Microbes Must Eat 59 



training microbiologists or in research investigations, increased 

 numbers and increased longevity of bacteria may be important. 

 We cannot do too effective a job by studying only a few of these 

 microscopic forms, but masses of actively growing cultures can 

 provide valuable information relative to the physiology and the 

 ultimate identity of a given species. 



In addition to preparing a diet which is most favorable for 

 as manv species of bacteria as possible, we can also blend chemi- 

 cals with our nutrient materials in such a manner that selectivity 

 of growth can be accomplished. Often, particularly in clinical 

 laboratories when we are attempting to isolate the etiological agent 

 in a given disease, it is highly desirable to inhibit the growth of 

 abundant organisms which we know are normally not pathogenic, 

 and to encourage the cultivation of our suspected pathogens. Such 

 a medium emplovs the principle of bacteriostasis for its selectivitv. 

 No culture medium has yet been devised which will allow all 

 known bacteria to grow under one set of conditions. To meet 

 this problem, microbiologists have been forced to concoct hundreds, 

 even thousands, of combinations of substances to fulfill growth 

 requirements. There are a few basic media to provide general 

 uniformity in running such standardized tests as milk and water 

 analyses, otherwise no two laboratories would be able to compare 

 a given sample with reproducible results. A great boon to 

 uniformity has been the manufacture by biological supply houses 

 of dehydrated media, and standardized procedures play an im- 

 portant part in modern laboratory technics. 



PREREQUISITES FOR A GOOD MICROBIOLOGICAL MEDIUM 



Any material employed to grow bacteria is termed a culture 

 MEDIUM, and the resultant growth is termed a culture. A satis- 

 factory microbiological medium should fulfill the following pre- 

 requisites. It must: 



1. Have the proper moisture content. 



2. Contain readily available food materials. 



3. Have the correct acid-base balance, called pH. 



