Expression of the rfa, LPS biosynthesis promoter in Salmonella typhimuriumduring invasion of intestinal epithelial cells

Salmonella regulates transcription of many of its genes in response to envi ronmental conditions encountered inside or outside the eukaryotic cells it infects. In this paper, we examined Salmonella typhimurium gene expression within epithelial cells, by using bacterial luciferase as a reporter. We fo cused on gene expression controlled by Salmonella rfa promoter, using lac p romoter as a control. We observed down regulation for both promoters during the initial 2 h of invasion. The decreased levels of luciferase activity a ppeared to be due to metabolic changes, since we observed similar results w ith tissue culture medium alone. Gene expression stabilized to a new steady state for the Salmonella rfa promoter, while a lac promoter activity stead ily decreased. Bacterial luciferase activity was a good indicator of intrac ellular numbers and allowed us to detect as few as 1000 bacterial cells/inf ected monolayer. Both promoters were not dependent on host protein synthesi s for expression.