GAS PRODUCING LACTIC BACTERIA. 



6 9 



totally absent (100). Hence milk uncontaminated by ex- 

 ternal material contains them in very small numbers or not 

 at all (38). If the milk, however, has been subject to much 

 contamination by filth the number, 

 even at the start, may be considerably 

 greater. 



Although these bacteria are not 

 particularly abundant in milk they are 

 so widely and universally distributed 

 that in the ordinary process of milk- 

 ing they are almost sure to find en- 

 trance into the milk at least in small 

 numbers. In milk which has stood 

 for several hours, as we shall notice 

 in the next chapter, they become very 

 abundant, and in old milk they are by 

 far the most abundant bacteria. 



II. An Aerobic Type of Lactic Bac- 

 teria. Gas Producers. The distinc- 

 tion between this type and the first is 

 that the bacteria grouped here grow 

 best in contact with air. They gro.w 

 luxuriantly upon the surface of cul- 

 ture media and better in shallow than 

 in deep dishes (Fig. 14, B). Their 

 morphology is, however, almost 

 identical with that of the first type. 

 They commonly, though not always, 

 possess the power of producing gas in 

 considerable quantity, and when they 



curdle milk the curd, instead of being showing the method of growth 

 smooth and solid, is apt to be broken in gelatin of the two types of 



< 1 r-n 1 1 -11 lactic bacteria. A is the faculta- 



and more or less filled with gas bub- tive anaerobic typ e and grows 



bleS (Fig. 13, a). They play a part only below the surface. B is the 

 1 . aerobic type growing well on the 



in the ripening of cheese .which is surface. 



