I02 Elements of Water Bacteriology. 



It is, of course, not always certain that organisms resem- 

 bling B. coli, but failing, for example, to reduce nitrates or 

 to form indol, have been derived from typical colon bacilli 

 by any recent process of modification. Organisms of 

 this sort may be found which for generations breed .true 

 to their characteristics and are apparently definitely dif- 

 ferent in one property or another from the true B. coli. 



The more of such atypical forms which are included 

 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 fulfill a particular set of tests, and class as pseudo- 

 colon organisms those which do not. If we find, having 

 estabhshed 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 ques^on of specific values and make confident 

 use of the practical test. In order that results may rest 

 on a sound basis of comparable data for various waters, 

 it is of course however essential 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 has been made 

 possible by the establishment of standard methods of 

 procedure by bodies of authoritative position, both in 

 England and America. In 1904 an English Committee, 



