THE NUMBER OF COLONIES ALLOWABLE ON 

 SATISFACTORY AGAR PLATES 



ROBERT S. BREED and W. D. DOTTERRER' 



New York Agricultural Experiment Station, Geneva, New York 



INTRODUCTION 



A point which is of much importance in making bacteriological 

 counts is the hmit in the number of colonies that may be allowed 

 to grow on a plate without introducing serious errors. Probably 

 every bacteriological worker has this point in mind in making 

 counts and has his own opinion based on experience; but there 

 are few pubhshed data on the subject. The matter has been 

 specially under discussion in connection with the proposed re- 

 vision of standard methods of milk analysis. This investiga- 

 tion was made in order to increase the amount of information 

 available for the use of the Committees who have undertaken 

 the work of revision. 



HISTORICAL 



It is interesting to note the published opinions of different 

 workers on this point. In 1895 Neisser (1895) pubhshed an 

 article in which he reached the conclusion that plates should 

 be so made that they will have about 10,000 colonies per plate, 

 which numbers should then be estimated by means of the low 

 power lenses of a compound microscope. He undoubtedly 

 believed that each bacterial cell put into an agar plate would 

 produce a colony regardless of overcrowding. Three years 

 later Hesse and Niedner, (1898) reahzing, to some extent at 



' The senior author of this paper is responsible for the original suggestion of 

 this problem, for direction in carrying it out, and aid in preparing the esults 

 for publication. The junior author has carried out the laboratory work and has 

 helped in preparing the results for publication. 



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