Aims: To study the effect of acid shock in sporulation on the production of acid-shock proteins, and on the heat resistance and germination characteristics of the spores formed subsequently. Methods and Results: Bacillus subtilis wild-type (SASP-alpha(+)beta(+) ) and mutant (SASP-alpha(-)beta(-) ) cells in 2 x SG medium at 30degreesC were acid-shocked with HCl (pH 4, 4.3, 5 and 6 against a control pH of 6.2) for 30 min, 1 h into sporulation. The D-85-value of B. subtilis wild-type (but not mutant) spores formed from sporulating cells acid-shocked at pH 5 increased from 46.5 min to 78.8 min, and there was also an increase in the resistance of wild-type acid-shocked spores at both 90degreesC and 95degreesC. ALA- or AGFK-initiated germination of pH 5-shocked spores was the same as that of non-acid-shocked spores. Two-dimensional gel electrophoresis showed only one novel acid-shock protein, identified as a vegetative catalase 1 (KatA), which appeared 30 min after acid shock but was lost later in sporulation. Conclusions: Acid shock at pH 5 increased the heat resistance of spores subsequently formed in B. subtilis wild type. The catalase, KatA, was induced by acid shock early in sporulation, but since it was degraded later in sporulation, it appears to act to increase heat resistance by altering spore structure. Significance and Impact of the Study: This is the first proteomic study of acid shock in sporulating B. subtilis cells. The increasing spore heat resistance produced by acid shock may have significance for the heat resistance of spores formed in the food industry.