Salmonella Typhimurium outer membrane protein A (OmpA) renders protection against nitrosative stress by promoting SCV stability in murine macrophage
AbstractPorins are highly conserved bacterial outer membrane proteins having β barrel structure and are majorly involved in the selective transport of charged molecules across the membrane. Despite having huge contributions in the pathogenesis of many gram-negative bacteria, their role remains elusive in salmonellosis. In this study, we have characterized the pathogenic role of porins majorly found on the outer membrane of Salmonella Typhimurium (such as OmpA, OmpC, OmpD, and OmpF) paying the utmost importance to OmpA. The outer membrane protein A (OmpA) of Salmonella Typhimurium has shown a multifaceted role in our study. We have observed that deletion of ompA from wildtype Salmonella has made it more prone to phagocytosis and weakly proliferative in macrophages. Whereas, in epithelial cells STM ΔompA was found to be invasion deficient and hyper-proliferative. The poor colocalization of STM ΔompA with LAMP-1 confirmed impaired stability of SCV membrane around the intracellular bacteria, which further resulted in the release of the knockout strain to the cytosol of macrophage where it is bombarded with reactive nitrogen intermediates (RNI). The cytosolic localization of STM ΔompA was found to be responsible for the downregulation of SPI-2 encoded virulent factor SpiC which is required for suppressing the activity of iNOS. The reduced recruitment of nitrotyrosine on wildtype Salmonella staying in the cytosol of macrophage by ectopically expressing Listeriolysin O (LLO) strongly proves the pro-bacterial role of OmpA against host nitrosative stress. The time-dependent increase in 405/ 488 ratio of STM ΔompA pQE60-Grx1-roGFP2 exposed to in vitro acidified nitrite suggested RNI dependent redox burst which further answered the reason for its enhanced sensitivity towards nitrosative stress. Our study further demonstrated loss of integrity and enhanced porosity in the bacterial outer membrane in absence of OmpA. The enhanced porosity of the bacterial outer membrane was further attributed to the upregulated expression of larger porins namely ompC, ompD, and ompF. In comparison with STM ΔompA ΔompC and ΔompA ΔompF, the enhanced uptake of nitrite and greater recruitment of nitrotyrosine on intracellular STM ΔompA ΔompD demonstrate the involvement of OmpC and OmpF in the entry of excess nitrite in ompA deficient bacteria.