56 BACTERIA AND SOIL FERTILITY 



are often referred to as autotrophic bateria; those which feed up- 

 on dead plants and animals are called heterotrophic or safrofhytes^ 

 those which feed upon living plants and animals we designate 

 as farasites. To the first two classes belong the beneficial soil 

 organisms, whereas the parasites are the disease producers. These 

 are often spoken of as pathogens, A microorganism may live at 

 one time as a parasite, at another as a saprophyte. For example, 

 the tuberculosis organism growing in the lungs of man, cattle, or 

 chickens is a parasite, but when growing in the test tube of the 

 bacteriolgist it is a saprophyte. Some few bacteria appear to live 

 only as parasites. In time even these we may learn how to grow 

 in the test tube. 



Water. — Man can live for days without food but only for 

 hours without water. The same is true of bacteria. Remove all 

 the water from the medium in which bacteria are growing and 

 they soon die or pass into the spore stage. The pneumonia coccus, 

 the cholera spirillum, and the PfeifFer bacillus die in a few hours 

 on drying, whereas the typhoid, diphtheria, and tuberculosis or- 

 ganisms may survive days. The tetanus, anthrax, and many soil 

 bacteria resist drying for months or even years. In these latter 

 cases the water is never completely removed j otherwise the organ- 

 isms would quickly die. 



Water plavs a number of parts in bacterial life, (i) It 

 dissolves the food so it can be taken up by the bacteria. When 

 the food is not already soluble the microbe throws off an enzyme 

 which brings the food into solution so it may be taken up by the 

 cell. (2) Water carries the waste products from the cell. (3) 

 It enters into nearly all the changes going on within the cell. 

 When starch is being digested water enters and the starch breaks 

 into sugar. Fats break into glycerine and an acid through the 

 taking up of water. In the digestion of proteins every step is 

 accompanied by the taking up of water. When bacteria are 

 building up in place of breaking down, water is the pivot around 

 which all reactions turn. If food burns within the body or with- 

 in the stove water is continually being evolved. (4) Water is 

 the whip which speeds up all reactions. If a match becomes wet 



