DORMANCY OF BACTERIAL ENDOSPORE 



73 



cells, some of which can substitute for Ca-+ in producing heat- 

 resistant forms^^. 



The second correlation followed from the discovery that 

 spores contain massive quantities of a chelating agent, dipi- 

 colinic acid (DPA), which is absent in vegetative cells^-. This 

 unique compound, which is found just prior to or coincidental 

 with heat resistance, has attracted considerable interest. The 

 DPA level varies from 5-15% depending on the species under 

 study. The discovery several years ago that the DPA content 

 could be varied by changes in the growth supplements of the 

 medium!^ provided an opportunity for relating this to heat 

 resistance. The rate of heat inactivation of some of these spores 

 is shown in Fig. 1 . Single heat inactivation curves were observed 

 with considerable variations in heat resistance. The correlation 

 of their dipicolinic acid content with viability and heat resist- 

 ance is shown in Fig. 2. Above 1 % DPA, the spores are viable 

 and their heat resistance is proportional to DPA content. Below 



1 











Viability 



Rate of 

 inactivation 



25 50 75 



fxg DPA/mg dry spores 



100 



50 



en 

 (U 



s_ 

 O 



a 



JD 



D 



> 



Fig. 2. Effect of the dipicolinic acid content on the viability and heat 

 resistance of B. cereus strain T spores. The rate of inactivation was measured 



at 80 \ 



References p. 94 



