328 Mutation in Micro- Organisms 



red, B. typhosus colourless colonies \ In the absence of extraneous 

 colouring matters, both organisms form colourless or whitish colonies. 

 We may begin our description of the mutations observed in the 

 coli-typhosus group with a consideration of the fundamentally im- 

 portant work of Massini (1907)2. From a case of enteritis, this 

 worker obtained an organism which at first grew as whitish colonies 

 (like typhosus) on Endo's medium. After the third day of growth, 

 however, minute red nodules appeared in the whitish colonies — the 

 nodules increasing in number in the course of time until certain 

 colonies contained as many as 200. These nodules were found to 

 be solid masses of bacteria, constituting daughter-colonies within 

 the parent-colony : and, as their colour indicated, they consisted of 

 organisms which possessed the power of fermenting lactose (like coli). 

 One would naturally suppose that this peculiar phenomenon was due 

 to the impurity of the original culture — its two different constituents 

 having simply separated out in the older cultures. By carefully plating 

 out^ the original culture, and by other tests, Massini convinced himself 



^ To understand what follows it is necessary to understand the principles involved 

 in these methods, which are in everyday use for the identification of members of the 

 coli-typhosus group. The principles upon which the Drigalski-Conradi and Endo mediums 

 are compounded are similar. Both consist essentially of an agar medium containing 

 lactose and a colour indicator. The medium of v. Drigalski and Conradi is slightly 

 alkaline and contains litmus as an indicator. B. typhosus does not attack lactose, and 

 therefore grows as a blue colony on the medium. B. coli, on the contrary, splits the 

 lactose with the formation of lactic acid and gas. The acid produced by a growing colony 

 turns the litmus red, the coli colonies thus being distinguished by assuming this colour. 

 Endo's medium contains fuchsin, reduced by Na2S03 to a colourless leuco-compound. 

 B. typhosus accordingly grows on the medium in the form of colourless colonies : whereas 

 B. coli, by splitting the lactose and forming acid, converts the reduced fuchsin back 

 to its characteristic red colour. Colonies of B. coli on this medium are therefore dis- 

 tinguished by possessing the deep red colour with a green shimmer characteristic of 

 fuchsin. It will be understood, therefore, that when in the following account a bacillus 

 is said to form — let us say — red colonies on Drigalski-Conradi agar, what is implied is 

 not that the bacteria themselves are red organisms, but that they possess the power of 

 fermenting lactose, i.e. produce a lactase. 



^ Massini's investigations were carried out in the laboratory of Neisser. According 

 to Pringsheim (1910) similar observations had been previously made upon yeasts by 

 Hartmann, whose records I have not been able to consult {Wochenschr. f. Brauerei, 

 1903). 



' "Plating out" is a method frequently used for testing the purity of cultures. 

 It consists essentially in "diluting" a suspension of organisms with a large volume of 

 culture medium, and then spreading it out on glass plates. The individuals are thus 

 separated as far as possible, and the colonies which develop are known to be derived 

 from single, or at most a few, individuals. By repeating the process a number of times 

 a mixed culture can be detected and pure races isolated. 



