NATURE OF THE " ESSENTIAL SUBSTANCE " 99 



substance to egg before any growth of Johne's bacillus 

 can be obtained is that Johne's bacillus has lost its 

 power of producing the necessary oxidizing enzyme, 

 such an enzyme being unnecessary in the animal body, 

 where the special acid (or acids) is produced by the 

 ordinary cell metabolism of the host. It is not to be 

 presumed that the acid is formed for the benefit of the 

 bacterium, but that the acid being formed by the host, 

 the bacterium, when in remote ages it commenced to 

 become pathogenic, either found it suitable for its own 

 metabolism, or it mutated, so that it could utilize the 

 special acid present in the host. In either case the 

 bacillus would no longer require an enzyme to pro- 

 duce the acid, and as it became more strictly patho- 

 genic, it would no doubt cease to be capable of forming 

 both the enzyme and the acid. We know that bac- 

 teria do vary, or mutate, in this manner ; for instance, 

 in the typhoid-coli group the most pathogenic mem- 

 bers, compared with the non- pathogenic varieties, 

 have very largely, though not completely, lost their 

 power of producing acids. On the other hand, one 

 of us (F. W. T.) has induced the typhoid bacillus to 

 acquire the power of producing acid from lactose ; 

 this was done by subculturing the bacilli for a long 

 period of time in a poor medium, containing lactose 

 and no other sugar, thus forcing the bacilli either to 

 utilize the lactose, or hunger strike and perish. Now, 

 the special acid in the animal body may not be a waste 

 product of the cell, but may be essential to the host, 

 although it may only be present in the tissues in an 

 exceedingly small quantity. This, if true, might 

 account for the long incubation period in such con- 

 ditions as Johne's disease and leprosy; for the small 

 supply of acid would soon be used up by the bacilli, 

 and there would be no more available until the host 



