Abstract: Objective: To investigate the effect of repeated citric acid treatment on the acid resistance, cell membrane and membrane protein of Salmonella typhimurium CGMCC 1.1190. Methods: Three acid-resistant strains were obtained after CGMCC 1.1190 was subjected to acid stress in trypticase soy broth (TSB) adjusted to pH 3.0, 2.7 or 2.5 with citric acid, then transferred and incubated repeatedly for 12 times. The D-values, colony morphology, cellular morphology, membrane permeability, membrane fluidity and membrane protein of these acid-resistant strains were determined and compared with each other. Results: The D-values of the acid-resistant strains were increased with increasing number of acid stress treatments. The lower the pH, the larger the D-values and the more significant the differences in the morphology of colonies and individual cells among the three strains. Compared with the original strain, the acid-resistant strains showed a significant decrease in the percentage of dead cells stained by propidium iodide (PI) at pH 2.0 (P < 0.05). It was indicated that acid stress at lower pH led to lower cell membrane permeability to H+ together with an increase in the phase transition temperature (Tm) of cell membrane phospholipids in the acid-resistant strains, suggesting decreased cell membrane fluidity. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated increased expression of cell membrane proteins with molecular mass of 35 to 180 kDa in the acid-resistant strains accompanied by the appearance of new bands with molecular mass of 135 and 180 kDa. Conclusion: Acid stress at lower pH increased the acid resistance of CGMCC 1.1190, made the colonies smaller and the individual cells longer, reduced the cell membrane permeability and fluidity, and increased the expression of some cell membrane proteins and the types of expressed proteins. These changes could be beneficial for the growth of this strain under acid stress, improving its survival.

Key words: Salmonella typhimurium; acid resistance; individual morphology; cell membrane; sodium dodecyl sulfate-polyacrylamide gel electrophoresis