QUALITATIVE EXAMINATION 857 



watched and sub-cultures sown when necessary for the identification of a 

 particular colony. 



In the case of beginners the identification of the various micro -organic 

 species involves a very considerable amount of labour : each colony must 

 be separately examined with the naked eye and under the microscope, the 

 morphology of the organism must be studied and its cultural characteristics 

 investigated together with its effect on animals. A little practice however 

 soon confers sufficient knowledge to enable the greater number of the colonies 

 to be recognized quite easily. 



It is not within the province of this book to enter upon a detailed description of 

 the saprophytic micro-organisms likely to be encountered in water, but it may be 

 said that while some of them are absolutely harmless others (e.g. Proteus vulgaris, 

 Micrococcus prodigiosus) elaborate soluble products which may give rise to symptoms 

 of toxaemia in man and the lower animals. These species tend to inhabit decom- 

 posing animal matter so that their isolation from a water to be used for domestic 

 purposes would be an unfavourable indication. The odour again which is given 

 off from the gelatin plates should be noted for it is not unusual to find that bacteria 

 associated with putrefactive processes give rise to disagreeable ammoniacal 

 emanations. 



B. The detection of pathogenic species. 



As has been said above the gelatin-plate method is not, speaking generally, 

 an efficient method for the detection of pathogenic species, and for the isola- 

 tion of such organisms the author has adopted for several years now with 

 considerable success the following technique which he recommends should 

 form an integral part of every water examination. 



Sow 0*5-2 c.c. of the water in tubes of broth or better Metchnikoff's liquid 

 peptone-gelatin medium and incubate the tubes forthwith at 38 C. In 

 some cases no growth whatever will be visible after 24 hours : the experi- 

 ment need not then be pursued further, and the results may be regarded as 

 negative. Much more frequently however a cloudiness will appear in the 

 tubes after incubating for 5-8 hours. This rapidly-appearing turbidity is 

 almost always due either to the colon bacillus or to other species of pathogenic 

 organisms, since saprophytic bacteria grow more slowly at this temperature. 

 When the cloudiness is well-marked within 6-10 hours of sowing transfer a 

 loopful of fluid to a fresh tube of medium and after mixing sow a loopful into 

 a second tube and incubate again. As soon as the second tube becomes 

 cloudy plate a trace of the growth on agar by the parallel stroke method for 

 the purpose of getting single colonies. Incubate the plates in a moist 

 chamber at 37 C. which affords conditions particularly favourable to the 

 rapid multiplication of pathogenic organisms. This method has enabled the 

 author to isolate from different waters Bacillus pyocyaneus, the pneumo- 

 bacillus of Friedlander, the colon bacillus and the various micro-organisms 

 of suppuration. 



The isolation of the colon bacillus or of closely related bacteria from a 

 water is a matter of frequent occurrence and formerly so much importance 

 was attached to the fact that any water in which it was found was con- 

 demned. At the present time however since with more perfected methods 

 the colon bacillus can be isolated from a very large number of waters there 

 is a tendency to go to the opposite extreme, to attach no importance what- 

 ever to its occurrence and to regard it as an harmless saprophyte. In the 

 author's opinion the truth lies between these extremes, for though in some 

 cases the presence of a few colon-like bacteria may be without significance, 

 in other cases it may indicate direct pollution with matter of excretal origin. 

 Further it must not be forgotten that this organism is found in a large 



