MathBench > Microbiology

Viable Plate Count, or...
How to count to a million

Picking the plate count

thumbs upFor this, we have any easy rule of thumb: the plate you count should have between 30 and 300 colonies. Use your mouse below to see how this works.

This rule of thumb has the undeniable advantage of being easy to use. But why does it work?

Let's imagine that we have a sample with exactly 42 million cells per mL. And let's say that our technique is flawless. If we count a 1:1,000,000 diluted plate, we should find exactly 42 CFUs (remember, we have magically flawless technique). But the exact same thing would happen if our sample started with 42,000,001 cells. Or 42,001,000 cells. Or even 42,100,000 cells. In fact, even with our absolutely flawless technique, the best we can say is that there are between 41,500,000 and 42,500,000 cells. So our absolutely unavoidable error is plus or minus 500K/42million -- about 1.1%. That's pretty good.

Let's look at some other ways we could count the same 42 million cell sample:

 

Dilution Factor plate count resolution ease of counting
1:10,000 4200

41,005,000 to 41,005,000

+/- 5000 = 0.011%

great!!

NOPE, can't do it, this is too much to count
1:100,000 420

41,050,000 to 41,050,000

+/- 50,000 = 0.11%

still great

SORRY, still too hard to count
1:1,000,000 42

41,500,000 to 41,500,000

+/- 500,000 = 1.1%

not too bad

easy to count
1:10,000,000 4

35,000,000 to 45,000,000

+/- 5 million= 11%

ACK! mayday!

chimpanzees can count this plate!

chimp

 

The more we dilute, the easier the counting, but the higher the error. So you can see that the 30 to 300 guideline is really a compromise between countability and error.