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Microbiology: Laboratory Theory & Application, Brief 3rd Edition

Microbiology: Laboratory Theory & Application, Brief (3rd Edition)

Book Edition3rd Edition
Author(s)Leboffe, Pierce
PublisherMorton Publishing

Chapter 6, Section 6-2, Observations and Interpretations, Exercise 1

Page 415


The colony of microorganisms is deduced to be one because there can be one or more cells forming a colony. The number of colonies formed on the plate indicates the probable cell number in a diluted sample. The increase in dilution decreases the cell count by ten times. The cells in sample 1 containing one million cells are diluted five [one milliliter of sample in ten milliliters of distilled water, then one milliliter of this sample in ten milliliters of distilled water, and so on performed thrice again] times to give a concentration of 10 cells in 10 ml. This can be verified by plating 10-4 and 10-5 on the agar plates and counting the number of colonies. The first plate with 10-5 will contain 80 to 120 colonies, while the second plate with 10-6 will contain 5 to 20 colonies. Depending upon the theoretical colony number, a dilution is selected for plating that can provide countable colonies.


The colonies of Escherichia coli [E. coli] on plates C1 and C2 are countable. The colony count is proportional to the broth dilution. Agar plates have been prepared by spreading the sample taken from the different dilution tubes. Usually, less diluted samples have a higher number of colonies, while more diluted samples have a lower number of colonies of countable range [30 to 300]. 


The sample plated on the agar plates A and B is from the tubes with dilution 10-4 and 10-5, respectively. The colonies in plates A1 and A2 fall under TNTC [Too numerous to count]. In B1 and B2 tubes, the counts are around 300 and 290, respectively. In tubes D1 and D2  [10-7], the colony counts are 15 and 13, thereby falling under TFTC [Too few to count]. 

Sample Response


Plates with countable coloniesA1A2B1B2C1C2D1D2
Colony countTNTCTNTC300290901101513
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