^8S General Botany 



to the oxygen that dissolves in the water surrounding them. At 

 room temperature this forms an extremely dilute solution (0.0009 

 per cent). If the oxygen content is increased artificially to 

 thirty times this amount (0.027 P^^ cent), practically all bacteria 

 die. In other words, oxygen is about as poisonous to bacteria 

 as formaldehyde and corrosive sublimate, two of the commonly 

 used disinfectants. 



Nevertheless, small amounts of oxygen favor the growth of 

 most bacteria. On this account they are called aerobes (Greek : 

 aer, air, and bios, life). Some bacteria, like the germ of lockjaw 

 and the bacteria that produce the rancid taste of butter, can 

 grow only when the oxygen content of the medium is extremely 

 low and when there are organic substances available containing 

 combined oxygen. These bacteria are called anaerobes (Greek : 

 an, without). Anaerobic bacteria occur in poorly drained soils, 

 in the bottoms of lakes, and in the deep waters of the ocean. 



The effectiveness of hydrogen peroxide in dressing wounds 

 and cleaning teeth depends upon the fact that it releases oxygen 

 readily.^ 



Food supply. Almost all bacteria require organic foods, and 

 live usually as saprophytes or parasites. They all depend upon 

 the oxidation of a part of these foods for their energy. They 

 differ widely in their food requirements and in their effects upon 

 the medium in which they live. 



The most important exceptions to this general rule of requiring 

 organic foods are found in the nitrifying bacteria of soils. These 

 resemble green plants in the fact that they can synthesize organic 

 compounds from CO2, H2O, and mineral salts, but differ in that 

 they cannot utilize sunlight. 



Structure and reproduction. Bacteria consist of one-celled 

 individuals, that occur usually in masses on or in the food- 



^ Hydrogen peroxide changes to water and oxygen on exposure to the air : 



2H2O2 ^2H20+02 



Hydrogen peroxide — >■ water + oxygen 



J 



