Abstract
During colonization of the human nasopharynx, multiple strains of the Gram-positive pathogen Streptococcus pneumoniae coexist and compete with each other using secreted antimicrobial peptides called bacteriocins. The major class of pneumococcal bacteriocins is encoded by the blp operon, whose transcription is controlled by the secretion and detection of a polymorphic family of quorum sensing (QS) peptides. We examined the genomic association between blp QS signals (BlpC) and receptors (BlpH) across 4,096 pneumococcal genomes. Imperfect concordance between nine QS signal peptide types and five phylogenetically-related QS receptor groups suggested extensive crosstalk between signals (where cells produce signals that non-clonal cells can detect) and eavesdropping (where cells respond to signals that they do not produce). To test these possibilities, we quantified the response of reporter strains containing each of six different blp QS receptor variants to cognate and non-cognate synthetic peptide signals. As predicted, we found evidence for crosstalk in five of six tested signals and for eavesdropping in four of these receptors. These in vitro results were confirmed during interactions between pneumococcal colonies grown adjacent to one another, providing direct evidence that crosstalk and eavesdropping occur at endogenous, ecologically-relevant, levels of signal secretion. Finally, using a spatially explicit stochastic model, we show that eavesdropping genotypes gain evolutionary advantages during inter-strain competition, even when their affinity to non-cognate signals is as low as 10% of the affinity to their cognate signal. Our results highlight the importance of social interactions in mediating intraspecific competition among bacteria and clarify that diverse competitive interactions can be mediated by polymorphism in QS systems.
Author Summary Quorum sensing (QS), where bacteria secrete and respond to chemical signals to coordinate population-wide behaviors, has revealed that bacteria are highly social. Here, we use a multifaceted approach to investigate how diversity in QS signals and receptors can modify social interactions controlled by the QS system that regulates antimicrobial peptide secretion in Streptococcus pneumoniae. We experimentally confirmed that single QS receptors can respond to multiple signals (eavesdropping) and that single QS signals activate multiple receptors (crosstalk).We also show that QS eavesdropping can differentially affect gene expression in neighboring colonies. Eavesdropping and crosstalk can potentially explain our finding from surveys of 4,096 pneumococcal genomes that 16.7% of strains express QS receptors that may be unable to detect the QS signal that they produce. Simulations of QS strains producing antimicrobial peptides revealed that eavesdropping can be evolutionarily beneficial even when their affinity for non-cognate signals is very weak. Our results demonstrate the importance of eavesdropping and crosstalk as drivers of the outcome of competitive interactions mediated by bacterial quorum sensing.