Water Relations of Foodborne Bacterial Pathogens - An Updated Review

The effects of a_w limitation on growth and metabolic activities of foodborne bacterial pathogens continue to be actively investigated in laboratories throughout the world. Perhaps the most intensive work over the past 10 years has centered on growth of Clostridium botulinum in multicomponent systems. This emphasis undoubtedly has been the result of concern about the role played by sodium nitrate in formation of nitrosamines and the possibility of a prohibition of the addition of this preservative to foods. While investigations have continued on C. botulinum and more "traditional" foodborne pathogens, a "new" group of pathogens, some of them opportunistic, has emerged. Several of these organisms are covered in this review whereas others are not for the simple reason that the water requirements of these organisms have not as yet been investigated. Particularly surprising is the lack of a_w-related information on Listeria monocytogenes , Aeromonas hydrophila and colohemmorrhagic Escherichia coli . In fact, the water requirements of gram-negative bacteria in general and the Enterobacteriacae in particular seem to have been somewhat neglected. Researchers intending to do a_w-related research should consider trends in the American diet and in commercial food processors that supply much of it. For example, no one can deny that consumption of fish and seafood products has increased in the diets of many Americans yet potential pathogens indigenous to these products have received little investigative work in terms of their water requirements. Scombroid poisoning, a form of histamine poisoning, may be caused by several species of gram-negative bacteria, yet we know nothing of the effect of a_w on these organisms, their heat resistance, combinations with modified atmospheres, pH, preservatives, etc. Similarly, limited application of gamma irradiation for sterilization of spices has been approved by the FDA, however, the effect of a_w in these irradiated systems is largely unknown. Certainly greater thermal resistance at low a_w levels has been reported; however, it is surprising that investigators have not searched for a similar effect with irradiation. Despite these shortcomings, a sizeable body of literature and knowledge of a_w and its effects on microorganisms has emerged. Some of the research has only begun to exploit the hard-won knowledge of how microorganisms adapt to, and cope with, environments of low a_w. Based on extrapolations of these efforts and our greater awareness of these physiological facts, one can only predict even greater advancements during the next decade.