Question

A student was given 6 plates taken from the incubator from a serial dilution done two days earlier and told to get a standard plate count.  The student, not sure what to do, attempted to count the colonies on all the plates, then add the counts from all plates and get an average.  Was this the correct procedure? Yes or No? Explain!

Answer 1

No, the procedure followed by the student is incorrect.

Standard plate count is a method for assessing the microbial population or load in the sample. Bacteria divide so faster that it becomes difficult to count them directly from the sample and so they are serially diluted and plated on a agar medium and incubated for 37 degree Celcius, 24 hours. When we leave the plate for more than one day, they start over populating making the procedure invalid.

So after incubation, the plates with colonies ranging from 30-300 CFU/mL is taken for further calculation. Minimum of 30 is required to prevent error and maximum of 300 is only considered because above that the plate becomes over populated.

The formula for calculating Bacteria per mL or CFU/mL of the sample

= No.Of colonies counted / dilution factor * volume transferred

By using this formula, we can find the CFU/mL and give the result for standard plate count.