262 Develop^nent of a Standardised Agar Medium 



unsuitable for use in bacterial count work, owing to the very rapid 

 development of moulds which took place on it. 



Tests were also made with media containing glycocoU and alanine. 

 These media were found to give results comparable with but not better 

 than those obtainable with asparagine. 



The entire omission of organic nitrogen was found greatly to reduce 

 the spreading, but the number of bacterial colonies which developed 

 from a suspension of Rothamsted soil, when plated on such a medium, 

 was so much reduced by the omission of organic nitrogen, that the 

 medium was unsuitable for counting work. 



It was therefore decided to use asparagine as the organic nitrogen 

 supply in the medium, and trials were made to ascertain the concentra- 



4 5 6 7 

 Days growth. 



Fig. 9. B. dendmidcs. Effect of asparagine content on spreading. 



tion which produced the least spreading of B. dendroides, while allowing 

 the best colony development when the medium was used for counting 

 other bacteria. In these trials medium CV (p. 259) made up with 1-5 per 

 cent, agar was used. The technique used to estimate the spreading was 

 similar to that described above. Fig. 9 shows the grow^th area of B. 

 dendroides with 0-1 per cent, and 0-05 per cent, asparagine. The colony 

 development on these two media was also tested, eight parallel platings 

 of a suspension of Broadbalk soil being made on each medium. The 

 colony counts obtained were significantly higher on the medium con- 

 taining 0-05 per cent, asparagine. 



The increase in the asparagine content is thus detrimental as it both 

 favours the spreading of B. dendroides and is harmful to the develop- 

 ment of other organisms. 



