Question

A serial dilution is performed on a culture of Saccharomyces cerevisiae (Brewer's yeast). A 1:10 dilution was performed in series seven times. You plated and spread 100 microliters of the fifth dilution to a YEPD agar plate to grow overnight. After incubation, you count 142 colonies on the plate. What was the viable count of the stock culture

Answer 1

Answer: CFU/mL = 14200 CFU/mL

Explanation:

Microorganisms are very small and difficult to count directly. Instead, a laboratory technique is used to count CFUs (Colony Formation Units). This involves making dilutions of a microbial suspension and spreading it on thin layers of agar containing nutrients for growth in Petri dishes also called agar plates. After incubation under ideal conditions, some plates are completely covered with microbial growth; some have little or no growth; and some are covered with individual isolated "spots", which are microbial colonies. Since a colony may consist of a single organism or a group of microbes, the dots represent a CFU.

To calculate the viable count of the stock culture, you have to count the number of colonies, multiply by the dilution and the result will be the plate count in CFU.

So, the equation is:

CFU/mL = Number of colonies / (Volume trasferred to plate x dilution blank)

100 microliters = 0.1 mililiter

1/10 = 0.1 (dilution factor)

Replacing the information from the question:

CFU/mL = 142 / (0.1 mL x 0.1)

CFU/mL = 14200 CFU/mL