Picking the plate count
For this, we have any easy rule of thumb: the plate you count should have between 30 and 300 colonies.
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 183,000 cells per mL. And let's say that our technique is flawless. If we count a 1:1000 diluted plate, we should find 183 CFUs (remember, we have magically flawless technique). But the exact same thing would happen if our sample started with 183,001 cells. Or 183,100 cells. Or 183,400 cells. Or 182,800 cells. In fact, even with our absolutely flawless technique, the best we can say is that there are between 182,500 and 183,500 cells. So our absolutely unavoidable error is plus or minus 500/182,000 -- about 0.27%. That's pretty good.
Let's look at some other ways we could count the same 183,000 cell sample:
| Dilution Factor | plate count | resolution | ease of counting |
|---|---|---|---|
| 1:100 | 1830 | 182,950 to 183,050 +/- 50 = 0.027% great!! |
NOPE, can't do it, this is too much to count |
| 1:1000 | 183 | 182,500 to 183,500 +/- 500 = 0.27% still pretty good |
boring but count-able |
| 1:10,000 | 18 | 175,000 to 185,000 +/- 5000 = 2.7% not too good |
easy to count |
| 1:100,000 | 2 | 150,000 to 250,000 +/- 50000 = 27% ACK! |
chimpanzees can count this plate! |
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.
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