Pneumococcal pneumonia is driven by increased bacterial turnover due to bacteriocin-mediated intra-strain competition.

Using chromosomal barcoding, we observed that >97% of the Streptococcus pneumoniae (Spn) population turns over in the lung within 2 days post-inoculation in a murine model. This marked collapse of diversity and bacterial turnover was associated with acute inflammation (severe pneumococcal pneumonia), high bacterial numbers in the lungs, bacteremia, and mortality. Intra-strain competition mediated by the blp locus, which expresses bacteriocins in a quorum-sensing-dependent manner, was required for each of these effects. Bacterial turnover from the activity of Blp-bacteriocins increased the release of the pneumococcal toxin, pneumolysin (Ply), which was sufficient to account for the lung pathology. The ability of Ply to evade complement, rather than its pore-forming activity, prevented opsonophagocytic clearance of Spn enabling its multiplication in the lung, facilitating the inflammatory response and subsequent invasion into the bloodstream. Thus, our study demonstrates how an appreciation for bacterial population dynamics during infection provides new insight into pathogenesis.