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The effects of aw 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 aw-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 aw-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 aw 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 aw in these irradiated systems is largely unknown. Certainly greater thermal resistance at low aw 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 aw 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 aw. 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.

Water limitation techniques can be useful tools to preserve food materials not only from autodecomposition, but also from microbial decomposition. Besides those organisms that might render a food unfit to eat because they can produce adverse compositional and textural changes, food-borne bacterial pathogens are also influenced by the amount of available water. This review considers several major food-borne bacterial pathogens and the effect of water limitation on various aspects of their growth. Wherever possible, references have been included which pertain to the influence of water limitation on these organisms in foods. The data reviewed indicate that food-borne bacterial pathogens in general can grow at water activity levels of 0.83 to 0.999. The implications of this wide aw range for prevention of growth of food pathogens and thus, the safety of foods, are discussed.

Staphylococcus aureus C-243, an enterotoxin B-producing strain, was cultured on media adjusted to various water activity (a,) levels by means of two different solute systems. Total numbers and rate of growth were diminished at low a, levels, and enterotoxin synthesis was extremely sensitive to reduction in aw. A reduction of a, from 0.99 to 0.98 in one medium and from 0.99 to 0.97 in the other medium resulted in extremely low levels of enterotoxin in spent culture media despite the attainment of high numbers of staphylococci.

Previous studies indicated that enterotoxin B production by staphylococci was strongly inhibited by slight reductions in water activity (a,) levels. Similar studies reported herein, employing an enterotoxin A-producing strain, indicated that this organism was capable of producing enterotoxin at a much lower a, level than that required for enterotoxin B production. Staphylococcal growth rates were slowed by decreased a, levels in all media tested; however, final cell counts did not drop below 108/ml in the media with the lowest a. levels.

The effect of water activity (aw) reduction on growth and acid and diacetyl production by three lactic streptococci was studied. In addition, the influence of low moisture conditions on several bacteria of significance in the fermentation of sauerkraut was examined. The minimal aw supporting growth of dairy lactics was 0.93 in a medium adjusted with glycerol. Media adjusted with sucrose generally were more inhibitory than those in which glycerol was the humectant. Titratable acidity, although not related to the type of humectant, did depend on the aw of the medium and was directly related to the extent of growth. Diacetyl concentration increased in cultures of reduced a, when the media were adjusted with both humectants; however, the effect was greatest with glycerol. A lactic strain associated with sauerkraut fermentation appeared to grow at a lower miniimal aw in a glycerol-adjusted medium than in a system adjusted with NaCl; however, none of the sauerkraut organisms grew at a, levels of <0.95 when NaCl was the solute. Acid production appeared to be related to the presence and extent of growth at all of the aw levels studied.

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