Table of Contents

• Prequiz
• 1: Scaling up
• 2: Scaling up small round things
• 3: The Princess and the Pea (Soup)
• 4: Serial Dilution: How it works
• 5: Pick your plate
• 6: Using serial dilution to count bacteria
• 7: Once more with resolution
• 8: You be the scientist
• 9: Review

Using serial dilution to count bacteria

So let's put this all together. We would like to count the bacteria in a particular sample, and we think that there should be in the neighborhood of 10 million cells per mL.

Ideally, therefore, we would like to get 100 bacteria on the plate that we count. That would be a dilution factor of 100:10,000,000, or 1:100,000. And to give ourselves a little wiggle room, we should start at least 1 dilution before that, so 1:10,000. Then we'll do three more 1:10 dilutions to get our series.

In the table below, click on the button corresponding to the plate that should be used to estimate the original concentration of bacteria:

dilution factor	1:10,000	1:100,000	1:1,000,000	1:10,000,000
count	475	48	4	0
answer

 

Pretty straightforward, right? Below, you can practice a couple more times. I also included a few unusual situations ... things do go wrong, of course, and you should be able to recognize that. If you think the dilution series is not valid, click on "re-do the dilution".

 

dilution factor	1:100,000	1:1,000,000	1:10,000,000	1:100,000,000
count	TNTC	674	68	7
answer

 

dilution factor	1:100,000	1:1,000,000	1:10,000,000	1:100,000,000
count	375	105	11	1
answer

 

dilution factor	1:100	1:1000	1:10,000	1:100,000
count	TNTC	249	24	0
answer

 

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dilution factor	1:10	1:100	1:1000	1:10,000
count	954	96	10	0
answer

 

dilution factor	1:10,000	1:100,000	1:1,000,000	1:10,000,000	1:100,000,000
count	TNTC	567	58	12	1
answer