46 RUSSELL J. BEERS 



tion capacity. Less information is available regarding vegetative growth, 

 but for the species for which this has been determined, higher moisture activi- 

 ties are required than for germination of their spores. 



Bacteria are known to be considerably more exacting in their moisture 

 requirements for growth than the majority of fungi. About 40% moisture 

 has been considered necessary for growth on meats (Fabian, 1951), corre- 

 sponding to about 95% moisture activity, and this latter figure is often quoted 

 as that below which bacteria in general will not grow. In a few growth 

 studies on individual species (Scott, 1953; Christian and Scott, 1953) values 

 in the neighborhood of 95%-97% have been found. 



Moisture requirements for the germination of bacterial endospores have 

 been almost entirely neglected. Bullock and Tallentire (1952) prepared 

 mixtures of spores of Bacillus suhtilis with peptone, lactose and kaolin, and 

 exposed samples of these mixtures to atmospheres of 100% relative humidity. 

 Samples were withdrawn at intervals for plate counts and determinations of 

 moisture uptake. As moisture uptake in the peptone powders proceeded, a 

 point was reached where the spores lost heat resistance but remained viable. 

 At a second point some of the spores germinated and died ( both spore and 

 total counts dropped), while at a third spores germinated and multiplied. 

 These findings would indicate that spores of bacteria, like those of fungi, 

 require higher moisture activities for growth than for germination. In kaolin 

 only the first of these phenomena occurred, while in lactose the second took 

 place but almost simultaneously with the first. The percentages of moisture 

 uptake for the first change are given and they are far apart, as would be 

 expected from the radically different hygroscopicities of the materials; these 

 workers chose well from the standpoint of a wide range of moisture affinities 

 and nutrients, but they did not determine moisture activities, so their data 

 do not help us in the determination of critical moisture levels for this 

 organism. Their main conclusion was that the range between the second and 

 third points might be put to use in ridding such products as pharmaceutical 

 powders, in which they were interested, of at least some of their spores. 



Waldhalm and Halvorson (1954) found that spores of Bacillus cereus var. 

 ierminalis, when dried and rehydrated by storing in an atmosphere of 100% 

 relative humidity until equilibrium was reached, possessed a higher moisture 

 activity than vegetative cells of the same organism similarly treated. This 

 indicated that the spores had less affinity for water than the cells, in oppo- 

 sition to the theory that spores contain an abnormal amount of "bound 

 water." In an extension of this work Halvorson and Waldhalm (1953) 

 reported preliminary experiments in which they mixed spores with dried 

 wheat flour and exposed the mixtures to moist atmospheres so that different 

 moisture activities were created. The mixtures were incubated and sampled 



