THE USE OF WASHED AGAR IN CULTURE MEDIA 



S. HENRY AYERS, COURTLAND S. MUDGE, and PHILIP RUPP 



From the Research Laboratories of the Dairy Division, United States Department 



of Agriculture 



Received for publication May 27, 1920 



In connection with the preparation of milk-powder agar it 

 was found that it was necessary to wash the agar in order that 

 it might be used in the medium without causing a precipitate 

 during sterilization. Qualitative tests indicated that the calcium 

 and magnesium salts in the agar probably combined with the 

 phosphate used in the preparation of the milk-powder solution 

 and the phosphates were precipitated during sterilization. To 

 overcome this difficulty, the agar was washed in distilled water 

 in order to reduce its salt content. 



Soil bacteriologists have employed washed agar for many 

 years. It has been washed in order to remove as much as pos- 

 sible of the soluble organic matter which is believed to be detri- 

 mental to the growth of nitrifying bacteria. Sternberg in his 

 "Textbook of Bacteriology" recommended that agar be soaked 

 in cold water for twenty-four hours; this was advised in order 

 to facilitate dissolving and filtering. It is the practice in some 

 laboratories to wash agar, apparently more for the sake of a 

 cleaner preparation than for any other reason. 



Washed agar gave such satisfactory results in the milk-powder 

 medium that its use in the regular standard peptone extract 

 medium was tried. It is the purpose of this paper to present 

 briefly the results of this work and to show the effect of washing 

 on the calcium and magnesium content of agar. 



The results in table 1 show in a decided manner the effect on 

 the bacterial count of using washed agar. While there was not 

 an increase in all samples, there was in many cases an increase 

 beyond any experimental error. As a rule, the higher counts 

 were found when pasteurized milk was examined. 



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