Question

You perform serial dilutions to a dilution factor of 1,000,000 and aliquot 10uL on each media plate. The following table indicates the number of colonies on each plate at each dilution

Dilution and # colonies

neat TNTC (too numerous to count)

1:10 TNTC

1:100 TNTC

1:1000 TNTC

1:10,000 367

1:100,000 33

1:1,000,000 2

What is the concentration of the neat sample?

Answer 1

Colony forming units is a unit used to estimate the number of viable cells in a sample. The muber of CFUs/mL can be calculated based on the dilution as follows:

CFUs/mL = (Number of colonies * Dilution factor) / Volume of culture plated

The number of CFUs observed per dilution is as follows
10μL of Undiluted = TNTC (>300)
10μL of 1:10 = TNTC
10μL of 1:100 = TNTC
10μL of 1:1000 = TNTC
10μL of 1:10,000 = 367
10μL of 1:100,000 = 33
10μL of 1:1,000,000 = 2

Since, only 10μL of the serial dilutions of the culture was plated, the CFUs/mL in
1:10,000 = 367 * 100 CFU/mL, this corresponds to 36,700 * 10^4 = 3.67 * 10^8 CFU/mL
1:100,000 = 33 * 100 CFU/mL, this corresponds to 3,300 * 10^5 = 3.3 * 10^8 CFU/mL
1:1,000,000 = 2 * 100 CFU/mL, this corresponds to 200 * 10^6 = 2 * 10^8 CFU/mL

Therefore according to the data, the number of CFUs in 1mL of undiluted sample = 2.99 * 10^8 CFU/mL (average of the above three values)