72 



H. HALVORSON et Cll. 



dehydrated. Murrell and Scott^ arrived at similar conclusions in 

 considering the heat resistance of bacterial spores at various 

 water activities. These same workers, however, reported that 

 99 % of the volume of the spore was exchangeable with deute- 

 rium-labeled water^. 



The more interesting chemical differences are those which can 

 be correlated with the degree of dormancy. The first of these was 

 demonstrated by Curran et al.^ who found a high level of 

 calcium (5%) in bacterial spores compared to 0.54% in vegeta- 

 tive cells. Sporulation in low calcium medium resulted in heat- 

 sensitive spores. These findings have been confirmed by others'"^^. 

 Slightly higher levels of other bivalent cations, Fq^+, Ni2+, 

 Mn-+, Zn-+ and AP+, are found in spores than in vegetative 



45 60 75 



Minutes at 80° 



Fig. 1. Kinetics of heat inactivation of B. cereiis strain T spores containing 

 various levels of dipicolinic acid (DPA). The numbers refer to the per cent 



dry weight of DPA. 



