‘NUMBER OF COLONIES FOR A SATISFACTORY SOIL PLATE. 
H. A. Noyes and G. L. GRouNDS, Purdue University. 
The uniformity between the number of colonies developing on petri 
plates carrying equal sized aliquots has been used as the basis for ascer- 
taining the number of colonies satisfactory for one plate. Prucha’ said 
in 1916: “Further study is needed to give sufficient basis for drawing 
definite conclusions, but the results so far point to the conclusion that 
the average of three plates from the same dilution approaches, reason- 
ably closely, to the average of a hundred plates made from the same 
dilution, when that average is between one and two hundred colonies 
per plate.” 
The following points have served as the basis for determining the 
number of colonies satisfactory for a soil plate: Soil may be a medium 
for the growth of all kinds of micro-organisms; the rate at which dif- 
ferent bacteria multiply varies considerably, and the antagonisms be- 
tween organisms are affected by media, etc. 
The plan for determining the number of colonies for a satisfactory 
soil plate was: First; to make many dilutions and platings of a pre- 
pared soil and study the numbers of colonies developing in three, seven 
and ten days incubation. Second, to compare the number of colonies 
developing from the different dilutions for evidence that plates from the 
higher bacterial dilution carried one-tenth the number of colonies of the 
lower dilution when the lower dilution did not give above the maximum 
number of bacteria that covld be developed into colonies on the plates. 
Third, to give confirmation of the conclusions reached by routine labora- 
tory data. 
Unpublished results (obtained in this laboratory) show rather con- 
clusively that practically all micro-organisms can be grown on a simple 
media. Differences in growth, in addition to being due to the virulence 
of the organisms and their natural characteristics, result from the media 
becoming unfavorable for growth, due to the presence of acid or basie 
reacting substances and specific end products of bacterial metabolisms. 
The importance of the proper conditions of aeration cannot be over- 
emphasized. It has been noted that duplicate plates from pure cultures 
often agree well when even more than two hundred colonies are present 
per plate. Each organism in a pure culture multiplies under similar 
conditions and unfavorable media and end products stop rate and extent 
1Prucha, M. J. Journal of Bacteriology, Vel. 1, No. 1, p. 92. 
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