CHAPTER II 
MEDIA AND THEIR PREPARATION 
Bacterta are such minute organisms that their environment exerts 
great influence on their development. In the laboratory where it is 
desired to get as accurate information with regard to ‘their life histories 
as possible, every attempt should be made to create an environment 
which simulates the natural one for the variety under examination. 
The bacteriologist calls this a medium. The characteristics of a good 
medium are: 1. It must approach the isotonic solution for the bac- 
terial cells. 2. Must contain proper kinds and amounts of the essen- 
tial food substances such as: (a) Water concentration. (6) Nitrogen 
requirement, organic and inorganic. (c) Inorganic salts. 3. Should 
be used at optimum temperature for organism under study. 
All media should be made up according to standard methods. In 
America the Standard Methods for the Examination of Water and 
Sewage by a Committee of the American Public Health Association 
are taken as the fundamental basis upon which most official media 
are made. These may not be the ideal procedures but doubtless 
embody most of the essential points according to our present information. 
By following these standard methods the analyst is more certain of 
securing results which may be compared with those obtained in other 
laboratories, than if he adopted his own formule. Some objections 
have been raised with regard to parts of these standard methods, 
but constructive criticism is necessary if they are to be brought to a cor- 
rect stage, 
ConsTITUENTS OF MEDIA 
While there are many different substances which may be used 
for cultering bacteria, a certain few yield the best results and are 
therefore used at the present time in most of our common media. 
These substances furnish the optimum conditions for the average 
bacterium. For these bacteria, certain pathogenic forms for instance, 
which do not grow on the ordinary media, special media must be pre- 
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