188 STUDY OF INDIVIDUAL BACTERIA-STAINING 



way but these features are best determined on stcinied 

 preparations. 



"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 elementary student, 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 which are either imperfectly 

 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 ver>' weak one, 

 even resembling an adsorption 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 

 fixipg the dye in or on the organism stained. Such substances 

 are called mordants. The principal mordants used are alka- 

 lies, 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 

 are solutions of the anilin dyes. These dyes, as is well 



