484 JAMES M. SHERMAN 



bacterial examination of milk, probably the strongest point in 

 its favor is the difference in size of colonies which develop on the 

 two media. The colonies on lactose agar are always consider- 

 ably larger than those on plain agar, this being especially true 

 of the acid-forming organisms. Photographs were taken of the 

 petri dish cultures obtained and reproductions of some typical 

 plates are appended to this paper. As was stated before, the 

 plain agar plates were counted with the aid of a hand lens as 

 prescribed by the standard methods for the bacterial analysis of 

 milk, whereas the plates containing lactose agar were counted 

 with the naked eye. This practice was followed because the 

 colonies on lactose agar were of sufficient size to make the use of 

 a lens unnecessary. Many trials have failed to reveal colonies 

 on lactose agar under a hand lens which were not visible to the 

 unaided eye. This, however, might not be true in the case of a 

 worker with defective eyesight. 



In order to get some definite information on the difference in 

 size of colonies on the two media, pure cultures of B. lactis-acidi — 

 which is usually the predominating organism in ordinary market 

 milk — and a streptoccus — which is usually the predominating 

 organism in milk of a certified grade — were plated out on plain 

 and lactose agars, and the colonies which developed were meas- 

 ured after incubation at 37°C. for forty-eight hours. Only plates 

 which contained less than one hundred colonies were used so as 

 to eliminate as far as possible the factor of inhibition. The ten 

 largest colonies on each plate were measured. These data are 

 given in table 3. 



These differences in size of colonies are presented graphically 

 in figures 1,2, and 3. 



DISCUSSION 



It should be understood that the data here presented are in- 

 tended to apply only to the routine examination of milk for which 

 a short period of incubation is used. Although the few data at 

 hand indicate that quite similar results are obtained with lower 

 temperatures and longer periods of incubation, it is not within 

 the province of this paper to draw such conclusions. 



