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Appl Environ Microbiol. 1975 March; 29(3): 309-312
Copyright © 1975 American Society for Microbiology. All Rights Reserved.

Use of pH Gradient Plates for Increasing the Acid Tolerance of Salmonellae

Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Philadelphia, Pennsylvania 19118

ABSTRACT

Several strains of salmonellae survived higher concentrations of lactic acid after streaking on the surface of pH gradient plates. Most strains increased their acid tolerance by about 0.8 to 1.0 pH unit (9- to 10-fold), with Salmonella madelia showing the greatest differential, pH 5.2 in the wild strain and pH 4.2 after conditioning. The increased acid resistance was quickly lost after transferring to normal tryptic soy agar. Tests for survival in a liquid medium at pH values lower than those giving visible growth indicated that these pH values were bactericidal rather than bacteriostatic for both the wild and acid-conditioned strains.