The blp locus of Streptococcus pneumoniae plays a limited role in the selection of strains that can cocolonize the human nasopharynx

Nasopharyngeal colonization is important for Streptococcus pneumoniae evolution, providing the opportunity for horizontal gene transfer when multiple strains co-occur. Although colonization with more than one strain of pneumococcus is common, the factors that influence the ability of strains to coexist are not known. A highly variable blp (bacteriocin-like peptide) locus has been identified in all sequenced strains of S. pneumoniae. This locus controls the regulation and secretion of bacteriocins, small peptides that target other bacteria. In this study, we analyzed a series of cocolonizing isolates to evaluate the impact of the blp locus on human colonization to determine whether competitive phenotypes of bacteriocin secretion restrict cocolonization. We identified a collection of 135 nasopharyngeal samples cocolonized with two or more strains, totaling 285 isolates. The blp locus of all strains was characterized genetically with regard to pheromone type, bacteriocin/immunity content, and potential for locus functionality. Inhibitory phenotypes of bacteriocin secretion and locus activity were assessed through overlay assays. Isolates from single colonizations (n=298) were characterized for comparison. Cocolonizing strains had a high diversity of blp cassettes; approximately one-third displayed an inhibitory phenotype in vitro. Despite in vitro evidence of competition, pneumococci cocolonized the subjects independently of blp pheromone type (P=0.577), bacteriocin/immunity content, blp locus activity (P=0.798), and inhibitory phenotype (P=0.716). In addition, no significant differences were observed when single and cocolonizing strains were compared. Despite clear evidence of blp-mediated competition in experimental models, the results of our study suggest that the blp locus plays a limited role in restricting pneumococcal cocolonization in humans.