A Salmonella enterica conjugative plasmid impairs autophagic flux in infected macrophages.

pR(ST98) was originally isolated from Salmonella enterica serovar typhi and could be transferred among enteric bacilli by conjugation. Our previous studies indicated that it could intervene in autophagy of host cells, while the mechanism remained undefined. Here, we explored how pR(ST98) influenced the autophagic flux of murine macrophage-like cell line (J774A.1). S. enterica serovar typhimurium wild type strain (χ3306), harboring a 100 kb virulence plasmid, was used as a positive control. pR(ST98) was transferred into χ3306 virulence plasmid cured strain (χ3337) to create the transconjugant strain (χ3337/pR(ST98)). The bacterial strains incubated with J774A.1 revealed that survival rate of intracellular bacteria carrying pR(ST98) was higher than that of plasmid free strain; presence of pR(ST98) decreased the number of autophagy vacuoles, LC3 positive and p62 positive bacteria, and also the level of LC3-II and degradation of p62 in macrophages. After intervention with autophagy inhibitor chloroquine, the amount of LC3-II and autophagy vacuoles were still lower in macrophages infected with strains carrying pR(ST98). Our study suggested that pR(ST98) could block or delay the formation of autophagosome in the earlier autophagy process, but couldn't affect the function of autolysosome. This finding provided novel insights into the role of enteric conjugation plasmid in bacterial pathogenesis.