NATURE OF MUTATIONS 31 



of bacteria acquired the power of fermenting them. In 1897 

 Miss Peckham 1 succeeded in modifying the typhoid bacillus 

 so that it would produce indol, one of the diagnostic features 

 of this micro-organism being that it is a non-producer of indol. 

 In 1906 my late colleague Dr. Klotz, 2 now Professor of Pathology 

 at Pittsburgh, studied the assumption of these sugar-fermenting 

 powers, and the loss of the same, by a member of the typho-coh" 

 group, the B. perturbans. The following year Twort 3 demon- 

 strated that he could cultivate paratyphoid bacilli so that they 

 will ultimately ferment saccharose, typhoid bacilli to ferment 

 lactose and dulcite, dysentery bacilli to ferment saccharose. 

 Interestingly enough these observations date back to the founder 

 of the science of bacteriology, to Pasteur and his classical observa- 

 tions upon fermentation. You will remember how he discovered 

 that ordinary tartaric acid is optically inactive because it is 

 a combination of equal amounts of the two, dextro- and laevo- 

 rotatory, isomers, and that he obtained the laevo-rotatory form 

 in a pure state by the action of moulds, which used up the dextro- 

 rotatory acid leaving the other isomer untouched. But doing 

 this, he noted that if he continued the experiment the rotation 

 to the left reached a certain point and then steadily diminished ; 

 in other words, that after using up the food of election the mould 

 now turned itself to assimilate and use up the laevo-rotatory 

 food. Now, as a general rule, dextro-rotatory organic substances 

 alone are produced and utilized by living animals. 



The steps by which bacteria have been found to gain these 

 powers of adapting themselves to and utilizing unaccustomed 

 ' food-stufis are most instructive, and are best shown by the 

 observations of Massini, 4 Penfold, 5 and other observers of the 

 last few years. These observers employed a solid medium, a 

 peptone broth agar medium (containing, as usually made, a little 

 muscle sugar), upon which the bacteria studied can be grown 

 readily, and to this they added in the process of preparation the 

 particular foreign sugar or glucoside, and made surface growths of 

 the bacteria. Where this is done it is found that the surface 



1 Journal of Experimental Medicine, ii., 1897, 549. 



3 Journal of Infectious Diseases, ii. (Supplement), 1906, 35. 



* Proc. Royal Society, Biol, lxxix., 1907, 329. 



* Arch. f. Hygiene, Ixi., 1907, 250. 



6 Journ. of Hygiene, xii., 1912, 195. See also Mtiller, R., Gentrlb. f. Bakt. 

 xlii., 1908, 157, and ibid. Abt. 1 Orig. lviii., 1911, 97. 



