MORPHOLOGY AND PHYSIOLOGY OF BACTERIA, 121 



pathogenic. The adjectives facultative, or optional, and 

 obligate, or strict, are used to qualify the above terms and 

 many others. 



Size. Bacteria vary greatly in size. The micrococci are 

 usually i [J. or less in diameter. The short diameters of 

 bacilli and spirilla also are less than i fJ. as a rule, while 

 the length may be several p. . The anthrax bacillus (1.5 A* 

 X 3 to 10 p) and the spirillum of relapsing fever are the 

 largest bacteria known to be pathogenic to man. To say 

 that a micrococcus is i p. in diameter means that 25,000 

 end to end would make a line I inch long. It has been 

 estimated that I milligram of a pure culture of the staphy- 

 lococcus pyogenes aureus contains 8,000,000,000 micro- 

 cocci. 



There is good reason for believing that organisms exist, 

 which are too small to be visible with the most powerful 

 microscopes. The nature of these organisms is not known, 

 but it is not improbable that some of them are bacteria. 

 (See pleuro-pneumonia of cattle, etc., Part II., Chapter V.) 



In stained preparations the bodies of bacteria frequently 

 seem to be homogeneous. On the other hand, they may 

 exhibit certain spots which stain more intensely than others, 

 the stained spots alternating with clear areas. The dark- 

 staining granules may take a slightly different shade of color 

 from the rest (metachromatic granules, Babes-Ernst 

 bodies). Somewhat similar appearances may result from 

 changes in the density of the protoplasm of bacteria, leaving 

 vacuoles that do not stain (plasmolysis). 



In old cultures bacteria are likely to show deformed and 

 twisted outlines called involution forms. It is not uncom- 

 mon for bacteria to be enclosed in a kind of envelope of 

 some clear substance, which stains with difficulty or not at 

 all, called a capsule. The paired micrococci of pneumonia 

 are inclosed in such capsules. The capsule is more likely to 



