When is the measurement of turbidity a reliable indication of the number of visible cells per ml?

Answer 1

Turbidity works by measuring how much "stuff" gets in the way of the beam - more defraction = more cells

so turbidity works well when there isn't a lot of other "stuff" in the solution - a lot of protein or lysed cells can interfere with the beam and increase your reading

it is best when the solution doesn't have "stuff" that absorbs at 600 nM (most people measure turbidity at 600 nm O.D. but some use 320 nm - me? 600 nm)

clumpy cells can also cause problems - sometimes this can be fixed with a dilution series

hope this helps

Answer 2

this is called a McFarland standard. each standard corresponds to a specific range of CFU's (colony forming units). McFarland standards usually range as:
#0.5, #1, #2, #3, #4, #5
these standards are vital in the preparation of serial dilutions in microbiology and in accurate colony counts.