Abstract
Salmonella enterica
serovar Typhimurium causes self-limiting gastroenteritis in humans and a typhoid-like disease in mice that serves as a model for typhoid infections in humans. A critical step in
Salmonella
pathogenesis is the invasion of enterocytes and M cells of the small intestine via expression of a type III secretion system, encoded on
Salmonella
pathogenicity island 1 (SPI-1), that secretes effector proteins into host cells, leading to engulfment of the bacteria within large membrane ruffles. The in vitro regulation of invasion genes has been the subject of much scientific investigation. Transcription of the
hilA
gene, which encodes an OmpR/ToxR-type transcriptional activator of downstream invasion genes, is increased during growth under high-osmolarity and low-oxygen conditions, which presumably mimic the environment found within the small intestine. Several negative regulators of invasion gene expression have been identified, including HilE, Hha, and Lon protease. Mutations within the respective genes increase the expression of
hilA
when the bacteria are grown under environmental conditions that are not favorable for
hilA
expression and invasion. In this study, the intracellular expression of invasion genes was examined, after bacterial invasion of HEp-2 epithelial cells, using
Salmonella
strains containing plasmid-encoded short-half-life green fluorescent protein reporters of
hilA
,
hilD
,
hilC
, or
sicA
expression. Interestingly, the expression of SPI-1 genes was down-regulated after invasion, and this was important for the intracellular survival of the bacteria. In addition, the effects of mutations in genes encoding negative regulators of invasion on intracellular
hilA
expression were examined. Our results indicate that Lon protease is important for down-regulation of
hilA
expression and intracellular survival after the invasion of epithelial cells.