268 Development of a Standardised Agar Medium 



In investigating the effect of the H-ion concentration of the sub- 

 stratum on the number of colonies that developed thereon a medium of 

 the following composition was employed: 



The medium was filtered at 100° C. and divided into 200 c.c. portions 

 which were sterilised at 15 lbs. pressure for 15 minutes in the autoclave. 



120 



1101- 



100 



90 h 



80 



70 



60 



50 



^H 6-4 6-6 6-8 7 7-2 7 4 7-6 7-8 



H-ion concentration of medium. 

 Fig. 13. Effect of the reaction of the medium on colony development. 



The H-ion concentrations of the media were then adjusted to values 

 ranging from 'pR 645 to pR 7-8, with sterile iV/10 HCl and iV/10 NaOH, 

 using aseptic technique. A single diluted suspension of Rothamsted soil 

 was plated on each medium, six to eight parallel platings being made in 

 each case. The colonies were counted after 10 days incubation at 20° C. 

 In Fig. 13 the mean number of colonies per plate on each medium is 

 plotted against the pYL value. The differences in colony development on 

 media ranging from jpR 6-8 to pR 7-8 are barely significant, having regard 

 to the variance between parallel platings, there being, however, some 

 indication of an optimum reaction near neutrality. On the acid side, 



