156 BACTERIOLOGY OF WATER, AIR, MILK, ETC. 



however, one point of importance in connection with the quantitative 

 estimation of bacteria in milk, and that is the fact that in order to keep 

 the development of the bacteria within the limits of 10,000 to 50,000 

 per c.c., it is necessary that the requirements of cleanliness in milking 

 and the rapid cooling of the milk after obtaining it and the keeping of 

 the temperature below 5oC. be rigidly observed. If a milk has a 

 high count it shows some error in the handling of the milk. Anderson 

 has found that top milk contains from 10 to 500 times as many bacteria 

 as bottom milk. Centrifugally raised cream contains more bacteria 

 than that forming by gravity. In making a quantitative bacteriolog- 

 ical examination, the principle is the same as with water. 



Make a known dilution of the milk with sterile water; add definite quantities of 

 this diluted milk to tubes of melted agar or gelatin, and pour into plates. The 

 diluted milk may also be delivered in the center of the plate and the melted agar or 

 gelatin poured directly on it, mixing thoroughly. Always shake the bottle well 

 before taking sample. 



Example. Added i c.c. of milk to 199 c.c. of sterile water in a large flask (500 to 

 looo c.c.). After shaking thoroughly, take i c.c. of this i : 200 dilution and add it to 

 99 c.c. of sterile water. Shaking thoroughly, we have a dilution of i : 20,000. Of this 

 we added 0.5 c.c. to a tube of gelatin or agar. After incubation the plate showed 

 75 colonies. Therefore the milk continued in each c.c. 75 X 2 X 20,000 (dilution) 

 = 3,000,000 the number of bacteria in each c.c. of milk. 



Lactose litmus gelatin or agar is to be preferred in milk-work, as the normal lactic 

 acid bacteria produce reddish colonies which are very striking. A standard easily 

 attained for high-grade, certified milk would be 5000 to 10,000 per c.c. 



In the qualitative examination of milk, many dairies employ the fermentation 

 tube, any organism producing gas being considered undesirable. Again liquefying 

 organisms, as shown by the presence of such bacteria in the gelatin plates, are evidence 

 of probable contamination by faecal bacteria. A question which seems difficult to 

 decide is as to the general nature of the so-called normal lactic acid bacteria of milk. 

 Some describe them as very short, broad bacilli with very small colonies, fermenting 

 lactose with the formation of lactic acid. Others consider that the streptococci are 

 the organisms which are concerned with the normal fermentative changes. In 

 examining specimens of milk considered the best on the market, 1 have repeatedly 

 found the small red colonies on lactose litmus agar to be in chains of either Gram- 

 positive streptococci or streptobacilli. 



Acid Producing Organisms. Shippen considers the chief organism 

 concerned in the souring of milk as B. giintherii, but notes that it is 

 the same organism as S. lacticus. All authors note the difficulty of 

 deciding whether the morphology is coccal or bacillary. McGuire 

 found these organisms almost constantly present in the dung of cows. 

 The organisms he obtained from cow dung were chiefly members of 

 the coli-aerogenes group and S. lacticus. 



