INDUCTION OF ACID RESISTANCE OF SALMONELLA-TYPHIMURIUM BY EXPOSURE TOSHORT-CHAIN FATTY-ACIDS

Exposure to short-chain fatty acids (SCFA) is one of the stress condit ions Salmonella typhimurium encounters during its life cycle, because SCFA have been widely used as food preservatives and SCFA are also pre sent at high concentrations in the gastrointestinal tracts of host ani mals. The effects of SCFA on the acid resistance of the organism were examined in an attempt to understand the potential role of SCFA in the pathogenesis of S, typhimurium. The percent survival of S, typhimuriu m at pH 3.0 was determined after exposure to SCFA for 1 h at pH 7.0, T he percent acid survival, which varied depending on the SCFA species a nd the concentration used, was 42 after exposure to 100 mM propionate at pH 7.0 under aerobic incubation conditions, while less than 1% coul d survive without exposure. The SCFA-induced acid resistance was marke dly enhanced by anaerobiosis (64%), lowering pH conditions (138% at pH 5.0), or increasing incubation time (165% with 1 h) during exposure t o propionic acid, When protein synthesis during exposure to propionate was blocked by chloramphenicol, the percent acid survival was less th an I, indicating that the protein synthesis induced by exposure to pro pionate is required for the induction of the acid resistance, The perc ent acid survival determined with the isogenic mutant strains defectiv e in acid tolerance response revealed that AtrB protein is necessary f or the full induction of acid resistance by exposure to propionate, wh ile unexpectedly, inactivation of PhoP significantly increased acid re sistance over that of the wild type (P < 0.05), The results suggest th at the virulence of S, typhimurium may be enhanced by increasing acid resistance upon exposure to SCFA during its life cycle and further enh anced by anaerobiosis, low pH, and prolonged exposure time.