106 BACTERIA AND FERMENTATION 



different species of bacteria are capable of setting up lactic fer- 

 mentation, and what we have now said amply supports that view. 

 Indeed, it has been estimated that upwards of 100 different bacteria 

 possess this property.* 



Bacillus acidi lactici (Hiippe) consists of rods about 2 fj, long and '4 /x, wide, 

 occurring singly or in pairs, chains or threads. Its habitat is sour milk. 



It grows best at blood-heat, but much above that it fails to produce its fermenta- 

 tion. It ceases to grow under 10 C. It inverts milk-sugar and changes it to 

 dextrose, from which it then produces lactic acid. Sugars do, however, differ 

 considerably in the degrees to which they respond to the influence of the lactic 

 ferment, and some which are readily changed by the alcoholic ferment are un- 

 touched by Bacillus acidi lactici. 



Staining reaction Ordinary stains and slightly by Gram's method. 



MotilityNo flagella ; non-motile. 



Spore formation Absent. 



Biology: cultural characters (including biochemical features) Good growth at 

 room temperature and blood-heat. 



Bouillon Diffuse turbidity ; abundant sediment. 



Gelatine plates and tubes Colonies similar to B. coli; small, smooth, round, 

 white. 



Agar plates and tubes Colonies similar to B. coli; small, smooth, white growths, 

 moist and shiny. 



Potato A wavy, smooth-edged growth, elevated ; greyish-white or yellowish- 

 white in colour, sometimes turning brown. 



Milk Solid coagulation, leaving clear fluid ; occasionally some gas-bubbles. 

 Lactic and acetic acids are produced. Powers of acid coagulation of milk are 

 gradually lost after long cultivation upon gelatine or agar. 



Anaerobic or aerobic Grows well aerobically, and if sugar present in medium 

 anaerobically also. 



Non- pathogenic. 



The lactic fermentation bacteria are short rods, do not liquefy 

 gelatine, nor do they form spores. They grow readily on gelatine at 

 room temperature, forming as a rule small circular colonies, white or 

 grey in colour, with sometimes a tinge of yellow, and the surface of 

 the colony is smooth and glistening. The lactic acid organisms pro- 

 duce appreciable amounts of lactic acid only at somewhat elevated 

 temperatures. If the amount of acid rises much above 2 per cent., 

 the growth of the lactic acid bacteria is inhibited. Many other sub- 

 stances, as we have seen, are produced in addition to lactic acid (e.g. 

 acetic and ferric acids, alcohol, methane, C0 2 . H . N"., etc.). Lactic acid 

 organisms (as non-spore bearers) are readily killed by pasteurisation. 



* Delbriick, Zopf, Krause, Peters, Lindner, Weigmann, Storch, and Marpmann, 

 are amongst those who, in addition to workers we have named, have described 

 bacteria possessing the power of setting up lactic fermentation. Only provisional 

 classifications are possible at present, as, owing to variations in biology and 

 terminology, it is probable that certain lactic organisms are described under several 

 different terms. Generally, it may be said that some grow well in the presence of 

 oxygen, and others do not. The latter group, facultative anaerobes, are perhaps 

 the most common. They sour milk best in deep vessels, and produce a right- 

 handed lactic acid. They are widely distributed in nature, and may form 90 per 

 cent, of the total bacteria in milk. Some produce gas, others liquefy gelatine, and 

 yet others produce spores. 



