150 BACTERIA 



species. Among the enormous number of kinds of 

 bacteria, a few have become parasitic on the higher 

 animals and cause disease. Hence, as in the past, 

 new diseases may make their appearance when suit- 

 able circumstances occur, by the adaptation of a pre- 

 viously harmless form to new conditions in an animal 

 body, where it produces poisonous substances, giving 

 rise to the symptoms of the disease. 



Respiration and Fermentation. — Many bacteria, like 

 the vast majority of animals and plants, can only 

 live in the presence of free oxygen. These are called 

 aerobic ^ forms (cf. p. 132). Some, on the other hand, 

 like yeast, can do without free oxygen for a time, and 

 others, again, cannot live in its presence {anaerobic 

 forms), and obtain their energy by splitting up organic 

 substances without oxidation. 



Bacteria which live on complex organic substances, 

 as most of them do, nearly always cause profound 

 changes in the medium they inhabit. They break 

 down the organic substances of the medium (pre- 

 sumably by the excretion of enzymes) just as yeast 

 breaks down sugar. The term, fermentation is technically 

 applied to all such processes, and is not confined to 

 alcoholic fermentation as in popular usage. Different 

 kinds of bacteria ferment different substances. Thus 

 certain bacteria found in milk {Bacterium acidi lactici 

 and many others) ferment milk sugar (lactose), splitting 

 one molecule of lactose (with a molecule of water) 

 into four molecules of lactic acid : — 



C12H22O11 4- HgO = 4C3Hg03 



lactose water lactic acid 



and the accumulation of the lactic acid in the milk 

 turns it sour. Alcohol itself is split up by certain 



' Living in air, from Greek ds^p, air, and /3/of, life. 



