260 Development of a Standardised Agar Medium 



It therefore appeared probable that the amount of spreading would be 

 correspondingly decreased on a medium containing amino-acid as the 

 source of organic nitrogen. I therefore measured that increase in area 

 of surface growths of B. dendroides upon the media employed in the last 

 experiment, made up with 1-5 per cent. agar. A medium was also tested 

 containing equivalent nitrogen in the form of tyrosine. Tubes of each 

 medium were autoclaved and poured into sterile petri dishes. Each 



60|- 



50 



10 



C3 30 



20 



10 



" Lemco ' 



Peptone 



Asparagiiie 



Days growth. 

 Fig. 7. B. dendrtiiih's. Effect of the source of organic nitrogen on spreading growth. 



plate, when the agar had set, was inoculated at the centre with 0-01 c.c. 

 of a suspension of a 48-hour old culture of B. dendroides. The plates 

 were incubated at 20° C, and the area of growth on each plate was 

 measured at intervals, over squared paper. Eight parallel platings of 

 each medium were prepared and the mean areas of growth are shown 

 in Fig. 7, in which the area of growth is plotted against the time. 



The curves show that spreading growth is very much reduced when 

 peptone or meat'extract is replaced in the medium by a simple amino- 



