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?

The dilemma of the dead cell

So far we have one hard way (Direct Count) and one easy way (Photospec) to quantify Frank's meningicoccal load. But both ways share a big problem ... the dilemma of the dead cell.

If you were counting, say, cats ... this wouldn't be a problem. A dead cat and a live cat are generally not easily confused. If in doubt, you could poke the cat, and you would know for sure. However, bacterial cells are much less lively than cats. Yes, there is plenty of stuff going on inside, but in general you're not going to see that from the outside. Under a microscope, cells that are alive and cells that are dead look a lot alike. And to a spec, live and dead cells look exactly alike.

Based on the dilemma of the dead cell, which would you predict?

Direct count and photospec will...?

If we care about getting the number of live cells right, we need a better way...