4 GENERAL CONSIDERATIONS 



Such a characterisation is a very wide one and will include 

 a large number of organisms which certainly are not all alike, 

 while many of them will not be identical with the B. coli 

 communis of Escherich. On the other hand these characters 

 will separate the group from the Proteus, Gaertner, Dysentery 

 groups, etc. 



All the members of this large group have not an equal 

 significance as indicators of excretal contamination so that it 

 is important to carry the classification further. 



The cultural characters which have been employed for the 

 differentiation of the bacilli of this group are very numerous but 

 in regard to a number of them experience has shown that no 

 differentiation of value is likely to result from their employment 

 and they may be disregarded. Of the differentiation tests still 

 considered by many bacteriologists to be of value the following 

 are the most important : Production of indol, growth in litmus 

 milk, characters of the growth on gelatine slope, production of 

 fluorescence in neutral red media, Vosges and Proskauer's 

 reaction, and the fermentation of various sugars and alcohols. 



The value of these tests can be best gauged not only by their 

 ability to adequately differentiate the group as a whole but also 

 by the light they throw upon the distribution of the different 

 members of the group. If by their use differences of distribution 

 are made manifest, so that the finding of the one variety indicates 

 one kind of contamination while the presence of another variety 

 points to a different source of contamination it is obvious that 

 the tests which enable this to be done are of the utmost value. 



Excreta human and animal is the most important form of 

 pollution for which this group is used as a test and the value of 

 these differentiating reactions must largely depend upon how far 

 they satisfactorily differentiate the strains found in excreta from 

 those which have not recently been derived from the animal 

 intestine. 



The characters of B. coli as found in excreta have been 

 investigated by several workers of whom the following may 

 be mentioned. 



Houston 1 studied the characters of 101 Coli-like organisms 

 1 Local Government Board, Medical Officer's Report^ 1902-3, p. 511. 



