THE INFLUENCE OF SYMBIOSIS. 81 



and there is evidence tending to support the hypothesis that the higher 

 parasitism results from an evolution as it were, produced hy many 

 factors, of which symbiosis is one. Most of the primary lesions brought 

 about by the bacillus are formed in or near the skin or mucous surfaces 

 and are probably influenced symbiotically or otherwise by the complex 

 bacterial flora constantly present in these places and by the resulting 

 modified tissue. 



I may quote a few of the most important observations of others in 

 taking up somewhat more in detail the question of symbiosis between 

 bacteria. 



Ludwig Hektoen has stated as follows : "It is conceivable that in 

 some cases the combined action of two microorganisms may be necessary 

 to cause a disease. The nontoxic products of two may synthesize to form 

 a toxic substance." 



Richie, 5 in his discussion of the general pathology of infection, 

 remarks that "A study of the correlation and interaction between the 

 various organisms frequently or almost invariably found together, may 

 in the future explain many clinical phenomena and observations as yet 

 obscure." And again the same author writes as follows : "The metabolic 

 products of certain, it may be harmless, microorganisms are often capable 

 of rendering a nonpathogenic genus pathogenic." 



It is generally accepted that many of the bacteria normally found 

 in the intestine may under proper surroundings produce disease. Sym- 

 biosis is undoubtedly one factor in these changes. Vallagussa found 

 that colon bacilli from the intestines of cats were more virulent when a 

 vegetable rather than a meat diet was given to the animals and that the 

 same bacilli were more virulent when taken from decomposing faeces 

 than when secured directly from the intestine. Coco demonstrated that 

 the virulence of colon bacilli was . increased by the presence of strep- 

 tococci and staphylococci, for mixed cultures caused fever when introduced 

 into the intestines of animals, while pure cultures of either did not do so. 



Atlasoff ° found that B. typhosus grew better in acid media in sym- 

 biosis with Torula rosw and that when these symbiotic organisms were fed 

 together to young rabbits, a fairly typical typhoid disease was produced. 



M. Neisser cultivated on blood agar, colonies of influenza and xerosis 

 bacilli from a case of conjunctivitis caused by measles. He continued 

 the cultivation of the mixed bacteria on ordinary agar, and was suc- 

 cessful for twenty generations with an organism which, before his work, 

 had never passed beyond two generations. The influenza bacillus would 

 not grow on ordinary agar without B. xerosis. When dead xerosis bacilli 

 were used, the cultures failed and this fact proved that it is not the body 

 of the B. xerosis which aids the growth of the other organism, but rather 



5 Albutt's System Med. (1907), 2, pt. 1, 1-198. 

 6 Ann. Inst. Pasteur, 18, 701. 



