Viable Cell Count Once the spread plates are grown, the duplicate plates of the

Question

Viable Cell Count

Once the spread plates are grown, the duplicate plates of the dilution that gave the most countable number of colonies are used to determine the viable cell count. The viable cell count is calculated thus:

In order to determine the number of cells per ml per AU, the above value can simply be divided by the absorbance reading at the time the sample was removed from the exponentially growing culture. Determining this value would permit a more accurate estimate of cells/ml at any given AU for the target bacterium, so long as the same growth conditions are used.



(NOTE: Do not put units in your answers, if you do the computer will grade the answer as wrong and you will lose the points. Answers should be written in numerical form or if you want to write as scientific notation you can write as 1.22E8 or 1.22E-8.) (Another NOTE: Your answer must be within 5% of the correct value to get credit for this problem.)



You have removed a volume of a culture of E. coli during exponential growth in order to measure the viable cell count. The absorbance reading of the undiluted sample was 0.3. You then diluted the sample by a factor of 6000 and prepared two spread plates using 0.1 ml each of the dilution. One of the plates produced 120 colonies and the other produced 132 colonies.



What is the average number of colonies found per plate?

What is the viable cell count for this culture?

What is the actual value for the cells/ml per absorbance unit?

Explanation / Answer

average number of colonies per plate = (120+132)/2 = 126

viable cell count corresponds to the number of colonies in the plate, i.e, 126.

actual value of cells/ml = (126/0.1) x 6000 = 75,600

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