MICEOBES— INFECTION— IMMUNITY 453 



and lungs where they produce a pneumonia, and another has 

 been found in the mouth, Oidium albicans, where it causes 

 ' thrush.' This latter is now considered to be a saccharo- 

 myces. 



The Blastomycetes or Yeasts are connected with the moulds. 

 To this group belong the organisms which produce alco- 

 holic and other fermentation. A pathological organism 

 is the Saccharoviyces farciminosus, which produces Epizootic 

 lymphangitis in horses, a disease which in appearance 

 closely resembles farcy. 



Conditions of Bacterial Growth. — Bacteria are vegetable 

 organisms devoid of chlorophyll, and depend for their exist- 

 ence on organic matter; this being so it has been reasonable 

 to consider bacteria in the light either of parasites or sapro- 

 phytes. We now know that certain bacteria may derive 

 either their carbon or nitrogen directly from the air, while 

 others again can live on simple inorganic compounds which 

 contain no free nitrogen. Such bacteria are found in the 

 soil, and as nodules on the roots of leguminous plants, and 

 their existence facilitates our conception of how nutrition was 

 carried on when life first began on this planet. 



All bacteria require water, salts, carbon, and nitrogen. The 

 pathogenic bacteria prefer an alkaline or neutral medium 

 to grow in, while the moulds prefer one which is acid. 



Some bacteria require oxygen for their growth and are 

 known as aerobes, others can only live in the absence of 

 oxygen, anaerobes, while still a third group can live either 

 with or without oxygen. Thus there are obligatory aerobes 

 and obligatory anaerobes, facultative aerobes and facultative 

 anaerobes. Heat is necessary for the development of bacteria, 

 and it is found that all pathogenic bacteria thrive best at the 

 temperature of the animal's body. Most, but not all, 

 bacteria may be cultivated artificially apart from the body 

 by providing the necessary atmosphere, nutriment, and 

 heat. It is found experimentally that by altering the 

 temperature at which an organism grows, changes may 

 occur in its virulence, and advantage is taken of this fact in 

 preparing attenuated viruses. 



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