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?

Comparison of methods

We've talked about 3 methods so far: just plain counting under a scope, using a photospectometer to do the dirty work, or growing bacteria in a dish to count the piles of descendants they leave.

There is one other way that we could (theoretically) figure out how many cells there are: we could gather them all in one place and weigh them. This would be the biomass method. Biomass is an important way of measuring populations in the lab, but in Frank's case it would not help us -- we would need to collect about 10 million bacterial cells before we could weigh them, and if we could do that, we could probably just cure Frank then and there.

So, let's compare the 3 practical ways we have of measuring the infection:

	Direct Count	Spec	Viable Plate Count
Which one requires the least time (for example, the boss wants the answer before lunch)?
Which one requires the least effort (think lab intern pain)?
Which one can distinguish live from dead cells?

And one more question:

One day an overzealous intern decided to compare all 3 methods. Here's what she found, but she forgot to label which reading came from which method:

347 organisms/mL

520 organisms/mL

510 organisms/mL

Can you tell which reading is which? And what phase of growth (lag, growth, stable, death) is the population in?