114 A MANUAL OF BACTERIOLOGY 



alcohol. Leave for fifteen minutes, or, for convenience, 

 for any time up to forty -eight hours. 



The preparations may then be stained with methylene 

 blue, hsematoxylin and eosin, or with the Leishman or 

 Giemsa stain. (See also under " Malaria," Chapter XVIII.) 



Impression specimens. These are employed to preserve 

 permanently the colonies or growths of organisms so that 

 their characteristic formation may be observed. With 

 plate cultivations this is very simple. A clean cover-glass 

 is cautiously lowered on to a selected surface colony, 

 avoiding all lateral movement. It is then gently pressed 

 on to the colony and then carefully raised by means of a 

 couple of needles ; the colony should adhere to the glass, 

 and may be dried, fixed and stained. The colonies in 

 gelatin tube cultures may also be used if the gelatin is 

 removed from the tube. This can be done by dipping the 

 tube for a few seconds into hot water, the gelatin round 

 the walls of the tube will be melted, and the gelatin mass 

 can then be tilted out of the tube on to a glass dish or tile. 



Stains and Staining Methods 



Micro-organisms being so minute and transparent, it 

 is usual to stain or dye them, so that they can be more 

 readily examined. In some instances organisms may 

 have a peculiar staining reaction which may serve as an 

 aid to their identification. But when an organism is being 

 investigated, examination in the fresh and living condition 

 must never be omitted, for it is only thus that its motility 

 and life-history can be studied. Only general methods 

 are detailed here ; special ones will be given when they 

 are required. 

 (1) Loftier 's alkaline methylene blue : 



Saturated alcoholic solution of methylene blue 30 c.c. 

 Solution of caustic potash, 0-01 per cent. . 100 c.c. 



