176 TEXT-BOOK OF BACTERIOLOGY. 



pear like white stars, or Chinese asters, dotted over the gelatin. Al- 

 though this fungus permeates the milk in great masses and covers 

 the cream on the surface with a tightlj'-cohering, velvety, dense 

 coat, 3'et it seems to take no essential share in the important 

 changes which the milk so quickly undergoes. 



It is universally known that milk turns sour and curdles if it 

 stands for a certain time. The slightly alkaline or amphoteric re- 

 action of fresh unboiled milk disappears, and the albumin of the 

 milk — the casein — is separated from it. This change, which is usu- 

 ally called lactic-acid fermentation, is the work of certain micro- 

 organisms. Not one single species, but a whole series of different 

 bacteria — as, for example, the potato bacillus, already described — 

 possess the power of causing this decomposition of milk. 



In the great majority of cases, however, the change is due to 

 one and the same, regularly- occurring micro-organism — a bacUlus 

 carefully studied and precisely described by Hueppe, to which its 

 discoverer has given the name of bacillus of lactic acid fermenta- 

 tion — Bacillus acidi lactici. 



The rods are quite short and thick, scarcely longer than they 

 are broad, generally united by twos, rarelj^ in long chains. They 

 are non-motile, but in consequence of their small size generally 

 show the Brownian molecular movement very clearlj'^. Sporulation 

 has also been observed in them — small globular bodies with strong 

 refractive power at the ends of cells and able to withstand high de- 

 grees of heat, thereby showing their importance as enduring forms. 



The Bac. acid. lact. thrives at temperatures between 10° and io" 

 C; it belongs to the semi-anaerobic species and is little sensitive 

 to the want of oxygen. 



The cells are stained with the usual anUin colors. 



On the gelatin plate at first appear small white dots, afterward 

 grayish- white, shining blotches like glazed porcelain, with trans- 

 parent edges, which do not liquefy the gelatin. Under the micro- 

 scope, one sees in the deeper-ljang colonies small yellow masses, 

 without any particular characteristics; the surface colonies, how- 

 ever, look like flat, extended leaves, with irregularly-jagged, very 

 tender edges. In the middle they are yellowish, but the color fades 

 toward the margin and shows rather more distinctly an elegant 

 folded pattern. 



In the test-tube the gelatin even of old cultures is not lique- 

 fied. At first the growth proceeds equally throughout the whole 

 length of the puncture, forming a delicate layer composed of small 

 isolated granules. Later the development at the surface becomes 

 particuls.':-ly luxuriant, displaying a moderately thick, dry, brittle 



