Plasmid pRST98 is a hybrid resistance-virulence plasmid isolated from Salmonella enterica serovar Typhi (S. typhi). Previous studies demonstrated that pRST98 could enhance the virulence of its host bacteria. However, the mechanism of pRST98-increased bacterial virulence is still not fully elucidated. This study was designed to gain further insight into the roles of pRST98 in host responses.

Human-derived macrophage-like cell line THP-1 was infected with wild-type (ST8), pRST98-deletion (ST8-ΔpRST98), and complemented (ST8-c-pRST98) S. typhi strains. Macrophage autophagy was performed by extracting the membrane-unbound LC3-I protein from cells, followed by flow cytometric detection of the membrane-associated fraction of LC3-II. Intracellular bacterial growth was determined by colony-forming units (cfu) assay. Macrophage cell death was measured by flow cytometry after propidium iodide (PI) staining. Autophagy activator rapamycin (RAPA) was added to the medium 2h before infection to investigate the effect of autophagy on intracellular bacterial growth and macrophage cell death after S. typhi infection.

Plasmid pRST98 suppressed autophagy in infected macrophages and enhanced intracellular bacterial growth and S. typhi-induced macrophage cell death. Pretreatment with RAPA effectively restricted intracellular bacterial growth of ST8 and ST8-c-pRST98, and alleviated ST8 and ST8-c-pRST98-induced macrophage cell death, but had no significant effect on ST8-ΔpRST98.

Plasmid pRST98 enhances intracellular bacterial growth and S. typhi-induced macrophage cell death by suppressing autophagy.