no MANUAL OF BACTERIOLOGY. 



which exist upon the living tissues or fluids of any organism. 

 Nearly synonymous with the above words are those which do 

 not and those which do produce disease, or non- pathogenic and 

 pathogenic. The adjectives facultative, or optional, and ohli- 

 gate, or strict, are used to qualify the above terms and many 

 others. 



Size. — Bacteria vary greatly in size. The micrococci are 

 usually I /i or less in diameter. The short diameters of bacilli 

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

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

 the spirillum of relapsing fever are the largest bacteria known 

 to be pathogenic to man. To say that a micrococcus is i /i 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 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 (meta- 

 chromatic granules, Babes-Ernst bodies). Somewhat similar 

 appearances may result from changes in the density of the pro- 

 toplasm of bacteria, leaving vacuoles that do not stain (plas- 

 molysis). 



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. 



