270 Development of a Standardised Agar Medium 



gm. 



KNO. 



The following medium was used as a basis in these tests: 

 K2HPO4 

 MgSO^.THaO 

 CaCl ... 

 NaCl ... 

 FeClg ... 



Asparagine 



Agar 



Water 



0-5 gm. 

 0-5 „ 

 15-0 „ 

 1000 c.c. 



1-0 



0-2 



0-1 



0-1 



0-002 „ 



This medium was divided into five portions to which were added the 

 following compounds in 0- 1 per cent, concentration : 



A. Dextrose. B. Saccharose. C. Mannitol. D. Lactose. E. Gly- 

 cerine. 



The media were adjusted to a H-ion concentration of ^^H 7-05 imme- 

 diately before sterihsation for 15 minutes at 15 lbs. pressure. Directly 

 after autoclaving, the reaction was again measured. Six parallel platings 

 of a diluted suspension of Rothamsted soil were made on each medium. 

 Table V shows the changes of reaction during sterihsation and the mean 

 number of colonies per plate with each medium. Table VI shows the 

 results of another similar experiment in which glucose, saccharose and 

 mannitol media were compared. 



Table V. 

 Change in Reaction of Media on Sterilisation. 



Table VI. 



Change in Reaction of Media on Sterilisation. 



Energy 

 material 



Dextrose 



Saccharose 



Mannitol 



pH value 



before 



sterilisation 



7-2 

 7-2 

 7-2 



pH value 



after 



sterilisation 



6-7 

 6-7 

 6-95 



No. of colonies 

 on each plate 



14, 21, 13, 14, 14, 13, 14, 13 

 19, 17, 17, 16, 15, 14, 13 

 19, 19, 22, 22, 18, 17, 17, 17 



Mean 

 no. of 

 colonies 



14-5 

 15-8 

 18-9 



It will be seen that the medium containing mannitol changes least 

 in reaction during sterilisation and at the same time gives a good colony 

 development. With lactose, the number of colonies was somewhat re- 

 duced and the individual colonies were dwarfed. With glycerine, marked 



