THE COLON GROUP OF BACILLI 135 



into which the colon group may be divided by their 

 use, and the significance of the results obtained will 

 be discussed in Chapter VIII. 



Practical Routine Test for the Colon Group. As has 

 been pointed out above the aggregation of lactose- 

 fermenting bacilli which we call the colon group may 

 be almost indefinitely subdivided by the appUcation 

 of a more or less elaborate series of diagnostic tests. 

 Each observer in the past drew up a scheme of what 

 he believed to be essential tests and called all the 

 bacteria which failed to conform to them " atj-pical." 

 The more of such " atypical " forms a particular 

 worker includes the greater will be the number of positive 

 isolations. The definition of this or any other bacterial 

 species is more or less arbitrary; we consider as true 

 colon bacilli those which fulfil a particular set of tests, 

 and class as pseudo-colon organisms those which do 

 not. If we find, having estabUshed such an arbitrary 

 standard, that the colon bacillus, as determined by it, 

 is found in waters known to be polluted, and not, as 

 a rule, in those known to be free from pollution, the 

 sanitarian can afford to ignore the theoretical question 

 of specific values and make confident use of the practical 

 test. In order that results may rest on a soimd basis 

 of comparable data for various waters, it is of course 

 essential, however, that a standard set of reactions 

 should be agreed upon by sanitary bacteriologists. 



After a considerable period of uncertainty, in which 

 each observer used the procedure which happened to 

 appeal to him, the attainment of comparative results 

 was made possible by the establishment of standard 



