241 



ON THE DEVELOPMENT OF A STANDARDISED 



AGAR MEDIUM FOR COUNTING SOIL BACTERIA, 



WITH ESPECIAL REGARD TO THE REPRESSION 



OF SPREADING COLONIES i 



By H. G. THORNTON. 



{From the Bacteriological Department, 

 Rothamsted Experimental Station.) 



(With 13 Text-figures.) 



1. Introduction. 



The recent developments in our knowledge of soil organisms emphasise 

 the necessity for quantitative methods of research on this subject. 



An accurate method of estimating the rise and fall of bacterial 

 numbers in the soil, must underlie the study of soil micro-organisms, in 

 their relations both to each other and to the fertility of the soil. The 

 present work was undertaken with the object of increasing the accuracy 

 of the plate method for counting soil bacteria. 



When considering this method, it is necessary to bear in mind what 

 kind of information should be obtainable by its use. The method has 

 strict limitations. Thus, not all the physiological groups of soil bacteria 

 are able to develop and produce colonies on any single medium. Yet 

 the medium used should enable the great majority of bacteria in the soil 

 to form colonies upon it, and in speaking of bacterial numbers we usually 

 recognise the exclusion of those few groups which need special media. 

 There are, however, other sources of error in the method, which still 

 further reduce the number of colonies which appear on the platings. 

 Chief amongst these would seem to be the adherence of bacteria to soil 

 particles, the death of some of the organisms during the diluting and 

 plating processes, and the interference between colonies on the plate. 

 The under-estimation of bacterial numbers obtained by the plate method 

 is well shown by work, such as that of Breed and Stocking (i) on the 



^ A grant in aid of publication has been received for this communication. 

 Ann. Biol, ix 16 



