40 BACTERIOLOGY 



dish devised by Krauss/ or some similar dish, will be found useful 

 when the stain has to be heated. Experiments have shown that 

 the ordinary method of fixation in the flame, when applied to 

 bacteria spread upon shdes, has little effect on the vitality of many 

 species. The beginner is, therefore, advised to make his prepara- 

 tions on cover-glasses. 



When very resistant or dangerous pathogenic bacteria are 

 being handled, after fixation by heat upon the slide or cover- 

 glass, the preparation may, if desired, be immersed in i-iooo 

 solution of bichloride of mercury long enough to kill the bacteria, 

 without injuring the preparation or its staining properties. 



Staining Solutions. — The staining of bacteria is done for the 

 most part with the aniline dyes. The object of staining bacteria 

 is to give them artificially some color which makes them distinct 

 and easily visible without imparting this color to the substance 

 or medium in which they are imbedded. The substances known 

 as aniline dyes are derivatives of coal-tar, but not always of aniline. 

 These dyes are of great importance in bacteriological ' work. 

 Their number is very large, but only a few are in comm'on use. 



It is simplest to classify the aniline dyes as acid or basic. 

 Eosin, picric acid and acid fuchsin are acid dyes; they tend to stain 

 tissues diffusely. Fuchsin, gentian- violet and methylene blue are 

 basic dyes; they have an affinity for the nuclei of tissues and for 

 bacteria; they therefore are the dyes used chiefly in bacteriological 

 work. The other varieties may be employed as contrast-stains; 

 another contrast-stain frequently used is Bismarck brown. 

 It is best to keep on hand saturated solutions of the aniline dyes 

 in alcohol, which are permanent, but cannot be einployed directly 

 for staining. In order to prepare the simple staining solutions, 

 the alcoholic solution is diluted about ten times, or so as to make 

 a liquid which is just transparent in a layer about 12 mm. in 

 thickness, after filtering. ^These watery solutions deteriorate 

 after a few weeks. 



Fuchsin and gentian-violet stain rapidly and intensely. 



' Krauss, Jour. A. M. A., Apr. 6, 1912, Vol. LVIII, p. 1013. 



