Question

You are given the following 10^-5 plaque counts from three different serial dilutions (a, b and c). For each, fill in the table to show how many plaques you would estimate you would find on the plates with the dilution factors listed (10^-6, 10^-7, etc.) AND identify the dilution (ranging from 10^-5 to 10^-9)where theoretically the results should become untrustworthy (too few to count for a reliable titer calculation).

              10^-5 plaque count         10^-6                     10^-7                     10^-8                     10^-9

1.   55                                    5 or 6

2. 1025

3. 321

Explain the criteria you are using to determine when results would become untrustworthy.

Answer 1

In this case, serial dilution occurs & dilution factor in serial dilution is 10-1 dilution (As we will get 10-6 dilution by multiplying dilution factor 10-1 with 10-5 dilution. Similarly, we will get 10-7 dilution by multiplying dilution factor 10-1 with previous dilution, i.e., 10-6).

Now, we can get individual plaque count by multiplying dilution factor (10-1 dilution) with the previous dilution. For example, for plates with 55 colonies in 10-5 dilution will provide (expected) 55 x 10-1 or 5.5 6 plaques (Rounded off to nearest full number) in 10-6 dilution. In this way the table is filled below.

10-5 plate	10-6 plate	10-7 plate	10-8 plate	10-9 plate
a) 55	55 x 10-1 = 5.5 6 (Rounded off) (Untrustworthy)	55 x 10-2 = 0.55 1 (Rounded off) (Untrustworthy)	55 x 10-3 = 0.055 (Untrustworthy)	55 x 10-4 = 0.0055 (Untrustworthy)
b) 1025 (Untrustworthy)	1025 x 10-1 = 102.5 103 (Rounded off)	1025 x 10-2 = 10.25 10 (Rounded off) (Untrustworthy)	1025 x 10-3 = 1.025 1 (Rounded off) (Untrustworthy)	1025 x 10-4 = 0.1025 (Untrustworthy)
c) 321 (Untrustworthy)	321 x 10-1 = 32.1 32 (Rounded off)	321 x 10-2 = 3.21 3 (Rounded off) (Untrustworthy)	321 x 10-3 = 0.321 (Untrustworthy)	321 x 10-4 = 0.0321 (Untrustworthy)

Generally, plates containing between 30-300 colonies are considered statistically significant. Thus, plates that contains less than 30 colonies are considered untrustworthy. This is shown in above table.