Table of Contents

• 1: Recap the story
• 2: Count them all?
• 3: Direct count
• 4: Light scatter (spectrophotometer)
• 5: The dilemma of the dead cell
• 6: Viable plate count
• 7: Comparison of methods
• 8: Summary

Comparison of methods

We've talked about 3 methods so far: just plain counting under a microscope, using a spectrophotometer to do the dirty work, or growing bacteria in a dish to count how many piles of descendants (colonies) they produce.

There is another 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 this case it would not help us -- we could do it but it would not tell us if the cells were dead or alive, or distinguish between particles (eg soil particles) in the water and bacteria.

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

	Direct Count	Spectrophotometer	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 technician pain)?
Which one can distinguish live from dead cells?
Which one can distinguish the lowest number of bacteria?

And one more question:

One day an enthusiastic lab technician 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, exponential (or log), stationary, death) is the population in?