618 A MANUAL OF BACTERIOLOGY 



Examination of Milk 



Number of organisms per c.c. This is carried out by diluting the 

 milk to 1 in 1000 1 in 1,000,000 with sterile water, or preferably 

 nutrient broth, as a better mixture is obtained. Plates are then 

 made either in gelatin or in distilled water agar (1| grm. powdered 

 agar, distilled water 1 litre, Eastes), or preferably in both media. 



B. coli, B. Welchii, and streptococci. These are searched for 

 quantitatively by the methods detailed for " Water " (pp. 576-586). 

 Amounts of milk in decreasing decimal order from 100 c.c. to 

 0-000001 c.c. should be examined. The B. coli must be differentiated 

 from B. lactis aerogenes and B. acidi lactici (see pp. 389, 381). 



Pathogenic organisms. The detection of these, with the exception 

 of the tubercle bacillus, is difficult and uncertain. In all cases 

 the milk should be centrifuged and the deposit examined. 



1. For the detection of the tubercle bacillus * staining methods 

 are almost useless (except in cases of advanced tuberculosis of the 

 udder or when the milk of a single cow is examined) and inoculation 

 must be performed. At least 250 c.c. of the milk should be centri- 

 fuged at 2000 to 2500 revolutions per minute for an hour. As 

 many organisms become entangled in the cream, it is advisable to 

 stop the machine after half an hour, stir in the cream, and again 

 centrifuge. The fluid is poured or pipetted off carefully, so as 

 not to disturb the sediment, leaving about 3 c.c. in the tube. The 

 sediment and the remaining fluid are then well mixed and about 

 1 c.c. is inoculated subcutaneously and intraperitoneally into 

 two guinea-pigs respectively (see also p. 329). For staining, a 

 process of solution of the milk may be employed, 20 c.c. of the milk 

 being mixed with 1 c.c. of a 50 per cent, potash solution, and heated 

 in a water-bath until the solution turns brownish ; 20 c.c. of acetic 

 acid are then added. The mixture is shaken, heated in a water- 

 bath for three minutes, and centrifuged for ten minutes. The 

 fluid is poured off, 30 c.c. of hot water are added to the sediment, 

 and the mixture is again centrifuged. Films are then prepared 

 from the sediment, and stained for the tubercle bacillus (see also 

 p. 325), the films being always treated with alcohol as well as with 

 acid. 



Non-pathogenic acid-fast bacilli occur in milk (p. 340). 

 2. The diphtheria bacillus is searched for by making serum 

 cultures from, and inoculating guinea-pigs with, the sediment. 



1 See Delepine, Rep. Med. Off. Loc. Gov. Board for 1908-09, p. 134. 



