Table of Contents

• 1: Recap the story
• 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
• 8: Review

Light scatter (using spec)

Direct count is a straightforward way to figure out how many meningicocci Frank is harboring, but it is a bit labor intensive. Wouldn't it be nice to get a machine to do the counting for us?

One way we can do this is to use a photospectometer. A spec doesn't exactly count the cocci, but it does measure how much they interfere with a light beam, and based on that, we can use a calibration to decide how many cocci we've got. Then we can do the scaling up trick to get the total population size.

This is a great method if you already have a calibration curve -- perhaps you work in a lab that has been studying this critter for years... Otherwise, you may need to spend a couple of days doing the calibration (growing the cultures, taking the spec readings, comparing them to counts under the microscope... that sort of thing).

Here's an example of a calibration curve:

 

For example, if you prepared a sample and got a spec reading of 0.5, you would find 0.5 on the x-axis and read the bacterial count off the y-axis -- about 26 million

How about a spec reading of 0.9? (Be careful with the log scale for bacteria!!)

If your sample had about 12 million bacteria, what OD reading would you expect?