Question

You do a serial dilution in the same manner as the one in the video. A plate with a sample volume of 10-6 mL produced 43 colonies. What was the original cell density (CFU)?

Answer 1

Answer: 43 x 10^6  CFU.

Explanation:

Plate count is a method used to determine the number of viable microorganisms in a sample. When the bacterial concentration is high, serial dilutions are carried out in a 1:10 sequence. Then, the dilutions are successive dilutions keeping the dilution factor constant at each step. Small aliquots of these dilutions are seeded in medium contained in a Petri dish where the bacteria grow forming colonies. Then the plates are incubated and colonies develop both within the agar and on the surface.

If the concentration of the seeded aliquot is too high, the bacteria will grow too much and it will not be possible to distinguish the colonies. However, if the concentration is very low, the number of colony forming units (CFU) may be very low and thus can be counted.

Colony forming units are calculated:

CFU = Number of colonies / (volume seeded on the plate * dilution).

The dilution is 10^-6, the number of colonies counted is 43, and usually the volume plated is 0.1 mL, so we replace that in the equation:

CFU= 43 / (0.1 mL * 10^-6)

CFU= 43000000 = 43 x 10^6