Chapter 20 



Microorganisms 



In this chapter, to simplify the specificity of staining methods, the 

 microorganisms will be broken down into three groups: bacteria, vi- 

 ruses, and fiuigi. 



Bacteria. Bacteria are customarily studied by direct microscopic ob- 

 servation and differentiated by shape, grouping of cells, presence or 

 absence of certain structures, and the reaction of their cells to differen- 

 tial stains. Bacteria may be stained with aniline dyes; in a single dye, 

 in mixed dyes, in polychromed dyes, or by differential methods. One of 

 the most universally used stains was developed by the histologist Gram 

 while he was trying to differentiate the bacteria in tissue. His method 

 separates bacteria into two groups, (1) those that retain crystal violet 

 and are said to be Gram positive, and (2) those that decolorize to be 

 stained by a counterstain and are said to be Gram negative. 



Some bacteria of high lipid content cannot be stained by the usual 

 methods but require heat or long exposure to the stain. They are also 

 difficult to decolorize and resist acid alcohol. These have been given the 

 name acid fast. The spirochete forms will stain only faintly if at all and 

 must be colored by silver methods. 



Many bacteria form a capsule from the outer layer of the cell mem- 

 brane, and the capsule appears like a halo aroimd the organism, or over 

 a chain of cells. This capsule will not stain in the customary stains; 

 Hiss's stain (page 317), however, is simple and usually effective in this 

 situation. Some bacteria are able to form spores that can be extremely 

 resistant to injurious conditions (heat, chemicals). Boiling will destroy 

 some of these, but many spores are more resistant. Some bacteria have 

 flagella, filamentous appendages for locomotion. 



Bacteria can be classified into three groups according to shape. 



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