Bacterial Multiplication 151 



number of colonies by the figure found on the Hd of the dish. 

 For example, if the 1:10,000 plate appears to be countable, 

 count the number of visible colonies and multiply the result by 

 10,000, since the colonies arose from only one ten-thousandth 

 of a milliliter of the original milk. 



It is sometimes difficult for students to understand how it is 

 possible to secure a 1:100 plate and a 1:1000 plate from the same 

 1:100 dilution bottle. The labels on the petri plate represent the 

 fractions of a milliliter of the original milk actually transferred to 

 the petri dish; it is not necessarily a dilution as such. 



Experience shows the number of organisms normally to be ex- 

 pected in a given test material, and the types of bacteria, within 

 limits, may also be predicted. The number of dilutions required 

 and the extent of the dilutions must be estimated, with sufficient 

 plates being prepared to hit the correct dilution range. There are 

 occasions when this estimate may be incorrect, and none of the 

 prepared plates will fall within the prescribed 30-300 limit. In 

 such instances it becomes necessary to employ the plate coming 

 closest to being countable. It is better to get some idea of the 

 viable count than it is to discard the plates because they are too 

 crowded, or because they do not have as many as thirty colonies 

 on them. If the plate coming closest to being countable is very 

 crowded, it is not necessary to count every single colony on that 

 plate. It is permissible to count representative areas and to cal- 

 culate the total number of colonies on the plate. An average petri 

 dish has an area of about 65 square centimeters. If 5 representa- 

 tive square centimeter areas are counted and multiplied by 13, 

 the total number of colonies on the plate can be estimated (5 X 

 13 = 65 square cm.). The importance of adequate mixing of the 

 agar with the milk dilution in the plates becomes very obvious 

 when even distribution of colonies is desirable for counting 

 colonies. 



Colony-counting devices are available which provide adequate 

 diffused illumination and which have glass plates marked off in 

 square centimeter areas, and smaller areas, to facilitate counting 



