Characterisation of Salmonella enterica serovar Typhi lifestyle in Human like Macrophages

Abstract

Salmonella enterica is a common foodborne pathogen with diverse serovars exhibiting different host specificities and disease manifestations. The molecular mechanisms of Salmonella pathogenesis have been extensively studied in the broad host range serovar Typhimurium (STm), but less is known about the human-restricted serovar Typhi (STy), responsible for enteric fever. During infection the pathogen can reside either in a specialised acidic vacuole called the Salmonella containing vacuole (SCV) or the host cytosol. Previous studies from the lab have also described the role of cytoplasmic acidification of STm in response to the acidic environment of the SCV. The lowering of internal pH is an essential trigger for downstream processes like the ssrB mediated activation of Salmonella pathogenicity island-2 (SPI-2) genes, essential for intracellular survival and replication of STm. In this study, we aim to describe the intracellular lifestyle and the mechanisms employed by STy using a clinically relevant strain, H58, for successful infection of human macrophages. We show that STy primarily resides within the SCV during macrophagic infections. Using a novel pH detection probe, mCherryTYG, we demonstrate the cytoplasmic acidification of STy in response to the external acidic stress in macrophagic SCV. Further, we validate the acid-dependent expression of SPI-2 genes by ssrB. Finally, using TIMERbac, we show that loss of ssrB leads to high intracellular replication rates, which results in elevated host cell death. Through our observations, we propose a non-canonical role of ssrB for maintaining persistent infection of macrophages by STy, a hallmark of enteric fever.