Table of Contents

• Section 1. Frank's Story
• 1: Recap the story
• Section 2. How Could We Measure the Infection?
• 2: Count 'em all?
• 3: Direct count
• 4: Light scatter (using spec)
• 5: The dilemma of the dead cell
• 6: Viable plate count
• 7: Comparison of methods
• Section 3. How to Do a Viable Plate Count
• 8: Why we need to dilute
• 9: The dilution factor
• 10: Overall dilution
• 11: Interlude: Diluting Cold Press Coffee
• 12: How to scale back up
• 13: Why we plate more than one dilution
• 14: Picking the plate count
• Section 4. Practice Problems
• 15: Putting it all together
• 16: More dilution problems
• 17: Quickies
• Section 5. Back to Poor Frank
• 18: Is he dead yet?

Why we need to dilute

For the remainder of this module, we're going to stick with the Viable Plate Count method -- the only method that can tell a live cell from a dead one. But we still have another problem -- the possibility of lots of lots of colonies to count. For example, 500 colonies on a petri dish would look something like this:

 

What if you are trying to count a population in the thousands or millions? You could literally have a carpet of colonies (also known as a 'confluent lawn') growing on your petri dish. This is where dilution saves the day. Not just dilution, but serial dilution... meaning dilution over and over again.

Why do we dilute? To have less to count

Why do we do it repeatedly? Because we don't know how much dilution we need. Every time we dilute, we'll also make a new plate to incubate. So we might do 5 dilutions and grow up 5 plates. Then we'll end up throwing away 4 of them. Sound wasteful? Well, dilution and plating is quick and easy compared to the pain of starting your experiment all over again.