CHAPTER XII. 
THE ‘“‘ COLI ’’ METHOD COMPARED WITH THE SEPARATE SPECIES 
METHOD. 
From what has gone before it will be apparent that, 
rightly or wrongly, we appear to make use of very much 
more information in the judging of a sample of water than 
is customary amongst analysts at home. It would be well 
therefore to make a few remarks on what we may call the 
““coli’’ method of judging water. 
The term ‘coli communis” is, as MacConkey says, not 
a happy one because’ no two bacteriologists use the term in 
the same sense. Four interpretations of this term are given 
ete, 
(1) The report of the English Committee on the standard- 
ization of methods in the examination of water defines the 
characters of coli communis as follows :—A small motile non- 
sporing bacillus growing at 37°C. as well as at room tempera- 
ture. The motility is well observed in young cultures in a fluid 
glucose medium. It is decolourized by Gram’s method of 
staining. It never liquefies gelatine, and the cultures should 
be kept for at least 10 days in order to exclude a liquefying 
bacillus. It forms smooth thin surface growths and colonies 
on gelatine—not corrugated—growing well to the bottom of the 
stab (facultative anerobe). It produces permanent acidity 
Pe milk=which i curdles withii 7 days at 97° ©. . It 
ferments glucose and lactose with the production of both 
acid and gas. The typical coli bacillus must conform to the 
above description and tests. It generally also forms indol 
(best obtained in peptone water cultures), gives a big yellowish 
brown growth on potato (greatly depends on the character 
of the potato), sometimes ferments saccharose (about 50 per 
cent.), changes neutral red, reduces nitrates and half the gas 
produced by it from glucose is absorbable by caustic potash. 
(2) The American Committee on standard methods 
states, that the test for the presence of this bacillus in water 
