322 ROBERT S. BREED AND W. D. DOTTERRER 



least, the true state of affairs published an article in which 

 they claim that plates having more than 100 colonies should be 

 disregarded and that under these conditions the microscope 

 should not be used for counting. In 1897, Hill (Hill and Ellms, 

 1897) contended that overcrowded plates would not give re- 

 liable results in water analysis. In 1899, Jordan and Irons 

 (1899) independently urged the same thing. Again Hill (1908) 

 called attention to the point in a paper read before the labora- 

 tory section of the American Public Health Association in 1907, 

 in which he pointed out that wide discrepancies in counts might 

 be caused by different methods of computation and concluded 

 that only those plates having numbers of colonies falling be- 

 tween 40 and 200 per plate should be considered in reporting 

 results. These figures were adopted in the report presented 

 by the Committee on Standard Methods for the Bacterial 

 Examination of Milk at the Richmond meeting of the Ameri- 

 can Public Health Association (1910). In the Report pre- 

 sented at the Rochester meeting in September 1915 (Comm. 

 Stand. Meth. 1915) the lower limit in the number of colonies 

 allowable on agar plates was changed from 40 to 30, and the 

 limits of 30 and 200 were also accepted by the Committee on 

 Standard Methods of Bacterial Water Analysis in their Report 

 presented at the same meeting. 



STATEMENT OF PROBLEM 



It is generally recognized that the kind of bacteria present 

 in the material under examination will have an influence on 

 the size of the colonies, and, consequently, on the number that 

 can develop on a plate. Some of the commonest and most 

 important bacteria in milk do not produce colonies larger than 

 pin points on ordinary agars even when only a few are present. 

 Other colonies grow large and in the case of spreaders may cover 

 the entire plate. 



Just what prevents the development of all the bacteria into 

 colonies on crowded plates is not thoroughly understood. In 

 some cases it may be because the food material is all used up; 



