FOOD REQUIREMENTS OF BACTERIA 59 



toxins. Much of the nitrogen, however, is used in building that 

 wonderful little engine, the body of the microbe. They break 

 down the complex proteins into simple building blocks before 

 they are built over into their body just as man would reduce the 

 wall of an old church before building it over into a new school- 

 house. Soil bacteriology deals mainly with the organisms and 

 their relationship to the nitrogen supply. Many bacteria are busy 

 changing the nitrogen compounds into forms which are accept- 

 able to the higher plants. 



Oxygen. — Bacteria are like all other plants and animals in 

 requiring oxygen. The various classes, however, require it in 

 different forms. One great group of microorganisms require 

 their oxygen as it is in the atmosphere — free. To these are given 

 the name aerobes. Others require it in compounds as "carbohy- 

 drates. These are known as anaerobes. Some organisms grow 

 best in the presence of free oxygen but can become adapted to 

 the use of combined oxygen. These are known as faculative 

 anaerobes. Free oxygen is a poison to absolute anaerobes. True 

 aerobes are great lovers of oxygen, and if shut up within a little 

 cell containing a few drops of water in which is enclosed a 

 bubble of air, on examination under the microscope it will be 

 seen that the motile ones will make their way from all parts of 

 this tiny ocean and be clustering around the drop of air. 



Vitamins — We hear much today of the vitamin requirements 

 of man, and it would seem that bacteria may need similar 

 substances. The extracts of animal organs as well as those of 

 plant tissue are valuable nutrient material for some bacteria which 

 it is impossible to supply in any medium of known composition. 

 These substances have been termed vkaminsy or accessory growth 

 factors. Sometimes it is only when £rst taken from the body of 

 the animals that these substances are needed. The tubercle bacilli 

 on being taken from the lungs of a man grow slowly on even 

 blood serum, but after a time they adapt themselves to other 

 media. On the other hand, the bacillus causing leprosy always 

 requires the highly complex tis$ue for growth. 



Ash Requirements. — ^Animals kept on a diet of pure water, 



