THE KINDS OF MICRO-ORGANISMS IN MILK 351 



represented and start growth on equal terms. In exceptionally 

 clean milk they are sometimes very scarce, so that it may require 

 considerable quantities of milk to demonstrate their presence. 

 However, even the cleanest market milk will, as a rule, turn sour 

 on standing and, therefore, the presence of the two groups is 

 evidenced. Media containing dextrose favor the growth of Bacil- 

 lus coli and streptococci, whose detection is, therefore, facilitated 

 by the use of carbohydrate media. 



When milk sours the Bacillus coli group multiplies and forms 

 acids from the milk-sugar. When about 0.4 to 0.5 per cent, 

 acid has been formed the coli-aerogenes group begins to drop out 

 of competition, while the streptococcus group continues to flourish 

 until about 1 per cent, acid has been produced. At this stage 

 the limit of the "normal" souring process has been practically 

 reached. After this there is a gradual return to neutral and then 

 alkaline reaction in the milk (compare page 266). 



Temperature is of considerable influence in bringing about 

 the souring process, and according to the temperature the result 

 varies, as has been previously explained. At temperatures above 

 30 C. the Bacillus coli group is favored, and the curd is not of 

 the same desirable character as the curd that is produced at 

 lower temperature, when the streptococcus group overgrows the 

 Bacillus coli group more readily. 



Bacillus aerogenes belongs to Group 1 of Lohnis' classification, 

 and has the following general characteristics: It is Gram-negative 

 and frequently capsulated; it grows well on ordinary media and 

 coagulates milk promptly; it does not liquefy gelatin and does 

 not dissolve the casein in milk; it produces gas from dextrose, 

 lactose and saccharose, and some other carbohydrates; acid is 

 also produced from carbohydrates; the acid is composed of lactic 

 acid and volatile acids; the bacilli are arranged in irregular groups 

 and colonies are fairly large; they grow preferably under aerobic 

 conditions, which fact accounts for their presence chiefly in sur- 

 face layers of milk. 



Streptococcus lacticus belongs to the second group of Lohnis' 

 classification and has the following characteristics: It is an oval, 

 round, or flat coccus, inclined to anaerobiosis and chain formation; 

 the chains are usually short, the diplococcus form being most 

 prevalent, although in old milk cultures longer chains frequently 

 occur; it grows scantily on ordinary media, better on media con- 

 taining carbohydrates; on agar slants the surface growth is thin, 

 veil-like, or composed of drops; it is Gram-positive, coagulates 

 milk promptly, the coagulum being smooth and compact, with 

 separation of little or no whey; it does not liquefy gelatin or dis- 

 solve casein, as a rule, although rare forms have been reported 



