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Name: Asha
Status: Student
Grade:  9-12
Location: AR
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 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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