THE KINDS OF MICRO-ORGANISMS IN MILK 373 



lococcus (pyogenes) aureus, Rosenbach, and the micrococci found 

 in milk are either identical with Staphylococcus aureus in cultural 

 and biochemical properties or, at most, environmental modifica- 

 tions. They are Gram-positive, grow well on laboratory media, 

 and frequently produce pigments. The golden-yellow pigment is 

 the common one produced, but lemon-yellow, pink, and reddish- 

 brown pigments are not infrequent. Staphylococcus albus is the 

 variety that produces no pigment. The micrococci grow best at 

 about 35 to 37 C., but some varieties have a lower optimum 

 temperature, some as low as 20 C. They grow also at tempera- 

 tures near the freezing-point of water. 



The staphylococci found in milk produce, as a rule, three 

 enzyms, namely, one which attacks milk-sugar with the produc- 

 tion of acid; one which coagulates the casein; and a proteolytic 

 enzym. The micrococci are, therefore, of importance in the 

 ripening of some cheeses, notably Emmenthaler, Limburger, and 

 Camembert cheese. They produce volatile substances which in 

 part are responsible for the aroma in some cheeses. Gorini's 

 acid-coagulating micrococci belong to this group. 



As a rule the micrococci found in milk are non-pathogenic, but 

 occasionally they may cause an inflammation in the udder. One 

 type of mastitis is caused by the staphylococci. Miss Evans 

 occasionally found staphylococci that were pathogenic for rabbits 

 in the udder. Pathogenicity for man has not been established. 



The staphylococci usually coagulate milk promptly, although 

 they do not produce as much acid as the other groups of lactic 

 acid bacteria. Coagulation is brought about by the combined 

 action of acid and the coagulative enzym. 



SPORE-BEARING BACTERIA IN MILK 



Spore-bearing bacteria occur constantly in milk, and are either 

 aerobic or anaerobic. The aerobic spore-bearers are of the pro- 

 teus and the hay bacillus groups. The presence of these, espe- 

 cially of the latter type, favor the growth of anaerobes in milk, 

 since the pellicle formed at the surface aids in excluding the oxygen 

 of the air. 



The bacilli of the hay bacillus or the potato bacillus type are 

 common on hay, fodder, potatoes, and in manure. Their entrance 

 into milk is, therefore, readily explained. As a rule, they pro- 

 duce a coagulative enyzm, and after coagulation break up the 

 proteins. The milk assumes a brownish or reddish-brown hue 

 and becomes translucent. Studies of anaerobes in milk have been 

 published by Grassberger and Schattenfroh and by Brown. 



The bacteria of this group have come into prominence through 

 the work of Flugge, who isolated several types from pasteurized 



