Pneumolysin (PLS or PLY) or Intermedilysin (ILY), the shortest members of the CDC family (Gonzalez et al., 2008). It exhibits a broad range of conductances (El-Rachkidy et al., 2008) and localizes to the cell wall of S. pneumoniae (Price and Camilli, 2009). Binding of ILY to human CD59 initiates a series of conformational changes within the toxin that result in the conversion of the soluble monomer into an oligomeric membrane-embedded pore complex. The assembly intermediates increase the sensitivity of the host cell to lysis by its complement membrane attack complex, apparently by blocking the hCD59-binding site for complement proteins C8α and C9 (LaChapelle et al., 2009).  The herbal bioflavonoid, Apigenin, inhibits oligomerization of PLY and protects against pneumonia (Song et al. 2016).  Pneumolysin alters lysosomal integrity in epithelial cells, but not in macrophages, inducing lysosomal membrane permeabilization and release of lysosomal content (Malet et al. 2016). A four-step mechanism of membrane attachment and pore formation has been proosed (van Pee et al. 2016). Pneumolysin is both necessary and sufficient to promote inflammation, increasing shedding and causing transmission to others (Zafar et al. 2017). The release of pneumococcal autolysin, which promotes cell lysis and the release of pneumolysin, is inhibited by treatment with azithromycin and erythromycin, but recombinant autolysin restores the release of pneumolysin (Domon et al. 2018). Pneumolyin exhibits direct cardiotoxic and immunosuppressive activities, as well as indirect pro-inflammatory/pro-thrombotic activities (Anderson et al. 2018). The transmembrane beta-hairpins of the PLY pore are stable only for oligomers, forming a curtain-like membrane-spanning beta-sheet, and its hydrophilic inner face draws water into the protein-lipid interface, forcing lipids to recede (Vögele et al. 2019). Formation of pre-pore complexes of pneumolysin is accompanied by a decrease in short-range order of lipid molecules throughout vesicle bilayers (Faraj et al. 2020). Although pneumolysin-induced inflammation drives person-to-person transmission from the nasopharynx, the primary reservoir for pneumococcus, it also contributes to high mortality rates, creating a bottleneck that hampers widespread bacterial dissemination, thus acting as a double-edged sword (Badgujar et al. 2020). Serotype 1 ST306, a widespread pneumococcal clone, harbours a non-hemolytic variant of pneumolysin (Ply-NH). Crystal structural analyses of Ply-NH led to the identification of Y150H and T172I as key substitutions responsible for loss of its pore-forming activity. A novel inter-molecular cation-pi interaction governs formation of the transmembrane beta-hairpins (TMH) in the pore state of Ply, which can be applied to other CDCs. H150 in Ply-NH disrupts this interaction, while I172 provides structural rigidity to domain-3 through hydrophobic interactions, inhibiting TMH formation. Loss of pore-forming activity enables improved cellular invasion and autophagy evasion, promoting an atypical intracellular lifestyle for pneumococcus, a finding that was corroborated in in vivo infection models. Attenuation of inflammatory responses and tissue damage promoted tolerance of Ply-NH-expressing pneumococcus in the lower respiratory tract. Adoption of this altered lifestyle may be necessary for ST306 due to its limited nasopharyngeal carriage with Ply-NH, aided partly by loss of its pore forming ability, facilitating a benign association of SPN in an alternative, intracellular host niche (Badgujar et al. 2020). Apigenin protects mice from pneumococcal pneumonia by inhibiting the cytolytic activity of pneumolysin (Song et al. 2016). PLY can disrupt plasma membrane integrity, deregulating cellular homeostasis. At lytic concentrations, PLY causes cell death, but at sub-lytic concentrations, PLY triggers host cell survival pathways that cooperate to reseal the damaged plasma membrane and restore cell homeostasis (Pereira et al. 2022). While PLY is generally considered a pivotal factor promoting S. pneumoniae colonization and survival, it is also a powerful trigger of the innate and adaptive host immune response against bacterial infection. The dichotomy of PLY as both a key bacterial virulence factor and a trigger for host immune modulation allows the toxin to display both "Yin" and "Yang" properties during infection, promoting disease by membrane perforation and activating inflammatory pathways, while also mitigating damage by triggering host cell repair and initiating anti-inflammatory responses. Due to its cytolytic activity and diverse immunomodulatory properties, PLY is integral to every stage of S. pneumoniae pathogenesis and may tip the balance towards either the pathogen or the host depending on the context of infection (Pereira et al. 2022).