A Salmonella enterica serovar Typhi plasmid induces rapid and massive apoptosis in infected macrophages

pR_ST98 is a chimeric plasmid isolated from Salmonella enterica serovar Typhi (S. typhi) that mediates the functions of drug resistance and virulence. Previously, we reported that Salmonella plasmid virulence (spv) genes were present in S. typhi. In our current study, we investigated whether plasmid pR_ST98 exhibits significant cytotoxicity in macrophages. pR_ST98 was transferred into the avirulent Salmonella enterica serovar Typhimurium (S. typhimurium) strain RIA to create the transconjugant pR_ST98/RIA. The standard S. typhimurium virulent strain SR-11, which carries a 100-kb virulence plasmid, was used as a positive control. The bacterial strains were incubated with a murine macrophage-like cell line (J774A.1) in vitro. Apoptosis of J774A.1 cells was examined by electron microscopy and flow cytometry after annexin-V/propidium iodide labeling, and the survival of Salmonella strains in J774A.1 cells was determined. Results showed that macrophages infected with strain pR_ST98/RIA displayed greater levels of apoptosis than those infected with RIA and that pR_ST98 may increase bacterial survival in macrophages. Further studies showed that the pR_ST98-induced death of macrophages was associated with the loss of mitochondrial membrane potential and that pR_ST98 may activate caspase-9 and then caspase-3. The research data indicate that the virulence of bacteria that contain the pR_ST98 plasmid is enhanced; the presence of this plasmid increases the survival of the bacterial pathogen and acts through the mitochondrial pathway to mediate macrophage apoptosis.