THE METHODS OF CULTIVATING MICROBES 59 



plug that passes into the test-tube shall come in 

 contact with any source of infection other than the 

 air itself. At the same time this portion of the plug 

 is directed downwards, in order to avoid any falling 

 germs that may be present in the atmosphere. The 

 platinum or glass needle, with its charge of seed 

 material, is plunged straight into the gelatine mass, 

 then carefully withdrawn and the plug replaced. 

 Where the seed material is also in solid gelatine, the 

 two tubes may be held inverted in the left hand, 

 one between the thumb and finger, the other between 

 the first and second, the plugs being held between 

 the second and third and third and fourth fingers ' 

 (Woodhead). 



The macroscopical appearances of the test-tube 

 cultivations should always be noted, for many 

 microbes give rise to characteristic growths. Some 

 microbes wholly or partially liquefy the nutrient 

 medium, while others have not this property ; but 

 may give rise to pigments, etc., in the medium or 

 media in which they are growing. 



Cultivation Methods. If the original fluid under 

 examination contains different microbes, and it is 

 desired to separate them, so as to obtain pure culti- 

 vations of one or all of the microbes present in the 

 original fluid, one of three methods may be used for 

 this purpose. The three methods are known as 

 plate-cultivations, fractional cultivations, and the 

 dilution method. 



In order to utilise the method of plate-cultivation, 

 about three tubes containing sterilised nutrient 

 gelatine or agar-agar are placed in a water-bath 



