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But the real objection to calling a group of bacilli, made 
up of definite and distinct species, by the single term “‘ coli ”’ 
becomes more apparent when an attempt is made to interpret 
the results obtained by these methods. It is accepted that 
as long as a bacillus fulfils all the tests in the previous quota- 
tions it is looked upon as fecal ; but the difficulty arises when 
bacilli are isolated which either do not fulfil all the above 
conditions, or possess additional characteristics. Thus a 
fair number of the so-called ‘‘ coli group ’’ are non-motile ; 
some do not produce indol; others ferment saccharose. 
The difficulty is to estimate what is the significance of these 
aberrant or atypical varieties, and it is this point which 
demonstrates the weakness of the above system. Thus 
authorities interpret these facts differently : Houston says that 
the saccharose fermenting organisms are not to be looked 
upon as so “ significant of undesirable pollution ’’ as the non: 
saccharose fermenters. Savage, on the other hand, says 
that they are equally significant of fecal contamination. 
Here we have two contradictory opinions on a most vital 
point. In discussing this difficulty in estimating the value 
of these aberrant types, Savage says: ‘‘ undoubtedly this 
question of variation and loss of attributes complicates the 
subject and detracts from the value of this indicator ’’ (hex 
‘coli’ as an indicator of fecal contamination). Again, 
‘" There is the great difficulty which attaches to the precise - 
significance given to the bacilli which deviate from the char- 
acters ascribed to the coli communis.’’ Surely, therefore, 
it would be better to separate these organisms, by the use of 
well-defined fermentation reactions into different species, 
and to study their position in nature. By the use of 
MacConkey’s method we can not only easily recognize the 
individual varieties of fecal bacilli in a water, but we have 
shown in the early part of this book that some of these have 
very important natural characteristics which have nothing 
whatever to do with their laboratory reactions, and which 
are of great value to the water analyst. This knowledge could 
not have been acquired without separating species. As long 
as the method of calling all organisms that fulfil a few tests, 
‘* coli,’’ and those that do not fulfil them, or only fulfil them 
