Adaptation of Pathogenic Microorganisms to Dry Conditions
Nestlé Nutrition, Avenue Reller 22, Vevey, Switzerland.
Pathogenic microorganisms surviving in dry products are responsible for numerous foodborne disease outbreaks, but a complete understanding of physiological survival mechanisms is still lacking. The response of pathogens to low water activity is very complex and involves immediate physiological actions of the cell as well as a coordinated genetic response. Compatible solutes, such as trehalose, may play an important role in bacterial survival, but, as has been shown for Cronobacter spp., trehalose-negative strains survived dry stress as well as the wild-type strains, suggesting that other mechanisms may play a role. Another contributing factor thought to be associated with resistance to drying is the production of extracellular polysaccharides (EPS); nevertheless, it has also been shown that the role of EPS in drying tolerance is not clear-cut and that additional research is needed to understand its role in resistance against drying. The coordinated genetic response of bacteria to stress involves multiple regulons or specific genes (O’Byrne and Booth, Int J Food Microbiol 74:203–216, 2002; Pichereau et al., Int J Food Microbiol 55:19–25, 2000; Riedel and Lehner, Proteomics 7:1217–1231, 2007). However, to elucidate the exact role of the various genes in bacterial dry stress resistance will likely keep researchers busy for a very long time. In the last decade some sectors within the food industry have responded by significantly improving the hygiene in dry areas of food processing facilities by, for example, introducing strict dry cleaning procedures and better attention to hygienic zoning. A comprehensive approach has been developed and published by microbiologists through the Grocery Manufacturers Association (Scott et al., Food Prot Trends 342–353, 2009a, Scott et al., Food Prot Trends 435–445, 2009b; Chen et al., Food Prot Trends 493–434, 2009).