Microbiol Structures and Staining Reactions 107 



oro;anisms. These stains belong to the group of anihne (coal-tar) 

 dves and include crystal violet, basic fuchsin, safranin, methylene 

 blue, cosine, etc. After flooding a properly prepared smear on a 

 glass slide with the particular dye and allowing a contact of from 

 a few seconds to a minute, enough dye will be taken up by the 

 cell contents to make the organism readily visible under the 

 microscope. Stained slides are usually best examined under 

 the oil immersion objective, which allows magnifications of one 

 thousand or more diameters. Different stains and combinations of 

 stains have been devised for specific purposes. A brief discussion 

 of a few of these will follow. 



An important prerequisite for any staining operation is to have 

 a clean slide on which the smear is to be prepared. Greasy slides 

 do not allow even distribution of the test material, and much of the 

 bacterial film may wash off during the staining processes. Be- 

 ginners in bacteriology have a tendency to prepare films much too 

 crowded with organisms for satisfactory examination of individual 

 cells— the ultimate goal of any bacterial staining process. With a 

 little practice the novice soon learns that a little bacterial culture 

 will go a long way. 



Once the organisms have been introduced into the drop of 

 liquid on the slide, the film should be spread out to provide thick 

 as well as thin areas of material to be stained. Drying is usually 

 best accomplished by allowing the smears to air-dry, but if more 

 rapid drying is desired, no more heat should be applied to the slide 

 than would be comfortable for your own fingers. It it is too hot for 

 your hand, it is certainly too hot for the bacteria in the wet smear 

 where abundant moisture intensifies the heating action. Bacteria 

 can tolerate higher temperatures of dry heat than they can with- 

 stand moist heat. It's not the heat, it's the humidity! Over-heat- 

 ing wet smears is undesirable, otherwise staining mav not be 

 characteristic. Some persons advocate a quick passage of the 

 dried film through an open flame to "fix" the smear more firmlv to 

 the slide. The value of this technic is debatable. 



Stains may be classified into two major groups: (1) General 

 Stains in which basic dyes are usually employed to make bacteria 



