H. a. Thornton 247 



used both for bacteria and fungi, where rehance is jjlaced on such variable 

 impurities as may be present in the agar, to supply the electrolytes needed 

 for growth. In the present work the Ca, Mg and S salts used in Conn's 

 sodium asparaginate agar (3) have been employed with the addition of 

 NaCl as a source of sodium. 



2. The Spreading Growth of Organisms on Agar Plates. 



Unfortunately agar, when used in a count medium, has one defect 

 that is so serious as to have deterred some ^\*orkers from its use. Certain 

 commonly occurring soil bacteria form rapidly spreading surface colonies 

 on agar, which, in many platings, cover the agar surface and inhibit or 

 interfere with the development of other colonies. These organisms are so 

 abundant in Kothamsted soil that on meat-extract peptone agar a large 

 percentage of platings are spoilt, and accurate bacterial counts are im- 

 possible on such a medium. If the amount of organic nitrogen com- 

 pounds in the medium be reduced, there is less growth of the spreading 

 organisms. The fact has long been reahsed and led to the development 

 of such media as Lipman and Brown's agar medium (6), containing only 

 0-05 per cent, peptone. However, considerable "spreading" still takes 

 place on such media as the above. Conn (3) noticed this fact, which I 

 have also observed with this and with other peptone media. Less 

 "spreading" seemed to occur on media containing simpler organic 

 nitrogen compounds, such as Conn's sodium asparaginate agar. This 

 indicates that a mere reduction in the amount of organic nitrogen in 

 the medium is not an efficient means of checking spreading colonies, but 

 that the nature of the compounds used is of importance. A more exact 

 knowledge of the conditions which control the growth of spreading 

 colonies, and especially of the effect on them of the composition of the 

 medium, appeared necessary. 



It was therefore decided to study the behaviour in pure culture of 

 an organism which formed spreadiiig colonies, in the hope that the know- 

 ledge thus obtained would enable a medium to be developed upon which 

 the formation of spreading colonies would be restricted. By far the 

 most abundant of these spreading organisms in Rothamsted soil is spore- 

 forming bacillus which appears to be similar to B. dendroides described 

 by Holzmuller(9) in his paper "Die Gruppe des Bacillus mycoides." 

 The organism, however, would appear rather to belong to the B. suhtilis 

 group. The strain here used has the following characters^. 



^ I am indebted to Mr P. H. H. Gray for having worked out the characters of the 

 organism in this laboratory. 



