i;i-:m-:i.;al toxsiDi^KATioN of disfasjc. 55 



ical bacteria arc less than one micron in diameter, e. g., the 

 pyogenic staphylococci average .8 of a micron in diameter. 

 Different individuals of the same species may vary considera1)ly 

 in size, thus the ISacterium anthracis mav varv fmm four to 

 ten microns in length. Some diseases are probablv the result 

 of infection with micro-organisms that are so small the\- can- 

 not be detected by the use of present da^- microscopes ami they 

 also pass through the best known germ-proof filters. These in- 

 fectious agents are designated as invisible or ultra-microscopic 

 and may be present in a "filteraljle virus." 



IMorphologicalK- bacteria are \'ery simple. Three principal 

 types of bacteria are recognized according to their form, viz : 

 the rod shaped (Uacteria), spherical (Cocci), and the spirals, 

 (Spirilla). Another type, characterized Ijv branching forms. 

 ( Chlamydo-bacteria ), has a few re])resentatives but their classi- 

 ficatiiiu as bacteria has been questioned. The representatives 

 of each of the three j)rincipal groups, P.acteria, Cocci and 

 Spirilla, are constant in their morphologv so long as the environ- 

 ments remain the same, i. e., the progenv of bacilli are bacilli, 

 etc. Again, each indi\-idual is constant in its form, increase in 

 size being the (inly change that occurs. I^'requcntly, however, 

 unfa\'oi-able conditions may cause pleomor]ihism among indi- 

 vidual organisms. 



Perhaps the best accepted morphologic classification of bac- 

 teria is as follows : — 



1. Coccaceae, spherical shaped bacteria. 



2. Bacteriaceae, rod or cylindrical shaped bacteria. 



3. Spirillaceae. spiral shaped bacteria. 



4. ChlamA-do-bacteriaceae, branching or irregular forms r>f 

 bacteria. 



According to their biologic characteristics, Ijacteria may be 

 classified as follows : — 



Aerobic or anaerobic. 



Chromogenic or non-chromogenic. 



Zymogenic or non-zymogenic. 



Saprogenic or non-saprogenic. 



Photogenic or non-photogenic. 



Thermogenic or non-thermogenic. 



Saphrop'hytic or non-saphrophytic. 



Parasitic nr non-parasitic. 



Pathogenic or non-pathogenic. 



Pyogenic or non-pyogenic, etc. 



Bacteria, like other living things, grow and reproduce under 

 favorable conditions. They grow until they attain a certain size 



