158 THE ENZYME THEORY OF DISEASE [CH. xi 



temperature of the human body those of cold blooded animals 

 at much lower temperatures. The diastatic ferment of germ 

 barley is most effective at 60 C. a temperature at which 

 most enzymes are destroyed. The phosphorescence sometimes 

 observed in sea- water is produced by the Micrococcus phos- 

 phorescens through the agency of an enzyme the optimum 

 temperature of which is that of the sea. 



The enzymes which are associated with bacteria and bring 

 about the fermentation of carbohydrates show a similar 

 behaviour. We find that a strain of bacteria which will 

 ferment a certain " sugar " at one temperature will not do so 

 at another. For example, Wilson (1910) describes a strain of 

 B. typhosus which at 22 C. would ferment lactose within two 

 days but at 37 C. failed to do so in a month. Coplans (1909) 

 observed certain strains of B. coli which exhibited the reverse 

 phenomenon, fermenting dulcite more readily at 37 C. than 

 at 20 C. 



The property of virulence in pathogenic bacteria is like- 

 wise governed by temperature. Organisms which are virulent 

 when growing at one temperature lose their virulence when 

 grown at another. For example, B. diphtheriae, B. tetani, 

 B. anthracis and many others (vide p. 74) lose their virulence 

 at temperatures much above that of the body. The fact that 

 no bacterial disease in cold blooded animals is communicable 

 to man may possibly be explained on such grounds. 



Furthermore, just as the enzymes which ferment carbo- 

 hydrates are destroyed at temperatures much above 60 C., so 

 we find the property of virulence may be completely removed 

 by subjecting an organism to high temperatures, even though 

 the organism itself survives. The tetanus bacillus is deprived 

 altogether of toxicity by growth at 65 C. for one hour (Muir 

 and Ritchie). 



(d) Exposure to sunlight is another factor which influences 

 both the fermenting power and the virulence of organisms. 



(e) Finally symbiosis is not without influence. Diseased 

 conditions may result from a mixed infection which neither 

 of the organisms concerned is capable of producing alone. It 

 is said that a dog will not succumb to the infection of tetanus 



