116 



DISINFECTION 



Effect of Heat on Subsequent Multiplication. — In his studies on disinfection, 

 Koch noticed that spores which had been heated but not quite killed required 

 longer to germinate than unheated spores. Similar observations have been recorded 

 by numerous workers both with spores and with vegetative bacteria. The con- 

 clusion usually drawn is that during the process of heating, the organisms are 

 damaged in some way, so that their ability to multiply when subsequently trans- 

 ferred to suitable conditions is interfered with. Certain figures of Eijkman (1908) 

 lend support to this view ; he heated a suspension of Bad. coli in saline at 52° C. 

 and after varying intervals he made duplicate plates to ascertain the number of 

 organisms remaining alive. One set of plates was counted after 3 days' incu- 

 bation, and the other set after 15 days' incubation. The results are shown 

 in Table i. 



TABLE 4 



Length of heating at 52° C. 



No. of organisms developing after incubation for ; 



It will be noticed that during the first 30 seconds the heat, though 'killing 

 over 50 per cent, of the organisms, does not interfere with the reproduction of 

 the remainder. Subsequently, the longer the organisms are exposed, the greater 

 is the difference between the results of the two series of plates. This suggests 

 that a certain proportion of the remaining viable organisms are injured, and that 

 the longer they are subjected to heat, the greater is the interference with their 

 reproductive power. 



Similar results have been obtained by Allen (1923), who, working with milk, 

 foimd that the generation time of non-sporing organisms which had been pas- 

 teurized was longer than that of the untreated organisms, indicating an attenuation 

 of the pasteurized organisms. 



A different interpretation has, however, been proposed. Eckelmann (1917) 

 suggests that the reason why a certain proportion of heated organisms require a 

 long time to germinate is not because they are suffering from the effects of heat, 

 but because they are provided with a more resistant cell membrane, which, while 

 allowing them to withstand temperatures that prove lethal to their fellows, inter- 

 feres with their rapid reproduction. According to her, heat would act as a selec- 

 tive agency, killing off all the bacteria with thin cell membranes and a power of 

 rapid reproduction, and leaving intact the bacteria with thick, relatively imper- 

 meable cell membranes and a restricted power of reproduction. Burke (1923) 

 adheres to the same view. She found that the spores of CI. hotulinum frequently 

 took several days to germinate, even when placed under optimum conditions. 

 When ttie spores were heated, the germination period was increased, generally 



