586 A MANUAL OF BACTERIOLOGY 



in the milk (see p. 428) indicates the probable presence of the 

 organism. To make sure that the change is due to the B. Welchii 

 and not to the C. butyricum, 1 c.c. of the whey per 100 grm. of 

 body-weight should kill a guinea-pig in forty hours when injected 

 subcutaneously. 



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

 index of contamination, but in the writer'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 incubated 

 at 37 C. for twenty -four hours and injected intraperitoneally 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 

 detection of sewage or fsecal pollution ; the bacterioscopic 

 analysis does not give any information as to the suitability 

 of the water for household, trade, or factory purposes. 



Number of colonies on the gelatin plates. The number of 

 colonies represents approximately the number of organisms 

 in the original sample capable of development aerobically 

 at 20 C. in gelatin. This number in a good water rarely 

 exceeds 100 or 150 ; in pure waters, particularly those 

 coming from deep chalk-wells, there may be only a few 

 5 to 10 per c.c. (the results are always expressed in numbers 

 per cubic centimetre of the original water). In waters of 

 poorer quality the number may approach 500 per c.c. 

 Anything over this casts suspicion on the water, and 

 1000 per c.c. or more should probably condemn the sample, 

 always supposing, of course, that multiplication in vitro 

 can be excluded by the proper storage of the sample 



