60 REPORT OF DEPARTMENT OF BACTERIOLOGY:OF THE 
Medium 2.31.— This was the medium used in isolation of yeasts. 
Mcdium 3.00.— This is our plain peptone agar and like 2.00 was 
little used in these studies. 
COUNTING. 
The plates made as above described were ordinarily held ten days 
in the laboratory before counting. The temperature approximated 
70° F. (21° C.) but varied 10 degrees from this at different seasons 
of the year. The average of the numbers found on the plates at 
the end of ten days was taken as the germ content of the cheese at 
the date of sampling. The counting was done with a hand lens 
magnifying 4 diameters and in most cases the entire number on the 
plate was enumerated but where the numbers present were exces- 
sive the counting was restricted to ten square centimeters or in some 
few instances was done under the microscope using a magnification 
of 10 to 20 diameters. The ease and accuracy of the hand counting 
was increased by using a glass plate ruled in square centimeters and 
fractions as a guide and by an automatic recorder for enumerating 
the colonies. After the plates had been counted for total numbers 
they were recounted to find the number of each of the types of } 
colonies represented. 
DIFFERENTIATION OF TYPES. 
The colonies on the plates were studied with a view of recog- 
nizing and isolating all the different types of germs. This dif- 
ferentiation of bacteria by means of the colany growths on the 
gelatin plates was one of the most important parts of our work. 
Solid media, gelatin in particular, are universally used by bacteri- 
ologists for differential study. While the gelatin medium is a great 
improvement over the liquid media used by Duclaux in his cheese 
investigations, it has its limitations. 
A large number of forms necessarily find their way into the milk 
during its collection and its manufacture into cheese and they de- 
velop there at different rates depending on how well they are 
adapted to the conditions which prevail. When a sample of the 
cheese is plated it is probable that not all of the classes of germs 
present will be represented by colonies on the plate owing to the 
dilution used. In this work the dilutions in each cheese ranged 
from I-1000 to 1-200000. The kinds present in the cheese in num- 
