Pick your plate
So it looks like you could solve the whole problem by filtering large amounts of clean water, and small amounts of contaminated water. BUT... sadly we can’t know just by looking which is which. So the safest route is ... do both! Filtering is quick and easy, compared to needing to go back for extra samples. When you see the plates, then you decide.
Let’s try it. In each box, you’ll see the results of filtering and plating 2 samples from the same water body. The first was a 10 mL sample, and the second plate was a 100 mL sample. Which plate should you count? Remember, you want the counting to be easy, but not so easy that you’re sacrifiing accuracy. The rule of thumb is 20 to 60 colonies per plate make a good sample.
Once you pick your plate, what is the contamination level is CFU per 100 mL?
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