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Pneumococcal bacteriocins (pneumocins) are antibacterial toxins that mediate intra-species competition within the human host. However, the triggers of pneumocin expression are poorly understood. Using RNA-sequencing, we mapped the regulon of the pneumocin cluster ( blp ) of  Streptococcus pneumoniae  D39. Furthermore, by analogy with pneumococcal competence, we show that several antibiotics activate the  blp -genes. Using real-time gene expression measurements we show that while the promoter driving expression of the two-component regulatory system  blpR/H  is constitutive, the remaining  blp -promoters that control pneumocin expression, immunity and the inducer peptide BlpC, are pH-dependent and induced in the late exponential phase. Intriguingly, competence for genetic transformation, mediated by the paralogous ComD/E two-component quorum system, is induced by the same environmental cues. To test for interplay between these regulatory systems, we quantified the regulatory response to the addition of synthetic BlpC and competence-stimulating peptide (CSP). Supporting the idea of such interplay, we found that immediately upon addition of CSP, the  blp -promoters were activated in a  comD/E -dependent manner. After a delay,  blp -expression was highly induced and was strictly dependent on  blpRH  and  blpC . This raised the question of the mechanism of BlpC export, since bioinformatic analysis showed that the genes encoding the putative exporter for BlpC,  blpAB , are not intact in strain D39 and most other strains. By contrast, all sequenced pneumococcal strains contain intact  comAB  genes, encoding the transport system for CSP. Consistent with the idea that  comAB  mediate BlpC export, we finally show that high-level expression of the  blp -genes requires  comAB . Together, our results demonstrate that regulation of pneumocin expression is intertwined with competence, explaining why certain antibiotics induce  blp -expression. Antibiotic-induced pneumocin expression might therefore have unpredictable consequences on pneumococcal colonization dynamics by activating genes that mediate intra-specific interference competition.

Expression of Streptococcus pneumoniae Bacteriocins Is Induced by Antibiotics via Regulatory Interplay with the Competence System