INOCULATION OF MEDIA 69 



general turbidity and are not particularly characteristic, 

 but sometimes an organism produces a film on the surface 

 which another similar organism does not, or the medium 

 remains clear, the growth forming a flocculent deposit, 

 thus affording a distinction. Not only do the characters 

 of the growths of organisms on media differ more or less, 

 but in some instances chemical changes occur in the media 

 which afford valuable information in the differentiation 

 of species. Thus many organisms exert a peptonising 

 effect on gelatin, and render it fluid sooner or later, while 

 others have no such action. Milk is coagulated by some 

 organisms, the coagulation being brought about in one 

 of two ways, either by the production of acids and pre- 

 cipitation of the caseinogen, or by the action of a rennet- 

 like ferment with the formation of a clot of casein. Most 

 organisms which liquefy gelatin coagulate milk, but the 

 converse is not the case. Agar is carbohydrate, not 

 albuminoid, in nature, and only two or three organisms 

 are known which liquefy it. In fluid media, such as 

 broth and peptone water, chemical tests can be applied, 

 especially for indole, which is formed by some organisms 

 but not by others. 



Method of inoculating tubes. The following is the 

 procedure by which sub-cultures are prepared from an 

 original test-tube or other culture : Tubes of the culture 

 media selected are placed in a test-tube rack. Their 

 mouths are then singed by holding in the Bunsen flame 

 for a few seconds, and with a forceps, also sterilised by 

 heating in the flame, the wool plugs are loosened by a 

 rotatory motion, and then partially withdrawn. The 

 mouth of the original culture- tube is similarly singed and 

 its plug partially withdrawn. A platinum needle is 

 selected and carefully straightened. The original tube 

 is then taken in the left hand between the thumb and 

 index finger with the palm upwards, and is held obliquely, 



