BACTERIA IN WATER 153 



in carrying out the necessary manipulations at the spot where the sample 

 is collected. 



The tubes are incubated for forty-eight hours, and it is well to read the 

 results at the end of the first twenty-four hours also. The formation of 

 acid and gas in the tube is usually recognised as "presumptive evidence " 

 of the presence of members of the b. coli group, but it is usual to 

 farther investigate the bacteria giving rise to this change to determine 

 whether they are "typical" or "atypical" b. coli. With this end in 

 view, each bottle in which acid and gas is present is well shaken up, two 

 or three loopfulsare placed on a plate of MacConkey's neutral red bile-salt 

 lactose agar. .These loopfuls are spread over the surface by ; means of a 

 sterile spreader, made by taking a piece of glass rod and turning a portion 

 about 2 inches long at right angles to the shaft. The plates are incubated 

 for twenty-four hours. As typical b. coli produces acid in lactose, any 

 colonies of such an organism are of a rosy red colour. These are then 

 picked off, sloped agar tubes are inoculated and used for the further in- 

 vestigation of the properties of the bacterium isolated. 



The media inoculated should be gelatin, litmus milk, neutral-red 

 lactose bouillon, glucose broth, peptone water, dulcite peptone water, 

 adonite peptone water, mannite peptone water, inuline peptone water, 

 saccharose peptone water, and potato. 



It is well in dealing with the neutral-red lactose agar plates to inoculate 

 a lactose peptone water tube from all the kinds of colonies present, 

 whether these are red or not, as MacConkey rightly points out that some- 

 times an organism which is really a lactose fermenter does not produce 

 a red colour on the solid medium. There is another point to be noted 

 here, namely, that the naked-eye appearances of colonies on lactose agar 

 are not of value in identifying the kind of organism present. 



The object of growing suspicious colonies on a range of media such as 

 that given, is to enable typical b. coli to be recognised when present. At 

 the present time it cannot be said that bacteriologists are in agreement 

 as to what characters determine the type of organism most frequently 

 found in the human intestine — this, of course, being the important point 

 in judging of the contamination of a water supply. The subject will be 

 more fully discussed in the chapter on Typhoid Fever, Here it may be 

 said that for work on water two attitudes are taken up in this country. 

 First, that of Houston, who recognises as typical qualities the following : 

 fluorescence in neutral-red broth, production of acid and gas in lactose 

 peptone water, production of indol, .production of acid and clot in litmus 

 milk (so-called " flaginac " reaction). Secondly, that of the English 

 Committee of 1904, which, on the one hand, laid stress on the additional 

 factor of non-liquefaction of gelatin, and -on the other, attached less 

 importance to the production of indol and the occurrence of fluorescence 

 (see p. 358). 



With regard to saccharose fermentation, different strains of coli of 

 undoubted intestinal origin behave differently towards saccharose, 

 but when saccharose is fermented the occurrence is significant, as 

 indicating a great probability that the organism is intestinal in 

 origin. 



(b) B. enteritidis sporogenes amd streptococci. — As in the case of sewage, 

 the presence of these in a water may be sought for. The methods are 

 those which have already been given (p. 149). 



Much work has been devoted to the question of these faecal streptococci 

 presenting .specific characters by which they could be differentiated 



