CHAPTER VII 



CULTURE-MEDIA AND THEIR PREPARATION 



MICROSCOPIC examination alone is quite insufficient to 

 differentiate species of bacteria. By the aid of a microscope 

 one cannot readily recognize the differences, for example, between 

 the organisms which cause typhoid fever and certain of the normal 

 inhabitants of the intestinal tract. It is necessary, therefore, 

 in a study and differentiation of species, that we make use of 

 different kinds of culture-media in which the bacteria may be 

 grown. By the term medium is meant any nutrient substance or 

 mixture upon which or in which bacteria will multiply. The 

 bacteria in their development on the various media show certain 

 growth reactions which are very useful in their differentiation. 

 Some produce acids, others gas, alkalis, and proteolytic and 

 coagulative enzymes. 



Use of Normal Solutions of Acid and Alkali and Methods of 

 Expressing Reactions. In the chapter on Physiology we have 

 noted that many bacteria are extremely sensitive with respect to 

 the acidity or alkalinity of the medium in which they are grown. 

 Some organisms develop best in a medium which is approximately 

 neutral; some refuse to develop unless there is a slight excess of 

 alkali present. It is necessary, therefore, that some definite method 

 of expression of these acidities and alkalinities be adopted. For 

 this purpose it is customary to use normal solutions. 



A normal solution of a chemical may be defined as one in which 

 there is one gram of replaceable (acid) hydrogen or its equivalent 

 per liter of solution. For example, if we wish to prepare a normal 

 solution of HCl,we must so dilute the acid that it contains one gram 

 of hydrogen per liter of solution. This is best accomplished in 

 any substance that can be readily weighed by dissolving the 

 molecular weight expressed in grams in sufficient water to make a 

 liter of solution. If there is more than one atom of replaceable 

 hydrogen in the molecule, it is necessary to divide the amount 

 used by the number of any such atoms. For example, the molecu- 



