MILK I 33 



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

 this diluted milk to tubes of melted <. gar 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 1000 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 1:20,000. 

 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 2X 20,000 (dilu- 

 tion) = 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, is 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, I have repeatedly 

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

 positive streptococci or streptobacilli. Of the acid-forming bacilli in milk we have 

 i. the B. lactis acidi group. These are oval cells about 0.9 microns by 0.6 microns, 

 often in chains. They are Gram positive and nonmotile. They may be the same 

 as Streptococcus lacticus of Kruse. They curdle milk with a homogeneous clot this 

 being due to the fact that they do not produce gas in lactose media. 2. The B. 

 coli aerogenes group. These are gas producers. (See under water.) 3. The B. 

 bulgaricus group. In connection with the organisms present in the tablets used 

 for treating milk to produce lactic acid for the treatment of intestinal disorders, and 

 considered to be normal lactic acid bacteria, I have found both streptococci and 

 bacilli. These have all agreed, however, in not producing gas in either lactose or 

 glucose fermentation tubes. 



The organism upon which special stress is laid in these so-called lactic acid pro- 

 ducers is the B. bulgaricus. This is a large, nonmotile organism with square ends 

 like anthrax. It often occurs in long chains and does not possess spores. It is Gram 

 positive and often shows metachromatic granules like those of the diphtheria bacillus. 

 Colonies show in forty-eight hours which resemble streptococcus ones, but are 

 more contoured on the surface. It produces a deep vivid pink in litmus milk, while 

 milk streptococci only cause a light pink. It produces a very large amount of acid 

 (3%). Little or no growth on ordinary laboratory media or below O C. (Op. temp. 

 42 C.). 



Heinemann states that it occurs normally in human faeces and various fermented 

 milks also in gastric juice when HC1 is absent. To isolate, put milk or faeces into 

 a broth containing 0.5% acetic acid and 2% glucose. Transfer to litmus milk after 



