34 AGRICULTURAL BACTERIOLOGY 



the. chemical reaction be favorable for the organism to be 

 grown. 



The bacteriologist has succeeded in devising what may be 

 termed basal culture media, to which may. be added amend- 

 ments to adapt them better to the growth of specific organ- 

 isms. The great basal culture medium is broth or bouillon, 

 prepared by infusing one part of chopped lean meat (beef 

 or veal most commonly) in two parts of water. The sol- 

 uble constituents are extracted from the meat. The insol- 

 uble portion is filtered off, and to the filtrate is added 1 or 2 

 per cent, of peptone. The latter is prepared from meat or 

 blood by digesting it with one of the agents that are active 

 in the intestinal tract of animals until the meat or blood is 

 changed into a mixture of simpler products that are soluble 

 in water. The infusion of meat to which peptone has been 

 added is boiled to coagulate the albumens present. Its re- 

 action is then adjusted to the desired point by methods that 

 can not be discussed in sufficient detail to warrant their 

 treatment here. 



Acidity and alkalinity are due to the presence of hydro- 

 gen (H) and hydroxyl ions (OH) respectively. Any solu- 

 tion in which the former are in excess will be acid in reac- 

 tion. If the hydroxyl ions are in excess of the hydrogen 

 ions, the solution will be alkaline. If both are present in 

 equal numbers, the material will be neutral in reaction. 

 Pure water is the great example of a neutral substance. 

 For most microorganisms the nutrient medium should be ap- 

 proximately neutral. It is doubtful whether there is any 

 microorganism that will not grow in a neutral reaction when 

 other conditions are favorable. 



The broth is filtered, after its reaction has been corrected, 

 to remove all coagulated and suspended matter, since it is 

 desirable to have the medium clear in order that the growth 

 of bacteria therein can be more easily seen. 



