CULTIVATION OF MICROORGANISMS 41 



method will be successful only when the desired organism 

 is present in approximately as great numbers as the other 

 bacterial forms that can develop on the plate culture. If 

 only a few of the organisms are present, it will usually be 

 necessary to seed some special selective medium that is more 

 favorable for the desired organism than for the other types 

 with which it may be associated. This will enable the or- 

 ganism in question to gain the ascendancy in the culture, 

 which can then be plated with assured success. 



An animal that is susceptible to a disease may be inocu- 

 lated with a mixture of kinds of bacteria among which the 

 producer of the disease is present. The body of the animal 

 will act as a selective medium, and from its tissues a pure 

 cull ure can often be obtained. Many modifications are em- 

 ployed to obtain pure cultures of some of the bacteria. 

 More extended texts on laboratory methods must be con- 

 sulted for the methods employed. 



Quantitative methods. The plate culture is also used for 

 tin' determination of the number of microorganisms in any 

 material. If a known amount of the material is added to 

 the melted nutrient agar or gelatin, the number of colonies 

 developing in the plate culture can be counted, and from 

 these results the bacterial content of the original material 

 may be determined. Each colony must have developed 

 from at least one cell. It is thus possible to obtain a num- 

 ber that will, as a rule, approximate the number of cells 

 actually present in the material used. The method is used 

 constantly in the control of water and milk supplies. 



It is, of course, impossible to obtain a medium that will 

 enable all kinds of bacteria that may be present in such 

 materials as milk or water to develop. The results thus 

 obtained represent only the number of organisms that are 

 able to grow under the conditions present in the culture 

 plate. It is usually possible to adjust the conditions so 



