56 VETERINARY BACTERIOLOGY 



pressure of the solutes. The action of sugars, salts, etc., is in 

 the nature of a physical antiseptic. Physiological salt solution 

 is one having the same concentration of salt as do the body cells 

 of the particular organism to be studied. It usually contains 

 .85 per cent, of sodium chlorid. 



SYMBIOSIS, ANTIBIOSIS, AND COMMENSALISM 



Two organisms that live together and which are mutually 

 beneficial are said to live in symbiosis. Each organism is called 

 a symbion or symbiont. The symbionts are not necessarily closely 

 related forms and may belong to the most widely separated groups 

 of plants, as, for example, bacteria and members of the bean family 

 of the flowering plants. 



Antibiosis is that condition which obtains when organisms 

 prove inimical to each other's development. The growth of one 

 species of organism in a culture-medium may completely inhibit 

 the development of some other type. For example, the organism 

 (Streptococcus lacticus) which ordinarily sours milk prevents the 

 development of most other species. 



An organism which uses the by-products of another as food, 

 in other words, is parasitic without producing disease, is called a 

 commensal. Many of the bacteria found on the skin, in the mouth, 

 and in the intestinal tract of man and animals are of this character. 



All degrees of intergradation between symbiosis, true para- 

 sitism, and commensalism have been described for different 

 species. 



PIGMENT PRODUCTION BY MICROORGANISMS 



Molds, yeasts, and bacteria are frequently found to be chromo- 

 genic, that is, capable of producing pigments or coloring-matter. 

 The colors produced range through all the colors and even shades 

 of color of the spectrum. A few only of the pathogenic forms 

 produce pigments. Many organisms, particularly molds and 

 bacteria, excrete a pigment-forming substance which diffuses 

 through the culture-medium and colors it. Such an organism 

 is the Bacillus pyor>/<in( //*, which produces a diffuse pigment, 

 one that changes the medium first to a green, then to a brown. 

 Some organisms produce pigment granules outside the cell, such 

 are said to be chromoparous, for example, Bacillus prodigiosus, 



