On the Development of a Standardised Agar Medium for Counting Soil Bacteria, With Especial Regard to the Repression of Spreading Colonies1

For bacterial count work the first essential in a medium is that it should be uniform and reproducible in its results. In the medium here described, details of which are given on p. 271, reproducibility has been achieved by the use of pure chemical compounds in an agar medium and by selection of such constituents as will not produce a significant change of reaction during sterilisation. On agar media, surface spreading colonies interfere with the accuracy of the results. A special study was made of a common spreading organism, B. dendroides. This organism spreads by active motility, and the factors controlling its spread were found to be (1) the existence of a surface film of water on the agar, and (2) the rate of multiplication previous to the drying of this film. A medium was developed on which this rate of multiplication was greatly reduced and on which, consequently, spreading is greatly restricted. Tests of the medium have shown that the results obtained with it are uniform and can be reproduced in different batches of medium.