184 MILK AND THE PUBLIC HEALTH CHAP. 



In a later paper l further discussing these tests, MacConkey 

 arrived at the opinion that (in view of the better tests for that 

 substance now available) the indol test should be retained. 

 He also modified his opinion as to the value of Voges and 

 Proskauer's reaction. In addition, he introduced three further 

 tests, i.e. the fermentation of inulin, inosit, and mannite. 

 MacConkey states : 



The method of procedure suggested is that a sloped agar tube 

 should be inoculated from a single colony on a plate, the growth 

 being rubbed all over the surface of the medium and in the water 

 of condensation. After 4 to 6 hours' growth at 37 C. a drop of 

 the condensation water can be examined to ascertain the presence 

 or absence of motility. After 24 hours' incubation at 3T C. a 

 good loopful of the growth is put into tubes of gelatine, lactose, 

 saccharose, dulcit, adonit, and inulin. The agar tube is returned to 

 the incubator, together with the rest of the tubes, and is used later 

 for the indol test. An inosit tube and a glucose tube (for Yoges 

 and Proskauer's reaction) may be inoculated at the same time as 

 the others, or these two may be used as confirmatory tests. Voges 

 and Proskauer's reaction may be tested for at the end of 4 days. 

 The other tubes should be kept under observation as long as there 

 is no change in the reaction of the medium. 



As regards the value of these additional fermentation 

 tests, it may be said at once that for research purposes the 

 greater the number of cultural differentiation tests used the 

 more valuable the information obtained in regard to the dis- 

 tribution of certain strains and their ultimate significance. 

 For routine work the increased labour necessitated by the use 

 of a large series of differentiating tests is so considerable that 

 the number of such tests has to be restricted, and only those 

 of proved utility, from the immediate practical point of view, 

 can be adopted. 



The practical value of these additional tests turns upon 

 the light they shed as to the origin of the organisms to 

 which they are applied. For example, the dulcite test is fully 

 justified if it can be shown that a dulcite fermenting organism 

 is of greater or lesser significance, and indicates a different 

 distribution than an identical organism which does not ferment 

 that alcohol. Have the workers who advocate the use of 

 these extra tests been able to demonstrate such a proposition ? 



1 Journal of Hygiene, 1909, ix. p. 86. 



