The extracellular loops of Salmonella Typhimurium outer membrane protein A (OmpA) maintain the stability of Salmonella containing vacuole (SCV) in murine macrophages and protect the bacteria from autophagy-dependent lysosomal degradation

Abstract

After entering the host cells, Salmonella Typhimurium (STM) stays inside a modified membrane-bound compartment called Salmonella containing vacuole (SCV). The biogenesis and stability of SCV are crucial for the intracellular proliferation of Salmonella. Our research has provided a novel mechanistic view on the role of a bacterial porin OmpA in maintaining the stability of SCV. We found that the deletion of OmpA forces the bacteria to escape from the SCV during the immediate early stage of infection. In the absence of OmpA, the bacteria failed to retain the LAMP-1 and came into the host cell’s cytosol. Subsequently, the cytosolic population of STM ΔompA activated the host autophagy machinery after colocalizing with syntaxin 17 and LC3B. The autophagosomes carrying STM ΔompA were targeted to the lysosomes for degradation. Inhibition of autophagy pathway using bafilomycin A1 restored the intracellular proliferation of STM ΔompA. We further showed that the four extracellular loops of OmpA played a crucial role in holding the LAMP-1 pool around the SCV. We have altered the extracellular loop sequences of Salmonella OmpA by site-directed mutagenesis and observed that the bacteria failed to maintain the LAMP-1 pool around the SCV, which finally resulted in their release into the cytosol of the host macrophages. Surprisingly, the cytosolic population of Salmonella having mutations in the extracellular loops of OmpA didn’t activate the lysosomal degradation pathway like STM ΔompA, which helped them to survive within the murine macrophages. In summary, our study revealed an OmpA dependent novel strategy utilized by Salmonella to combat host autophagy by promoting the stability of SCV.