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 80° 
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 86 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 
f ir work with milk bacteria, either regarding the number 
present, or the species represented. 
*Bul. 172, New York Exp. Station. 
