IOWA ACADEMY OF SCIENCES. 143 



The comparisons between lactose agar and lactose gelatin 

 are not sufficient in number to show that either has the ad- 

 vantage in the number of the colonies developed. 



The results would indicate that other factors than the 

 media used controls in these comparisons. The acid 

 organisms develop about equally well in the two, but as 

 the bacteria constituting the remainder of the flora vary in 

 species, it is probable that some samples of milk contained 

 those developing best at the lower temperature of gelatin, 

 while others find the higher temperature of the agar most 

 favorable. 



Harding* uses lactose agar kept at a temperature of 30^ 

 C, in his quantitative work and finds that it gives a higher 

 number than gelatin at room temperature. In regard to 

 the relation of media to the kind of bacteria developed 

 and the kind repressed, one fact stands out clearly. In 

 media without lactose the acid organisms develop very 

 slowly, especially upon agar. In two cases this media 

 showed no acid germs present, while the lactose agar 

 showed that they constituted 36 per cent and 20 per cent of 

 the whole number. 



Peptone gelatin shows some acid organisms but com- 

 parative few of the number are present. The proportion of 

 acid bacteria developing upon lactose agar and lactose 

 gelatin is much the same. The data shows that all three 

 classes grow in less numbers upon peptone agar than 

 upon other media. The enzyme producing seem to develop 

 rather better as a rule on gelatin than on agar. Those 

 having no effect appear to find the lactose agar the most 

 suitable medium for growth. 



It is evident that erroneous conclusions may be drawn, 

 as some investigators have done, from using peptone media 

 in' work with milk bacteria, either regarding the number 

 present, or the species represented. 



*Bul. 172, New York Exp. Station. 



