224 AIDS TO BACTERIOLOGY 



and commencing with the lowest dilution (No. 6) and 

 working up to the dilution containing 10 per cent, of milk 

 (No. 1). Tubes showing gas and acid are worked up for 

 typical B. coli, as described under ' Water.' 



Enumeration of B. Welchii.-^Quantities of 100 and 

 10 c.c. of milk are placed in sterile tubes, and into milk- 

 tubes the following quantities are inoculated: 1 c.c., 

 0*1 c.c., 0*01 c.c., and O'OOl c.c. The tubes are then 

 treated as described on p. 87. 



Enumeration of Streptococci. The dilutions used in 

 the examination for B. coli can be plated out on such 

 solid media as Conradi-Drigalski or bile-salt neutral red 

 lactose agar plates (see remarks on bile-salt media for 

 streptococci, p. 233). Streptococci appear on both as 

 small colonies, which should be subcultured and further 

 examined. Or the dilutions of milk may be inoculated 

 into glucose neutral red broth (first recommended by 

 Savage), and the tubes examined for chains, after incuba- 

 ting at 37 C. for twenty-four to forty-eight hours. 



Houston separates the tests into two classes : multiply- 

 ing factors (B. coli and streptococcus tests, and enumera- 

 tion of organisms), and non-multiplying factors (B. Welchii 

 [Enteritidis sporogenes] and readings of volume of sedi- 

 ment). 



The Toronto Academy of Medicine has fixed the fol- 

 lowing standard : Milk shall not contain during the months 

 of June, July, August, and September, more than 10,000 

 bacteria per cubic centimetre, as shown by a forty-eight- 

 hour culture on nutrient agar medium at 37 C.; nor in 

 the remaining months of the year more than 5,000 bacteria 

 per cubic centimetre, as demonstrated by the same test. 



CHAPTER XXI 

 THE BACTERIOLOGY OF WATER 



CERTAIN organisms are so frequently found in water that 

 they may be regarded as normal inhabitants thereof, 

 others are recognised as soil bacteria that have been 

 washed in, while the third group (bacterial indicators of 

 pollution) comprises organisms of intestinal origin. The 

 bacterial alvine flora of animals and birds resembles that 



