THE EXAMINATION OF PLATE CULTURES 33 



whether the colonies are well isolated or run together (con- 

 fluent) ; describe the appearance of the individual colonies 

 (a) on the surface, (b) in the depth of the medium. 



Indicate their shape (see 51). Are the edges sharply 

 defined ? Is the margin even or irregular ? Give their 

 size (diameter in millimeters) and indicate their color (deter- 

 mine shade from a color chart 1 ) and consistence. Do the 

 surface colonies adhere to the medium or can they be easily 

 removed? Examine them with a low-power lens and describe 

 the surface markings, if any. Also indicate the difference 

 in color as observed with the unaided eye and with the 

 microscope. 



49. Estimating the number of colonies on plates. If the 

 number of colonies is not large (not exceeding 100), they may 

 all be counted and the exact number recorded. This may be 

 done with the third plate. When the number is larger it is 

 more convenient to divide the total area into smaller areas and 

 count the number of colonies in each of several (20 to 40) 

 of the small areas. Add these together and divide the sum by 

 the number of areas counted ; the quotient gives the average 

 number on one area. Multiply this quotient by the number 

 of areas containing colonies, and the product will be the num- 

 ber of colonies on the plate. This latter process, however, 

 gives the approximate number only. 



For dividing the area of the plate into smaller, equal areas, 

 it is convenient to use WolfThugel's counting apparatus. This 

 was devised more particularly for square or oblong plates 

 (Koch). In counting the colonies on the Petri dishes Parkes' 2 

 scheme modified by Jeffers 3 is more suitable. It consists of 

 a disk about 20 cm. in diameter, divided into areas of a square 

 centimeter each. Place the Petri dish over the disk, taking 

 care that it is centered. 



1 Saccardo, Chromotaxia sen Nomenclator Coloriim. 



2 Parkes, Journal of Pathology and Bacteriology, Vol. IV, p. 173. 

 8 Jeffers, Journal of Applied Microscopy ?J<3\. I, No. 3, 1898. 



