USE OP WASHED AGAR IN CULTURE MEDIA 



595 



certain milk-powder media the milk powder was dissolved in 

 such a way as to leave the calcium phosphate in the medium. 

 With such media low counts were noticed. 



Having these facts in mind and knowing that at least one of 

 the effects of washing agar was a partial removal of the calcium 

 and magnesium salts it occurred to us that this lowering of the 

 percentage of these salts might be one of the reasons for the 

 increased counts when washed agar was used in the standard 

 medium. To confirm this opinion, counts were made on three 

 media, all of which contained 0.5 per cent peptone and 0.3 per 

 cent extract, and had the same reaction but made up with differ- 

 ent kinds of agar. One contained regular shred agar, one washed 

 agar, and the other washed agar with sufficient CaS0 4 and MgS0 4 

 added to make up for the calcium and magnesium (calculated 

 as CaO and MgO) removed by washing. 



TABLE 3 

 Effect of addition of calcium and magnesium salts to a washed-agar medium 



A few samples of pasteurized milk were plated on these three 

 media. The results in table 3 indicate that the addition of these 

 salts to the washed-agar medium reduced the counts so that 

 they agreed with those on the regular agar medium. 



We do not intend to convey the idea that this is an explanation 

 of the higher counts. Perhaps similar results may never be 

 obtained again, but at least they indicate that these salts may 

 play an important part in culture media. 



