COLONIES ALLOWABLE ON AGAR PLATES 327 



plate, it is seen that there is a tendency for discrepancies caused 

 by having too many colonies on a plate to occur in all groups 

 having less than 50 colonies per plate (one exception to this 

 statement is seen in the group to 10 in table 1). In all cases 

 where more than 50 colonies occurred on the plates, the greater 

 number of discrepancies was caused by having too few colonies 

 on the plates. The tendency toward discrepancies caused by 

 having too few colonies on the plates becomes very marked 

 as soon as the limit of 200 colonies per plate is passed. 



These findings indicate that while the greater proportion of 

 the discrepancies on plates having less than 50 colonies per plate 

 are caused by the operations of the laws of choice and chance, 

 yet there is some factor present which tends to cause more 

 colonies to develop than should do so. In all probabihty this 

 factor is chance contamination from the air which occurs during 

 planting. As is well known, it is common for supposedly sterile 

 check plates to develop one, two or more colonies on prolonged 

 incubation. The presence of these colonies on inoculated plates 

 having fewer than 50 colonies per plate causes a relatively large 

 error in the counts which in some cases would cause the individual 

 plate count to exceed the 20 per cent limit specified here as neces- 

 sary before the plates were classed as satisfactory. 



The tendency for irregularities, due to having too few colo- 

 nies on plates, to occur in counts having 50 or more colonies 

 per plate is too well known to all bacteriologists to require ex- 

 tended discussion. These are undoubtedly caused by the 

 effect of overcrowding. The fact that not all of the discrepancies 

 on plates having more than 400 colonies per plate were of this 

 sort is more significant, for it shows that not all of the discrep- 

 ancies on plates having numerous colonies are due to over- 

 crowding. Irregularities in the number of bacteria used in 

 inoculating or chance contaminations are two things which 

 might produce plates having too many colonies even on crowded 

 plates. 



When all of these things are taken into consideration, it 

 becomes a difficult matter to decide upon the limits in number 

 of colonies which should be allowed on plates. It is at once 



