PLATE CULTIVATION 95 



" Enrichment " may sometimes be practised. This 

 means placing the material under such conditions that a 

 particular species grows and multiplies considerably in 

 the material. This is sometimes used for the isolation of 

 the meningococcus from cerebro-spinal fluid ; the fluid is 

 incubated at 37 C. for twenty -four hours and then 

 cultured, and for the isolation of the typhoid bacillus 

 from water, peptone is added to the water, which is 

 then incubated at 37 C. for twenty -four hours (see 

 Chapter XXI.). 



" Concentration " may also be utilised. This may be 

 done by sedimentation, by centrifuging at high speed, 

 by the formation of an inert precipitate and subsequent 

 sedimentation or centrifuging, or by filtering through a 

 porcelain filter. The organisms will tend to be con- 

 centrated in the deposit. 



Plate cultivations. The method of plate culture is one 

 of the most important in bacteriology. It is used for 

 three purposes : (1) for obtaining pure cultivations, i.e. 

 cultures containing a single species, from a mixture of 

 organisms ; (2) for the enumeration of organisms ; and 

 (3) for ascertaining the characters of the colonies of 

 organisms as an aid in the identification of species. 



Before the introduction of plate cultivations pure 

 cultures of many organisms could only be obtained by 

 chance, or by the " dilution method." The dilution 

 method consists in estimating approximately the number 

 of organisms in a given volume of fluid by a direct count 

 of the organisms in a measured droplet of the fluid 

 knowing the volume of the fluid under the cover-glass, 

 the area of the cover-glass, the area of the field, and the 

 average number of organisms per field, the number of 

 .organisms can be calculated. Now, an instrument on 

 the same principle as the hsematocytometer might be 

 used, or the counting be done by Wright's method (see 



