BIOLOGICAL DIFFERENTIATION 249 



cytometer, Fig. 138), and diluting the fluid by the addi- 

 tion of sterile water or bouillon until a given volume 

 (usually i c.c.) of the dilution contained but one 

 organism. By planting this volume of the fluid into 

 several tubes or flasks of nutrient media, it occasion- 

 ally happened that the resulting growth was the pro- 

 duct of one individual microbe. A method so uncer- 

 tain is now fortunately replaced by many others, 

 more reliable and convenient, and in those methods 

 selected for description here, the segregation and isola- 

 tion of the required bacteria may be effected 



A. By Mechanical Separation. 



i . By surface plate cultivation : 



(a) Gelatine. 



(b) Agar. 



(c) Serum agar. 



(d) Blood agar. 



(e) Hanging-drop or block. 

 [2. By Esmarch's roll cultivation : 



This archaic method (see page 226) is no longer 

 employed for the isolation of bacteria.] 



3. By serial cultivation. 



B. By Biological Differentiation. 



4. By differential media. 



(a) Selective. 



(b) Deterrent. 



5 . By differential incubation. 



6. By differential sterilisation. 



7. By differential atmosphere cultivation. 



8. By animal inoculation. 



The selection of the method to be employed in any 

 specific instance will depend upon a variety of cir- 

 cumstances, and often a combination of two or more 

 will ensure a quicker and more reliable result than a 

 rigid adherence to any one method. Experience is 

 the only reliable guide, but as a general rule the use of 



