130 BACTERIA 



phates. Very rapidly indeed the little isolated colonies 

 spread, and, becoming confluent, form a membrane or pellicle 

 over the whole area of fluid. When the surface is covered 

 the alcohol acidifies to vinegar. After this it is necessary 

 to add each day small quantities of alcohol. When the 

 oxidation is completed the vinegar is drawn off, and the 

 membrane is collected and washed, and is then again ready 

 for use. It ought not to remain long out of fermenting 

 liquid, nor ought it to be allowed to over-perform its func- 

 tion, for thus having oxidised all the alcohol it will com- 

 mence oxidation of the vinegar. 



In wort-gelatine Bacterium pasteurianum develops round 

 colonies with a smooth or wavy border, whilst B. aceti has a 

 tendency towards stellate arrangement. Spores have not 

 been observed, and from a morphological point of view the 

 two species behave alike. Neither produces any turbidity 

 in the liquid containing them. In order to flourish, B. 

 aceti requires a temperature of about 33 C. and a plentiful 

 supply of oxygen. In a cool store or cellar there is, there- 

 fore, nothing to fear from B. aceti. Frankland has isolated 

 a Bacillus ethaceticus, which is a fermentative organism pro- 

 ducing ethyl-alcohol and acetic acid. By oxidation the 

 ethyl-alcohol may be converted into acetic acid. 



3. Lactic Acid Fermentation. 



Cause, Bacillus acidi lactici ; medium, milk-sugar, cane-sugar, glucose, 

 dextrose, etc. ; result, lactic acid. 



The process set up by the lactic ferment is simply a de- 

 composition, an exact division of one molecule of sugar into 

 two molecules of lactic acid, there being neither oxidation 

 nor hydration. The conditions under which the ferment 

 acts are very similar to those we have already considered. 

 There is frequently carbonic acid gas formed ; there is a 

 cessation of fermentation when the medium becomes too 



