Anyone come across a table for a turbidometer that tells you what %light transmittance equates to population density of microbes in suspension?
It was my (perhaps incorrect) understanding that after zero-ing out your uninoculated vial with fluid, you add the organism sample and the rest should be standard since the only remaining variable would be the suspended particles. Am I mistaken?
You have the basic idea right, but I'm not aware of a convenient reference table. You might just have to do the experiment and make your own standard curve.
You can use the Mc Farland scale.Yes it is true that if you are setting the blank then you have to use the uninoculated broth and then at varying time you can check the absorbance/transmittance at 660 nm for the inoculated broth.
If you are looking for an easy way to standardize your inoculum concentration then you can use a McFarland standard like Ketna suggests. However, in case the cfu/ml value is imperative for your work, then plotting your own standard curve (like George's suggestion) becomes mandatory. A 0.5 McFarland is usually equated to 10^8 cfu/ml for E. coli. Nevertheless, these values change from cell to cell, depending on their cell shape, size, and presence of extracellular components.
Thank you very much. I've never heard of the McFarland scale, but my own enquiries brought me pretty much to the same conclusions. On the positive side it turns out that in actual practice for the method I'm considering, labs tend to just "eyeball" the turbidity, so perhaps I was worrying unneccessarly.
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