated antimicrobials may influence strain dominance, population structure, and potentially vaccine escape.