858 THE BACTERIOLOGICAL EXAMINATION OF WATER 



number of samples of typhoid -infected waters and it may altogether mask 

 the presence of the typhoid bacillus. 



Whenever a colon bacillus is isolated from a sample of water its charac- 

 teristics should be carefully studied. If these agree in all details with those 

 of the typical colon bacillus of Escherich (and the author attaches considerable 

 importance to the rapidity with which milk is coagulated) and more particu- 

 larly if the strain isolated prove to be pathogenic to guinea-pigs (using O5-1 

 c.c. of a 24-hour broth culture intra-peritoneally) no hesitation need be felt 

 in expressing an adverse opinion upon the water. The coincident occurrence 

 of bacteria associated with putrefaction renders it still more probable that 

 the pollution is excretal in origin. 



Great importance is to be attached to the inoculation of animals with 

 organisms isolated from water and grown at 37 C. This experiment should 

 never be omitted before coming to a final conclusion. 



Vincent attaches some importance to the numbers in which the colon bacillus is 

 found in a water and in his opinion a water which only contains 10 to 50 colon bacilli 

 per litre may be regarded as good in quality. For the purpose of ascertaining the 

 number present he adopts the dilution method using broth containing 0'70 per 1000 

 of carbolic acid. 



[An opinion upon a water must be based upon a number of data of which 

 the occurrence of the colon bacillus is merely one. The numbers in which 

 the colon bacillus or colon-like bacteria are present is, of course, a matter of 

 importance but its importance depends upon other factors : for these numbers 

 will vary according to the source of the water, and a number which would be 

 sufficient to condemn absolutely a deep well water would be disregarded in 

 an upland surface water. In effect an absolute standard cannot be fixed : 

 a mere laboratory examination of the water without a full knowledge of 

 its source is in the vast majority of cases of little or of no value. The real 

 value of a bacteriological examination will depend upon the consideration of 

 all the facts both as to source, method of collection, mode of transit, and the 

 nature and number of the various micro-organic species present ; and the 

 interpretation of these data is a matter requiring considerable knowledge 

 and experience. Moreover it is now agreed by all who are in a position to 

 express an opinion, that before a reliable conclusion as to the purity of a 

 water can be arrived at the laboratory examination must be systematic 

 extending over a period sufficient to cover all possible or likely sources of 

 contamination.] 



C. Systematic examination for certain pathogenic organisms. 



During an epidemic of enteric fever or cholera or if a number of cases of 

 anthrax occur, the specific organisms should be systematically sought for. 



The pathogenic organisms which have most frequently to be isolated are 

 the colon bacillus, the typhoid bacillus, the pneumobacillus, the anthrax 

 bacillus and the cholera vibrio. The methods to be adopted in each of these 

 cases have already been described in the chapters devoted to a detailed 

 consideration of these organisms. 



3. Houston's method of water examination. 1 



Houston some years ago introduced a method for the bacteriological 

 examination of water which in the United Kingdom and in many parts of 

 the Dominions beyond the Seas has now superseded all others. 

 The sample of water is examined with a view to determining 

 1. The total number of organisms present. 



1 This sub-section has been added. 



