STAINING 201 



way, but these features are best determined on stained prep- 

 arations in many instances. 



"Dark field" illumination and the ultramicroscope are 

 of great value in the study of living bacteria and other 

 minute objects, but apparatus of this type would scarcely 

 be used by the student in an introductory course, so that 

 they will not be discussed in the present volume. 



STAINING. 



The main use of the microscope in bacteriology is in the 

 study of stained preparations of the organisms. Staining 

 makes bacteria opaque and hence more easily seen than the 

 transparent unstained forms. Some methods of staining 

 also show morphological structures w^hich are either imper- 

 fectly recognized in the unstained cell, spores, or are not 

 visible at all— capsules, metachromatic granules, flagella. 

 Finally certain bacteria are colored by special methods of 

 staining which do not affect others, so that under proper 

 conditions these bacteria may be recognized by staining 

 methods alone— tubercle bacilli in the organs of animals. 



The phenomena of staining are essentially chemical, 

 though sometimes the chemical union is a very weak one, 

 even resembling an absorption of the dye rather than true 

 chemical union— most watery stains. In other cases the 

 chemical compounds formed are decidedly stable and are 

 not decomposed even by strong mineral acids— staining of 

 tubercle bacilli and other "acid-fast" organisms. In still 

 other cases the principal action is a precipitation on the 

 surface of the object stained— methods for staining flagella. 



In many methods of staining in addition to the dyes used 

 other substances are added to the solution which assist in 

 fixing the dye in or on the organism stained. Such sub- 

 stances are called mordants. The principal mordants used 

 are alkalies, anilin, carbolic acid, iodine, metallic salts, 

 tannic acid. 



While it is true that some bacteria may be stained by that 

 standard histological nuclear dye, hematoxylin, it is of little 

 value for this purpose. Practically all bacteriological stains 



