184 ELEMENTS OF WATER BACTERIOLOGY 



iso-dulcite mutants, that many paratyphoid and Gartner 

 group bacilli produce raffinose mutants, and that other 

 mutations also occur. The general phenomena are 

 the same in each case. A strain which normally fails 

 to ferment a given carbohydrate is grown upon a solid 

 medium containing that carbohydrate. As the colonies 

 develop there appear upon them raised papillae of a 

 different consistency from the rest of the colony and 

 colored red if litmus be present. Transplants from 

 the papillae give pure cultures of a strain fermenting 

 the carbohydrate in question and forming no papillae. 

 Transplants from the other portions of the colony 

 give the original strain, non-fermenting, but capable 

 of producing fermenting mutants as before. The 

 identity of derivative strain in all other respects has 

 been made clear by exhaustive cultural tests and serum 

 reactions ; and Penf old has shown that the whole proc- 

 ess may be repeated, starting from an isolated single 

 cell. 



The work of MacConkey and Clemesha, which will 

 be discussed shortly, is based on the assumption that 

 a great number of minute subdivisions of the colon 

 group, whether they have arisen by the gradual modify- 

 ing effect of an unfavorable environment, or by muta- 

 tions, or in some other way, are for practical purposes 

 fairly permanent entities which they describe and name 

 as definite species. 



Distribution of Types of the Colon Group in Waters 

 of Various Kinds. The sanitary importance of a study 

 of these minor types within the colon group depends 

 on the assumption that a certain set of characters is 



