GRAM'S STAIN. 99 



here that different organisms take up and hold different 

 stains with different degrees of intensity, and thus duration 

 of staining and degree of decolorisation must be varied. It 

 may be laid down as a general rule that, so long as 

 organisms retain the stain, the greater the decolorisation of 

 the tissues in which they lie, the clearer will be the results. 



Gram's Method and its Modifications. In the methods 

 already described the tissues, and more especially the 

 nuclei, usually retain some stain when decolorisation has 

 reached the point to which it can safely go without the 

 bacteria themselves being affected. In the method of 

 Gram, now to be detailed, this does not occur, for the stain 

 can here be removed completely from the ordinary tissues, 

 and left only in the bacteria. All kinds of bacteria, how- 

 ever, are not stained by this method, or rather, in the stage 

 of decolorisation some bacteria part with the stain as 

 readily as, or even more readily than, the tissues. This 

 fact is taken advantage of in certain circumstances to 

 differentiate between different species of bacteria ; and 

 therefore in the systematic description of any species it is 

 customary to state whether it is, or is not, stained by Gram's 

 method. By this is meant, as will be understood from 

 what has been said, whether the species retains the colour 

 after the latter has been completely removed from the 

 tissues. It has been said that, by Gram's method, the 

 stain can generally be removed from the tissues. It must, 

 however, be remarked that some tissue elements may retain 

 the stain as firmly as any bacteria, e.g., keratinised epi- 

 thelium, calcified particles, the granules of mast cells, and 

 sometimes altered red-blood corpuscles, etc. 



In Gram's method the essential feature is the treating of 

 the tissue, after staining, with a solution of iodine. This 

 solution is spoken of as Gram's solution, and has the 

 following composition : 



Iodine, . . i part. 



Potassium iodide, . . 2 parts. 



Distilled water, . . 300 parts. 



