106 ATLAS OF BACTERIOLOGY. 



thrive on the admission of air, if other aerobic varie- 

 ties are present (vide bacillus tetani). 



2. Certain chemical actions, for example, the de- 

 composition of nitrate into gaseous nitrogen cannot 

 be effected by some bacteria alone, while it can be 

 done by two forms in combination. This experience 

 is to be remembered in looking for the producers of 

 certain decompositions. When the isolated varieties 

 do not act singly or act incompletely, combinations 

 must be examined. 



3. In a similar way it has been observed, for ex- 

 ample, that among a series of soil bacteria each 

 single variety is not pathogenic, while certain com- 

 binations, when introduced into the animal, make 

 the latter sick. This experience also merits special 

 attention in the search for the producers of a new or 

 obscure disease. 



Some writers also assume the production of cholera 

 by two germs (diblastic theory). 



4. Feeble pathogenic varieties (for example, atten- 

 uated tetanus bacilli) are said to gain in virulence 

 when cultivated with bacterium vulgare. 



D. The Conditions of Formation and Germi- 

 nation of Spores. 



BIOLOGICAL CHARACTERS or SPORES. 



The extent of the formation of endogenous spores 

 appears to be imperfectly known at the present time. 

 Apart from a large group of bacilli which are re- 

 lated to bacillus anthracis and bacillus tetani, un- 

 doubted endogenous spores are known only in sarcina 



