264 Development of a Standardised Agar Medmm 



Table IV. 



Effect of Nitrate and Asparagine on the spreading of 

 Bacillus dendroides. 



It will be seen that complete omission of KNO3 in medium A is 

 without effect on the spreading, though the omission of asparagine checks 

 the spreading even where the KNO3 is increased to 0-1 per cent. A 

 reduction in the KNO3 content below 0-05 per cent, is therefore of no 

 assistance in reducing "spreading." 



The effect of a higher concentration of KNO3 was next tried. Two 

 media were compared, having nitrogen supplied as follows: 



A. 0-2 per cent. KNO3 0-05 per cent, asparagine. 



B. 0-05 per cent. KNO3 0-05 per cent, asparagine. 



Fig. 11 shows the increase in area of surface growth of B. dendroides 

 on these two media, each point on the curve representing the mean of 

 twenty parallel plates. 



It will be seen that in the presence of asparagine, the higher con- 

 centration of KNO3 increases the spreading on platings. A suspension 

 of Barnfield soil was plated on the above media, and on one containing 

 0-1 per cent. KNO3 + 0*05 per cent, asparagine, and no significant differ- 

 ence could be found in the number of colonies developing on the three 

 media. There is thus no advaiitage in lowering the concentration of 

 KNO3 below 0'05 per cent, while a higher concentration tends to stimu- 

 late spreading. This percentage was therefore adopted for use in the 

 medium. 



The additional source of energy in the medium was supplied as 

 mannitol. This compound was used in preference to a sugar for reasons, 

 dealt with below, connected with the change in reaction during sterilisa- 

 tion. Experiments on the effect of varying percentages of mannitol on 

 the spreading of B. dendroides were made. It was found that an increase 

 in the mannitol content from 0-05 per cent, to 0-1 per cent, did not stimu- 

 late spreading but that a further increase of 0-2 per cent, caused slightly 

 more spreading growth. Counts were also made of the number of colonies 



