ISOLATION OF B. WELCHII 711 



and non-pathogenic for mice. (See table, p. 279.) All but 

 lactose-fermenters should be neglected. 



BACILLUS WELCHII GROUP. As already stated, it is 

 not essential as a routine procedure to search for the 

 Bacillus Welchii group, though in certain instances it may 

 be of advantage to do so. A negative result in such 

 cases is probably of more value than a positive one. 



For the isolation of B. Welchii, 50-100 c.c. of the water are 

 added to 50-100 c.c. of sterile milk contained in an Erlenmeyer 

 flask of suitable size. Not less than 200 c.c., and preferably 

 500 c.c., of the water should be examined. The flasks are then 

 heated in a water-bath to 80 C. for fifteen to twenty minutes, 

 some sterilised liquid paraffin, oil or melted vaseline is poured on 

 the surface to exclude air, the flasks are cooled in water to 37 C. 

 or thereabouts, and incubated for forty-eight hours at 37 C. The 

 typical change in the milk (see p. 516) indicates the probable 

 presence of the organism. 1 The " filter-brushings " method 

 (No. 1, p. 718) may also be used, but is not so good as the fore- 

 going one. 



The virulence of a peptone-water culture has been suggested as an 

 index of contamination, but in the author's hands has not given 

 reliable results. If sufficient peptone and salt be added to a 

 measured volume of the water to form a 1 per cent, solution of the 

 former and a | per cent, solution of the latter, the mixture incu- 

 bated at 37 C. for twenty-four hours and injected intraperi- 

 toneally into a guinea-pig, a bad water is stated to kill, whereas a 

 good one does not. The amount to be injected is 2 c.c. and death 

 should .ensue within forty-eight hours. 



INTERPRETATION OF RESULTS. The interpretation of 

 the results of the bacterioscopic examination of water is a 

 difficult matter, for which experience is necessary. Just 

 as in chemical analysis, it is not possible to lay down an 

 absolute standard, a knowledge of the source and sur- 

 rounding conditions being of the greatest importance in 

 forming an opinion. The ultimate aim is, of course, the 



1 R. T. Hewlett, Trans. Path. Soc. Lond., vol. Iv, 1904, p. 123. 



