66 THE TYPES OF MILK BACTERIA. 



bacteria (Fig. 12). The types described as cocci, bacilli and 

 bacteria are, therefore, probably the same. Since none of them 

 grows readily in the presence of oxygen they grow better 

 under the surface than on the surface of culture media. 

 They sour milk more readily in deep than in shallow dishes, 

 the former condition furnishing them with less oxygen than 

 the latter. They also fail to grow readily in most artificial 

 culture media used for cultivating bacteria in the laboratory. 

 None of them produces gas and they all curdle the milk into 

 a solid, hard, acid curd, .without bubbles and with very little, 

 if any, odor (Fig. 13, &). The acid they produce is primary 

 right-handed lactic acid, and the amount which they develop 

 varies with the temperature. Most of them grow best at a 

 temperature between 70 and 98 F. The amount of acid 

 which they may develop at 98 F. is represented at about .88 

 per cent, to .90 per cent, (see p. for explanation of these 

 numbers). At a temperature of 83 they produce i.oi per 

 cent, in about 48 hours ; at a temperature of 65 they produce 

 i.io per cent., requiring six days for the purpose (56, 57). 

 It is impossible to say how many species belong to this 

 type. Members of the group have been described by many 

 bacteriologists and have been given a variety of names. The 

 following names have been used : Bacillus acidi lactici I. 

 and II. (Esten), Streptococcus acidi lactici (Grotenfelt), 

 Bacillus acidi lactici (Gunther), Bacterium lactis acidi 

 (Leichman), Bacillus lacteri (Dinwidie), Bacillus a (Freu- 

 denreich), and several others. All of these are so closely 

 alike that they are probably to be regarded as identical, so 

 that perhaps all these names apply to the same species. It 

 is true that there are some variations in the descriptions 

 given by the different authors, but it is also true that there 

 are variations in the characters of the same species as given 

 by the same author. For example, some cultures of the 

 same species are found to produce acid in such small quanti- 

 ties that they do not curdle milk, while others curdle it in a 



