The blp Locus of Streptococcus pneumoniae Plays a Limited Role in the Selection of Strains That Can Cocolonize the Human Nasopharynx

Abstract

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. IMPORTANCENasopharyngeal colonization with Streptococcus pneumoniae (pneumococcus) is important for pneumococcal evolution, as the nasopharynx represents the major site for horizontal gene transfer when multiple strains co-occur, a phenomenon known as cocolonization. Understanding how pneumococcal strains interact within the competitive environment of the nasopharynx is of chief importance in the context of pneumococcal ecology. In this study, we used an unbiased collection of naturally co-occurring pneumococcal strains and showed that a biological process frequently used by bacteria for competition-bacteriocin production-is not decisive in the coexistence of pneumococci in the host, in contrast to what has been shown in experimental models. Streptococcus pneumoniae is an important bacterial pathogen associated with high morbidity and mortality worldwide (1-3). Notwithstanding, disease is a rare event compared with the frequency of asymptomatic nasopharyngeal colonization (4).Nasopharyngeal colonization is particularly frequent among young children (5), and multiple strains can be simultaneously detected in the same sample (6-8). As the pneumoco...

Genetic organization of i Streptococcus salivarius i 24SMBc i blp i -like bacteriocin locus

“…NZ_CP009913.1.). The bacteriocin cassette in the 24SMBc strain shows a genetic structure similar to that of the other blp operon already well characterized in S. pneumoniae and S. thermophilus (18,19,20); particularly, it is made up of three main modules: i) the ABC-transporters that recognize the N-termini of bacteriocins and transport these peptides out of the cell with the cleavage of the double-glycine motif, ii) the bacteriocin immunity module to protect the producer bacteria from the effects of their own bacteriocins, and iii) the peptide pheromone with a typical leader peptide containing a double-glycine motif. orf8 (180 bp) appears to be a structural gene, blpU-like, encoding 59 amino acids and a pore forming peptide belonging to bacteriocin class II with a double-glycine leader that could be important for its processing to its shorter mature peptide form.…”

“…It is well known that bacteriocins play a fundamental role in the intra-and inter-species competition between the normal oral streptococcal species and pneumococci, contributing to maintain the complex equilibrium between a healthy state and the progression toward disease within the normal range (20,27) Streptococcus salivarius species is one of the pioneer strains, colonizing the oropharyngeal and gastrointestinal tracts in newborns, remaining predominant throughout the human life span, and capable of improving healthassociated oral and gut microbiota just by interfering with potential pathogens (28).…”

“…S. pyogenes and S. pneumoniae (11,16,17) Previous studies failed in the identification of the bacteriocin produced by S.salivarius 24SMBc while this study aimed at characterizing this unknown bacteriocin through the sequence of its genome. A blpU-like bacteriocin, belonging to a class IIc type with characteristics similar to peptides produced by S. pneumoniae and S. termophilus (8,19,20) was found.…”

Epidemiological dynamics of bacteriocin competition and antibiotic resistance

“…Bacteriocin production has been shown to inhibit competing strains in vitro and in murine models of colonisation (see, for example [17]). However, how this e↵ect translates to the epidemiological level is not entirely clear (see, for example [18]). In light of this uncertainty, we model two distinct ways in which the bacteriocin production could influence epidemiological dynamics in a homogeneously mixing host population.…”

Competitive Dominance within Biofilm Consortia Regulates the Relative Distribution of Pneumococcal Nasopharyngeal Density

“…A candidate for modulating densities of pneumococcal types had been the blp locus, which encodes a bacteriocin system that produces a potent bacteriocin, BlpC, but it has been recently demonstrated that BlpC plays a minor role in cocolonization (36,37). A secreted factor might not be involved in dominance of S19F strains by the S4 strain TIGR4, as demonstrated in experiments using Transwell systems, although our experimental design did not allow us to test for a factor(s) that could be only released when in close proximity.…”