Heterogeneity in pneumolysin expression governs the fate of Streptococcus pneumoniae during blood-brain barrier trafficking

Outcome of host-pathogen encounter is determined by the complex interplay between protective bacterial and host defense strategies. This complexity further amplifies with the existence of cell-to-cell phenotypic heterogeneity in pathogens which remains largely unexplored. In this study, we illustrated that heterogeneous expression of pneumolysin (Ply), a pore-forming toxin of the meningeal pathogen, S. pneumoniae (SPN) gives rise to stochastically different bacterial subpopulations with variable fate during passage across blood-brain barrier (BBB). We demonstrate that Ply mediated damage to pneumococcus containing vacuolar (PCV) membrane leads to recruitment of cytosolic “eat-me” signals, galectin-8 and ubiquitin, targeting SPN for autophagic clearance. However, a majority of high Ply producing subset extensively damages autophagosomes leading to pneumococcal escape into cytosol and efficient clearance by host ubiquitination machinery. Interestingly, a low Ply producing subset halts autophagosomal maturation and evades all intracellular defense mechanisms, promoting its prolonged survival and successful transcytosis across BBB, both in vitro and in vivo. Ply therefore acts as both, sword and shield implying that its smart regulation ensures optimal disease manifestation. Our elucidation of heterogeneity in Ply expression leading to disparate infection outcomes attempts to resolve the dubious role of Ply in pneumococcal pathogenesis.Author summary: Streptococcus pneumoniae, the Gram-positive diplococci, is the primary etiological agent of bacterial meningitis. In order to cause central nervous system (CNS) infections, SPN has to breach the blood-brain barrier, however, the pneumococcal determinants involved in this process remain unidentified. Here, we demonstrate that pneumolysin, a pore forming toxin secreted by SPN, plays a complex role in governing the fate of the pathogen while trafficking through BBB. Though presumed to be an important virulence attribute, the role of Ply in SPN pathogenesis, particularly in development of meningitis remained debatable. By revealing heterogeneity in Ply expression within isogenic SPN population, our findings attempt to resolve the uncertain role of Ply in SPN pathogenesis. We illustrate that heterogeneity in Ply expression gives rise to stochastically different pneumococcal subpopulations during BBB trafficking. This arises as a consequence of differential interactions of SPN with host microbicidal defense pathways like autophagy and ubiquitination machinery. Among the stochastic SPN population, a low Ply producing SPN subset not only shows improved persistence, but is also capable of successful transcytosis across the BBB resulting in CNS pathogenesis. Overall, our findings suggest that tight spatio-temporal regulation of Ply expression is key to SPN subsistence and dissemination within the host.