Commensal to pathogen switch in Streptococcus pneumoniae is governed by a thermosensing master regulator

Opportunistic pathogens switch from a commensal to pathogenic state by sensing and responding to a variety of environmental cues, including temperature fluctuations. Minor temperature oscillations can alert the pathogen to a changing niche ecosystem, necessitating efficient sensing and rapid integration to trigger behavioral change. This is typically achieved through master regulators, which act as umbrella systems dictating pleiotropic phenotypes. Here, we uncover a pivotal role of minor temperature shifts in transition of Streptococcus pneumoniae (SPN) from commensal to virulent lifestyles, mediated via an RNA thermosensing (RNAT) element within the untranslated region of the global regulator CiaR. By positively regulating the expression of the surface adhesin, Phosphorylcholine (PCho), in response to elevated temperature, CiaR potentiates pneumococcal infection. Engineering the RNAT structure to create translation restrictive or permissive versions allowed us to demonstrate how modulation in expression of CiaR could alter pneumococcal invasion capability, influencing infection outcomes. Moreover, intranasal administration of PCho mitigated SPN-induced bacteraemic pneumonia. Since a majority of opportunistic respiratory bacterial pathogens decorate their surface with PCho, this signaling arm could be exploited for anti-infective interventions. Significance statementOur nasopharyngeal area is dominated by opportunistic pathogens, such as Streptococcus pneumoniae (SPN), which typically reside as harmless bystanders. However, in individuals with heightened nasopharyngeal inflammation, resulting from allergy, viral infection or an immature or senescent immune system, these seemingly innocuous microbes, including SPN, switch to virulent lifestyles. We reveal an elegant mechanism for rewiring of SPN virulence genes upon sensing temperature oscillations in the nasopharynx by the master regulator CiaR. Elevated nasopharyngeal temperatures due to pathologic conditions is sensed by thermosensing ciaR mRNA. This primarily promotes SPN surface decoration by PCho which facilitates improved invasion, triggering virulence phenotypes. Our findings point towards adoption of a common mechanism for switch to virulent lifestyles by variety of microbes sharing the specific respiratory niche.