Counting Bacteria Lawns
Country: United States
Date: February 2006
Is there any way to calculate the number of
bacterial colonies in a bacterial lawn? I cannot use the dilution
method to count the bacteria because there are other things mixed
in with the agar that cannot be diluted.
Scrape off one square cm of bacterial lawn from an agar plate and resuspend
in 10 mL of diluting fluid. Vortex well to break apart the cells. Dilute the
cell suspension one to a million and plate 100 uL, incubate the plate at 37
deg C for 48 hours and count the colonies. If there are too many colonies to
count or too few, replate at a different dilution. Calculate the number of
cells in 1 sq.cm as follows: #of colonies x 10 x 10 x dilution factor. There
are two factors of 10 because you suspended the piece of lawn in 10 mL and
you plated one-tenth of an mL.
Ron Baker, Ph.D.
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Update: June 2012