114
At the end of 24 or 48 hours these plates were examined for Lactose fermenting colonies and if present a suitable number (4 to 8) were picked off and grown on Agar slopes and from these Agar slopes the detailed examination was made.
90.
In putting up the samples Gelatine plates were not poured after the first week or It was found so difficult to maintain an incubator at the low temperature necessary for incubation of these plates that the method was abandoned. In a Tropical water the Gelatine total count does not appear to me to be of much importance and nothing is lost by doing without it.
The picking off of the colonies of the Lactose Agar plates is an extremely uncertain matter, and in view of the idea held by some that B. coli communis colonies can easily be identified when growing on MacConkey's Agar medium, it will perhaps be well to note that MacConkey (1908) says in his paper on the Bile Salt media which bears his name: "I have repeatedly tried to distinguish one organism from another by the appear- ance of the colonies on Bile Salt Neutral Red Lactose Agar but invariably without success. The form of a colony on Bile Salt Agar is of no value in identifying a Lactose fermenting organism My experience in working out the characters of over a thousand Lactose fermenters isolated from this media entirely agrees with this statement.
A selection has therefore to be made in some way or another except for the smallest quantities of water ( cc, 1 cc) when the whole can be plated. This selection naturally introduces an element of uncertainty but extended over a long period it is more or less equalised.
Incubation, etc.
Throughout all cultures, with the exception of Gelatine, were incubated at 37° C. There appears to be no sufficient reason for raising the temperature to 42° C.
THE COLI GROUP.
This group will be briefly dealt with under the following headings :-
(1) The tests used by various authorities for the group.
(2) The tests used in this investigation.
(3) The value of the Coli group as an indicator of pollution in water,
It is impossible to give more than the briefest notes on the above points but without some notes on the subject it is impossible to interpret the results.
The terms "B. coli" or "the Coli group" have been used in such various ways that it is necessary to state definitely on what tests the diagnosis of Coli is made.
If the tests are few, a very wide group with many organisms of very doubtful importance will be included. On the other hand it is impossible to restrict attention to B. coli communis (Escherich) alone as it is not the only excretal organism.
It is also evident that the more tests which are employed the fewer samples can be worked out. A middle course of some kind must therefore be taken and where the line is to be drawn varies with the worker.
1. The tests used by various Authorities for the Coli group.
(a.) Gage (1903) used the following tests for the Coli group :-Agar streak-gas in Dextrose Bouillion--coagulation of milk-Reduction of Nitrates Produc- tion of indol and non-liquefaction of gelatine in 14 days.
(b.) Boyce-Grünbaun-MacConkey and Hill (1902) defined the group as :-non sporing-non liquefying-giving; acid and gas on Glucose and Lactose and may do so on Saccharose-acid in milk and usually clot also-grow well at 42° C. and give acid and gas ou Glucose Bile Salt Broth.