NUTRIENT MEDIA 33 



Test-Tubes. 6 x inches is the most useful size. For 

 special purposes smaller and larger ones are required. 



Cornet forceps for cover-glasses, dissecting forceps of 

 various patterns, and other instruments, are needed, 

 together with flasks for holding media. 



CHAPTER III 



THE PREPARATION AND USE OF NUTRIENT 

 MEDIA 



IT is necessary, in order to obtain a satisfactory know- 

 ledge of the biological characters of a micro-organism, 

 to obtain a pure culture that is, a culture containing 

 one species only. 



When, by exposure to air or by other means, isolated 

 bacteria lodge on the surface of a nutrient medium, 

 they are fixed in situ and commence to grow, each organism 

 producing a colony which eventually becomes visible 

 macroscopically. This character is used for the isolation 

 of organisms. Pure cultures of bacteria are hardly ever 

 met in practice, and a very common method of separating 

 individual bacteria is to disperse the liquid containing 

 them over or through some solid medium in such a dilution 

 that individual bacteria can form sufficiently large colonies 

 without their meeting. This is generally effected in 

 Petri dishes. 



By the introduction of a part of a colony into a tube of 

 sterile medium, a pure culture is obtained after incubation, 

 and the larger quantity of growth provides more material 

 for examination. The appearances of the growths on 

 various media constitute important, and often positive, 

 means of identification. 



In culture, the store of nutrient material becomes 

 gradually used up, and reproduction stops. It is necessary, 

 therefore, to reinoculate them from time to time into fresh 

 media. Bacteria are artificially cultivated in both liquid 

 and solid culture media. 



Nutrient media are employed in test-tubes, small conical 

 (Erlenmeyer's) or other flasks, or Petri dishes. All test- 

 tubes, flasks, etc., are thoroughly cleansed with 25 per cent. 



3 



