Cortex modulation shapes UV-C and heat resistance properties of Bacillus cereus endospores

UV-C and heat are widely used for hygienic design and decontamination in food processing, yet bacterial endospores of foodborne pathogens can adapt in ways that undermine process efficacy. As such, it was recently shown that Bacillus cereus ATCC 14579 can readily acquire enhanced spore UV-C resistance through phase variation in its pdaA gene, which encodes an N-acetylmuramic acid deacetylase involved in spore cortex formation. Specifically, disruption of the pdaA open reading frame (causing a PdaA^OFF state) results in spores with strongly increased UV-C resistance but reduced heat resistance and viable spore yield. In this study, we show that recurring heat stress on pdaA^- spores of ATCC 14579 readily selects for attenuated variants of the cortex-shaping CwlD muramoyl-L-alanine amidase that are able to restore spore viability and heat resistance to wildtype (i.e. PdaA^ON) levels, while still retaining slightly improved UV-C resistance. As such, our findings link structure-function changes in the cortex to survival of B. cereus under processing stresses.