COUNTING COLONIES IN PETRI DISH CULTURES 57 



possible with that of one of the circles on the counting 

 plate. 



3. Using the tripod lens count the colonies in each sector 

 of the smallest circle, then in each division between the 

 concentric circles. 



Note 1. The tripod counting lens must be used if the colonies 

 are very small, as they/ otherwise may be confused with air bubbles 

 in the medium. If there are less than 500 colonies present, the 

 entire plate should be counted. If the number is much greater, 

 from ten to twenty divisions, in some definite order, should be 

 counted, an average taken, and the results multiplied by the area 

 of the plate in square centimeters. 



Note 2. Wolfhugel's counting plate is very desirable for 

 counting a large number of colonies. It is ruled in square centi- 

 meters and the squares on the diagonals of the plates are sub- 

 divided into smaller squares. The colonies appearing in from ten 

 to twenty of these smaller squares may be. counted, an average 

 taken and the result multiplied by the number of small squares in 

 1 sq. cm. times the area of the Petri dish in square centimeters. 

 (The entire area of the plate may be obtained most quickly by 

 placing a Jeffer plate upon the Petri dish in question.) 



4. Ascertain the number of colonies per cubic centi- 

 meter in the original sample by multiplying the whole num- 

 ber of colonies on the plate by the dilution; e.g., if there 

 are 386 colonies on the plate and the original culture was 

 diluted 1 in 1000, the number of colonies contained in each 

 cubic centimeter of the original sample is 386,000. 



Note. When there is an excessive number of colonies on a plate 

 the vigorous microorganisms will inhibit the growth of the less vigorous 

 and thus the number of colonies counted is smaller than the number 

 of microorganisms present. Moreover, the colonies may become 

 confluent and the counts will again be in error. 



