22 CONDITIONS MODIFYING [CH. ii 



unable to establish high and low acid producing races of 

 streptococcus lacticus. Glenn (1911) failed to produce high 

 and low acid producing strains of B. proteus. Other observers 

 have failed in attempts by selection to develop a particular 

 morphological type of the diphtheria bacillus (Clark, 1910) 

 and to modify the agglutination reactions of B. typhosus 

 (Moon, 1911). 



11. Symbiosis is known to influence the behaviour of 

 bacteria and has in many cases a marked eflect on the character 

 of one -or other of the organisms growing together. The 

 phenomenon of symbiosis is a familiar one in vegetable life. 

 The individual struggle for existence is observed to give place 

 occasionallyto a permanent partnership between two organisms 

 for their mutual benefit An example of such cooperation is 

 furnished by the Lichens each of which is a dual organism 

 composed of a fungus and an Alga. Vegetable life as a whole 

 is dependent upon the activity of nitrifying organisms in the 

 soil and in some cases a definite alliance is formed between 

 the two parties, as in the case of the Leguminosae and the 

 nitrifying bacteria which take up their residence in the root 

 nodules of these plants. 



All forms of mutual parasitism are in reality examples of 

 symbiosis. One species of bacteria may, however, be dependent 

 for its growth upon another species without necessarily being 

 parasitic. Thus Pasteur advanced the theory that aerobic 

 bacteria by exhausting the supply of oxygen gave anaerobic 

 bacteria a chance of growing. 



Allen (1910) found that a strain of B. inflvenzas, which 

 could not be grown on ordinary media, grew luxuriantly 

 on sterilised media in which the staphylococcus albus had 

 previously grown. Neisser was able to cultivate the same 

 bacillus on plain agar for several generations by growing the 

 Xerosis bacillus with it, though a dead culture of the latter 

 had not the same favouring effect (Muir and Ritchie). 



In other cases the growth of one species is inimical to the 

 growth of another. B. typhosfus wiU not grow in a filtered 

 broth culture of staphylococcus albus, nor in that of many 

 «ther organisms (Freudenriech, 1888). The meningococcus is 



