28 



INHIBITORY ACTION OF PARATYPHOID BACILLI 



The Effect of the Concentration of Lactose. — In the foregoing tests 

 1 per cent lactose was used throughout. It was thought, however, 

 that less or more might affect the uniformity of results. Before 

 taking up the experiment it became necessary to consider what mini- 

 mum per cent of lactose was necessary for Bacillus coli to produce 

 the maximum amount of gas. Earlier, repeated trials by one of us 

 had shown that 0.5 per cent lactose yields about as much gas as 1 per 

 cent or more. Table IV shows that the same result is obtained 

 whether a concentration as low as 0.5 per cent or as high as 2 per 

 cent lactose is used. Numerous tests of the gas formula in these 

 experiments indicated no appreciable change in the relative amounts 

 of H and CO2. These tests have therefore been omitted from the 

 tables. 



TABLE IV. 



Effect of Different Concentrations of Lactose on Secondary Fermentation by B. coli, 



Calf 302. 



Primary culture. 



Paratyphoid 297, 

 Hog-cholera XII 

 Paratyphoid 297 

 Hog-cholera XII 



The absence of lactose in the primary culture does not interfere 

 with the establishment of inhibition. Cultures of paratyphoid bacilli 

 in fermented bouillon without lactose will inhibit subsequent gas 

 production by Bacillus coli when the latter and lactose are added 

 at the same time after the primary culture has grown 4 or more days. 

 To ensure the above results, there must be distinct clouding of the 

 closed arm by the paratyphoid bacilli. Absence of multiplication 

 due to a too thorough removal of muscle sugar fails to establish 

 inhibition. 



Effect of the Age of the Primary Culture. — Certain experiments had 

 indicated that the age of the primary culture before inoculation with 

 Bacillus coli was not an indifferent factor. Taking the combination 



