22 ISOLATION OF INDICATOR ORGANISMS 



discussed under B. coli, while in addition there is possibility of 

 error in that the streptococci may fail to grow or be overgrown 

 or that they may fail to be detected when present. 



The last is of some importance and can only be diminished 

 by careful examination in hanging drop preparation, followed in 

 doubtful cases by examination of the stained centrifugalised 

 deposit. 



An alternative method, as for B. coli enumerations, is to use 

 solid media over which definite fractions of the substance under 

 examination are brushed and then incubated. All colonies 

 likely to be those of streptococci are subcultivated and carefully 

 investigated. Several media have been used as suitable for this 

 purpose, the most useful being ordinary agar and the medium 

 of Drigalski and Conrad i. 



The enumeration of streptococci by this means is not very 

 satisfactory. Only small quantities (o - i to ro c.c.) can be used for 

 plating since concentration methods are time consuming and 

 not satisfactory. Also none of the media employed are capable 

 of differentiating in any satisfactory way the probable strepto- 

 coccus colonies so that in practice a very large number of 

 colonies have to be subcultivated and investigated, very few of 

 which are streptococci. This makes the procedure very tedious. 

 It is also probable that not all streptococci will grow on such 

 plates, particularly when the other colonies are numerous. When 

 it is a question of studying the biological properties of the 

 isolated streptococci this method must be adopted. 



The determining tests to apply to isolated streptococci have 

 already been discussed in Chapter I and are further referred to 

 in the subsequent chapters. 



Bacillus enteritidis sporogenes. 



The estimation is based, not upon the number of these 

 bacilli present, but upon the number of their spores. The 

 cultural feature which is used as pointing to the presumptive 

 presence of this organism is the characteristic change in milk 

 described in Chapter I. 



Various amounts of the substances under examination or, 

 if concentration of the bacteria has been effected, definite 



