CONTAMINATION OF MILK 189 



is boiled and filtered in the ordinary way ; 1 per cent of the sugar, 

 alcohol, or glucoside is added to portions of this stock solution to 

 make the different media. The tubes are sterilised in current 

 steam for three successive days. 



For some purposes it is of great value to ascertain the 

 pathogenicity of isolated strains of streptococci. This is con- 

 veniently done by injecting mice subcutaneously or intra- 

 peritoneally. 



The presence or absence of streptococci in milk may 

 also be studied by a careful examination of the centri- 

 fugalised deposit stained by methylene blue. Failure to find 

 streptococcus chains does not mean they are absent, but only 

 suggests they are not present in considerable numbers. The 

 stained deposits from samples of vended milk usually show 

 numerous streptococci, but in those made from fresh byre 

 milk samples they are, as a rule, not to be demonstrated. 



IV. Estimation of the Numbek of B. bnteritidis 

 Sporouenes Spokes 



The milk itself is directly incubated. The usual quantities 

 of milk to examine are 1, 10, and 20 c.c, the smallest amount 

 being added to a tube of freshly sterilised whole milk, while 

 the other quantities are placed in empty sterile test tubes. 

 The milk tubes are heated in a water-bath to 80° C, and kept 

 at that temperature for ten minutes, then cooled and incubated 

 anaerobically at 37° C. The anaerobic cultivation is most 

 conveniently done by absorbing the oxygen with pyrogallic 

 acid and potash contained in a wide-mouthed bottle in which 

 the tubes are placed. 



The amounts given above are too wide apart to yield a 

 satisfactory estimation, and the following method is advocated 

 by the writer : ^ Quite small, narrow (4 by \ inch), sterile 

 empty test tubes are used in batches of ten for each estimation. 

 20 c.c. of milk is employed for each test, 2 c.c. being added 

 by sterile pipette to each tube. The ten tubes are heated for 

 1 minutes at 80° C, rapidly cooled and incubated anaerobically 

 in specimen jars with ground-glass stoppers, just large enough 

 to take the ten tubes, the oxygen being absorbed by the usual 

 potash and pyrogallic acid mixture. The tubes are examined 



1 Report of Medical Officer, Local Government Board, 1909-10, p. 477. 



