According aspathogens possessing the potential to cause infectious disease.

According to Charles Darwin’s: Origin of Biology, evolution remains an imperative lawof life and it quotes that “It is not the strongest of the species that survives, nor the mostintelligent, but the one that is most adaptable to change”. Every form of life extending from themicroscopic prokaryotes to the macroscopic mammals display complex strategies to develop,forage and evolve for its survival. Among microorganisms, bacteria comprises of diverse domainof microbes. Bacteria inhabit as a commensal in appropriate host models while some act aspathogens possessing the potential to cause infectious disease. Bacterial pathogenesis is theprocess by which bacteria infect and cause disease in a host. Infectious diseases are the leadingcause of morbidity and mortality world-wide.Selection pressure exerted within the host environment including host defensemechanisms, competition among the microflora and medical interventions cumulativelycontribute to the evolution of bacterial pathogens. When the bacterium enters the host, the hostpathogen interaction initiates an attack-defense strategy from both the ends for their respectivesurvival. As the host advances to mount new line of defense tactics to shield the pathogen’sattack, the pathogen in turn strategize novel attack mechanism to overcome the heightened hostdefense mechanism. Thus, arms-races between the host-pathogen interactions create selectionpressure that contributes to shaping up of the bacterial genome and contributes to the emergenceof new virulence factors.Streptococcus pneumoniae, the gram positive bacterium resides as an asymptomaticcolonizer in the nasopharynx of humans. More than 94 types of Streptococcus pneumoniae areknown to exist out of which strains belonging to serotype 1, 4, 7 and 15 are known to inhabit asprimary pathogens unlike its commensal cousins. Strains belonging to different serotypes arefurther classified based on multi locus sequence typing (MLST) technique giving rise to varioussequence type. MLST306 strains belonging to serotype 1 first appeared in 1998 from clinical2nasopharyngeal swabs and proceeded to become the most prevalent sequence type (>80%).Streptococcus pneumoniae produces a host of virulence factors which contributes to itspathogenesis. Pneumolysin (Ply), a cholesterol dependent cytolysin produced by all strains ofStreptococcus pneumoniae bears the ability to create pores in the eukaryotic membrane.MLST306 exhibits heterogeneity in contrast to other serotypes based upon the functional aspectof pneumolysin i.e. it produces a non-pore forming pneumolysin. The pore forming potential ofthe pneumolysin produced by wild type Streptococcus pneumoniae triggers autophagy mediatedkilling of the bacteria by the host. We hypothesize that possession of a non-pore formingpneumolysin variant confers higher persistence rates to MLST306 inside host cells. Thereforethe aim of our study is to investigate the role of non-hemolytic pneumolysin variant in theintracellular life of Streptococcus pneumoniae.