4G VETERINARY BACTERIOLOGY 



energy of the rays of sunlight in much the same manner probably 

 as do green plants. The coloring-matter in these forms, however, 

 is purple or red (bacteriopurpuriri) . Other forms living in water 

 which contains hydrogen sulphid, as in the sulphur springs, oxidize 

 the hydrogen sulfid to free sulphur and even sulphuric acid and 

 gain energy for the manufacture of their foods from carbon dioxid, 

 water, and other compounds by this process. Still other forms 

 are believed to make use of iron, ammonia, nitrites, and other 

 inorganic substances, and by their oxidation secure the necessary 

 energy. Organisms which can manufacture their own food out 

 of inorganic substances are said to be prototrophic. The proto- 

 trophic microorganisms so far as known are all bacteria or molds. 

 Most microorganisms utilize organic matter in a dead or living 

 condition for food. Those which utilize dead organic matter are 

 called metatrophic, while those requiring living material or complex 

 protein foods are called paratrophic. The latter are frequently dis- 

 ease producing. It must not be supposed that these division lines 

 are strictly drawn. For example, certain bacteria seem to make use 

 of the oxidation of carbohydrates and other organic substances to 

 enable them to take up the nitrogen from the air and to convert it 

 into usable form. Such are both prototrophic and metatrophic. 



The peculiar food requirements of the different species must 

 be kept in mind in the preparation of nutrient media for their 

 growth. Some organisms will not grow in the presence of organic 

 materials, while others require such specialized media as blood- 

 serum or hemoglobin. 



A second grouping of microorganisms commonly used is based 

 upon the relationship to other living organisms. Those which 

 do not require a living host (animal or plant) are called sapro- 

 phytes if bacteria, yeasts, or molds, and saprozoites if protozoa; 

 those which require a living host are called parasites. Those 

 parasites which do not produce disease are termed commensals. 



MOISTURE RELATIONSHIPS OF MICROORGANISMS 



Microorganisms require considerable amounts of water for 

 their development. The optimum condition for growth in 

 most cases is saturation. There is great variation in ability 

 to resist desiccation (drying). The spores of some bacteria and 



