MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 121 



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 p or less in diameter. The short diameters of bacilli 

 and spirilla also are less than i n, as a rule, while the length may 

 be several micra. The anthrax bacillus (1.5 /* X 3 to 10 yu) and 

 the spirillum of relapsing fever are the largest bacteria known 

 to be pathogenic to man. To say that a microccus is i n 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 staphylococcus pyogenes aureus contains 

 8,000,000,000 micrococci. 



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 ex- 

 hibit 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). Some- 

 what 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 irregular and often 

 bizarre shapes, and these are called involution forms. It is- not 

 uncommon 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 

 enclosed in such capsules. The capsule is more likely to be 

 demonstrated when the bacteria are obtained from the fluids 

 derived from an animal's body than when they have been 



