BACTERIOLOGICAL TECHNIC 97 



3. Colorless gelatin liquefying colonies per cc. 



4. Neutral red reduction, + or — . 



5. Indol reaction, + or — . 



6. Gram stain behavior, + or — . 



7. Gas (hydrogen) formula. 



III. Agglutinating tests for Typhoid Germs. 



8. Staining Bacteria 



Staining consists of the infiltration of the cell-substance with solutions 

 of various coloring materials obtained for the most part from the group of 

 coal-tar derivatives known as the aniline dyes. As is generally known, 

 different cells and different portions of one and the same cell react differ- 

 ently with the various dyes used. This peculiar behavior brings out 

 contrasts in appearances which aid very materially in determining the 

 morphological characters. Xhe prime object, therefore, in using stains is 

 to aid in the study of cell morphology. Different bacteria react differ- 

 ently with the several stains used. Some species take certain stains very 

 readily, while they are quite indifferent to other stains. The vegetative 

 cell stains much more readily than do the spores. In fact, spores are 

 stained with great difficulty; however, after they are once thoroughly 

 stained they hold the color persistently. 



The dyes which may be used in bacteriologic work are of many kinds, 

 differing as to color and as to staining powers with different cells, cell-con- 

 tents, and cell-parts. They are usually classified as acid or basic. Eosin, 

 acid fuchsin, and picric acid are acid stains, and are said to be diffuse 

 in their effects, having no special affinity for any special cell structure, 

 fuchsin, methylene blue, and gentian violet are basic, and appear to have 

 special attraction for bacteria and for plasmic and nuclear substances of 

 cells generally, for which reasons they are most generally employed as 

 bacterial stains. Fuchsin is, in fact, about the only efficient stain for 

 endospores, while gentian violet and methylene blue are excellent stains 

 for the bacterial cell-wall. 



It is known that certain substances possess the property of preparing 

 the bacterial cells in such a way as to induce them to take up the dye 

 more readily, thus intensifying the stain, as aniUne oil and carboUc acid. 

 Such substances are called mordants, and may be used separately or added 

 directly to the stain itself. 



Certain liquids or solutions remove the stain from the bacterial cell 

 more or less readily, as water and alcohol, but more particularly solutions 

 of acids. Such substances^re quite generally employed for removing any 

 excess of stain from the bacterial cell or from the matrix in which the bac- 



