188 AGRICULTURAL BACTERIOLOGY 



in milk. No gas is formed by this organism. It produces 

 chiefly lactic acid and small quantities of volatile acids, 

 such as formic and acetic. Other substances of unknown 

 nature are formed in minute amounts. 



The effect of its growth in milk is to produce a jellylike 

 curd that has an agreeable odor and a mild acid taste. 

 When the curdled milk is shaken, the curd becomes very 

 finely divided and the milk assumes a creamy consistency. 

 The milk fermented by this organism forms an appetizing 

 and healthful food. 



The second group is commonly known as B. coli-aerogenes 

 group. The organisms are facultative and non-spore-form- 

 ing. Some produce sufficient acid to curdle milk; others 

 do not. The chief characteristic is the formation of a mix- 

 ture of carbon-dioxide and hydrogen in greater or less 

 abundance. The formation of gas in curdled milk causes 

 an open or spongy curd. The milk in which these organ- 

 isms have grown usually has a disagreeable taste and odor, 

 for which reason they are dreaded by butter- and 

 cheese-makers. 



The organisms of the first group may be said to be de- 

 .sirable, since they protect the milk from the action of 

 other less desirable forms and are of great value in dairy 

 manufacturing. The coli-aerogenes organisms are ' unde- 

 sirable from every point of view. 



If the milk has been produced with due regard to cleanli- 

 ness, the lactic bacteria will generally predominate, but if 

 quantities of manure and dirt find their way into the milk, 

 the colon group of bacteria will gain the ascendency. It 

 is often asserted that milk fermented by the colon group 

 of bacteria is unhealthful. Whether this is true may be 

 questioned, but every consideration, both esthetic and eco- 

 nomic, demands that the milk be produced under conditions 

 that will render contamination as small as possible. 



