24 CONDITIONS MODIFYING [CH. n 



In other cases the prolonged growth of two species together 

 appears to produce no change whatever in either of them. 

 Williams (1902) grew a virulent streptococcus and a virulent 

 diphtheria bacillus together, transplanting every three or 

 four days for 90 such "generations" without influencing the 

 characters of either organism. Horrocks (1911) grew B. typho- 

 sus and B. fluorescens non-liquefaciens together for a period 

 of four months. Examinations made at intervals of one week 

 throughout this period revealed no alteration in the character 

 or agglutination properties of the B. typhosus. 



The methods of studying the effect of symbiosis are various. 

 Simultaneous growth can be studied by "sowing" different 

 species of bacteria together indiscriminately on the surface of 

 the medium ; or distinct colonies may be grown on a plate so 

 that at first a considerable space intervenes between colonies 

 of the different species, the interval gradually lessening as the 

 colonies extend until it is finally obliterated ; a third method 

 is that of "criss-cross " planting, the effects of symbiosis being 

 seen at the intersection of the lines of growth ; or fourthly, 

 one species may be grown on the surface of the medium and 

 another deep to it in the form of buried colonies ; or fifthly, 

 a double celluloid sac may be utilised in which the products 

 of bacterial growth can diffuse from one compartment to the 

 other; finally, successive growth offers a further means of 

 investigation, the medium being sterilised after the growth 

 of one species and then planted with the other. 



12. The methods of modifying bacteria which remain to 

 be described all involve the agency of the living tissues and 

 may be regarded as forms of parasitism. 



(a) Transmission through the alimentary canal is said in 

 some cases to bring about modifications in character. It has 

 been suggested that the bacillus of Aertryck may assume 

 the characters and agglutinative properties of B. enteritidis 

 Gaertner after transmission through the intestine of the 

 mouse. 



Bahr (1912) found that the fermenting powers of certain 

 strains of dysentery bacilli were modified after they had been 

 passed through the intestine of the fly, although for nine 



