A low-pH medium in vitro or the environment within a macrophage decreases the transcriptional levels of fimA , fimZ and lrp in Salmonella enterica serovar Typhimurium

Many Salmonella Typhimurium isolates produce type 1 fimbriae and exhibit fimbrial phase variation in vitro . Static broth culture favours the production of fimbriae, while solid agar medium inhibits the generation of these appendages. Little information is available regarding whether S . Typhimurium continues to produce type 1 fimbriae during in vivo growth. We used a type 1 fimbrial phase-variable strain S . Typhimurium LB5010 and its derivatives to infect RAW 264.7 macrophages. Following entry into macrophages, S . Typhimurium LB5010 gradually decreased the transcript levels of fimbrial subunit gene fimA , positive regulatory gene fimZ , and global regulatory gene lrp . A similar decrease in transcript levels was detected by RT-PCR when the pH of static broth medium was shifted from pH 7 to a more acidic pH 4. A fimA -deleted strain continued to multiply within macrophages as did the parental strain. An lrp deletion strain was unimpaired for in vitro growth at pH 7 or pH 4, while a strain harboring an lrp -containing plasmid exhibited impaired in vitro growth at pH 4. We propose that acidic medium, which resembles one aspect of the intracellular environment in a macrophage, inhibits type 1 fimbrial production by down-regulation of the expression of lrp, fimZ and fimA .