180 BACTERIOLOGY 



out that the conversion of sugar into lactic acid (lactic acid 

 fermentation) is due to the action of micro-organisms. 

 Hueppe has investigated this fermentation more thoroughly, 

 and has described a microbe possessing in an eminent degree 

 the property of curdling milk. It consists of short, plump, 

 immotile rods, which usually occur in pairs but are occa- 

 sionally united in longer chains. Growth can take place in 

 the absence of oxygen, as well as when it is present. On 

 the gelatine plate little white points appear, having a surface 

 like porcelain, which do not liquefy the medium ; they show 

 under the microscope a yellowish coloration in the centre, 

 which becomes more transparent and paler towards the 

 margin. Growth appears along the needle-track in thrust- 

 cultures ; a coating of considerable extent soon forms on 

 the surface, and crystals of salt separate out from the 

 nutrient medium. A whitish layer develops on agar, and a 

 thick brown dirty deposit on potatoes. 



If a small quantity of a pure cultivation of Bacillus 

 lacticus be added to milk, formation of acid occurs, with 

 curdling of the caseine, the milk-sugar being split up into 

 carbonic and lactic acids. This action on milk seems 

 to be capable of taking place only when air has access, 

 although the growth of the micro-organism is independent 

 of it. 



Micrococcus acidi lactici. The micrococcus of this name 

 described by Marpmann closely approximates in its action to 

 the Bacillus lacticus. It consists of non-motile cocci which 

 occur singly or in pairs. In twenty-four hours yellowish- 

 white punctiform colonies appear on the gelatine plate, 

 while in thrust-cultures a superficial layer develops which 

 is thicker in the centre than at the edge, and is rather slow 

 in growing. Keddening of the milk takes place in twelve 

 hours, and in twenty-four coagulation and lactic acid fer- 

 mentation. 



