BACTERIA AND FERMENTATION 265 



larly Bacillus carotovoriis causes " soft rot " of Carrot, but does 

 not attack Parsnips. 



The evil effects of parasitic Bacteria are due to poisons {toxins) 

 wliich they produce ; these are proteins in nature, and those 

 which liave been isolated are the most poisonous chemical sub- 

 stances known. The infected animal resists the invading or- 

 ganism, partly by the amceboid cells of the blood (leucocytes) 

 which devour them, and partly by producing chemical substances 

 which neutralise the toxins. The power of the body to make 

 a successful resistance is much improved by practice. Hence 

 one attack of an infectious disease often enables a person to 

 destroy that particular bacillus at once if it gets into the body 

 a second time, and so another attack of the disease is avoided. 



A large number of Bacteria obtain energy ' by bringing about 

 processes oi fermentation. Thus the Vinegar Bacteria [Bacterium 

 aceticiim, etc.) convert alcohol into acetic acid," whilst the lactic 

 acid Bacteria cause the souring of milk, changing the milk sugar 

 (lactose) into lactic acid. Another product of bacterial activity 

 is butyric acid, which is the chief cause of the rancid character 

 of bad butter. The putrefaction of meat is likewise due to the 

 agency of Bacteria, which in this case decompose protein sub- 

 stances. Each type of cheese is the product of a definite bacterial 

 and Fungus flora. In many cases fermentation is due to a group 

 of organisms ; for instance, the so-called ginger-beer plant, used 

 in the manufacture of the beverage of that name, consists of a 

 Yeast (p. 255) associated with certain Bacteria. 



It is to Bacteria, moreover, that we owe the decomposition 

 of the cellulose in dead leaves, etc., whereby undue accumulation 

 is prevented, and the carbon is again brought into circulation 

 as carbon dioxide. The processes of decay involved in the " ret- 

 ting " of fibres (e.g. Flax, Hemp, Jute), the " curing " of tobacco, 

 and the conversion of sewage are likewise brought about by 

 members of this group. 



The nitrates of the soil, upon which plants are dependent 



1 In some cases part of this energy is dissipated as heat (cf. p. 261 ) 

 or light (e.g. the phosphorescence of bad meat). 



^ In the commercial production of vinegar either wine or spirit is 

 used as the raw material, and to these vinegar is added in order to in- 

 troduce the necessary Bacteria. 



