Re: How do you measure zones of inhibition?

Date: Sun Feb 25 17:58:44 2001
Posted By: Lynn Bry, MD/PhD, Department of Pathology, Brigham & Women's Hospital, Harvard Medical School
Area of science: Microbiology
ID: 979009604.Mi

Message:

Hi Dylan - Zones of inhibition are always measured in millimters, as the diameter of the full zone. By "full zone," I mean the area of clearing around your disk, plus the diameter of the disk itself. If your zone looked like so (C=cleared area and D=disk):

                       
                    CCCCCCCC
                   CCCCDDCCCC
                  ------------  --> Take measurement across the diameter.
                  CCCCDDDDCCCC
                   CCCCDDCCCC
                    CCCCCCCC

You would measure both sides of the clear area, as well as the middle distance covered by the disk. The easiest way to make the measurement is to first turn the plate over, so you're measuring against the plastic of the Petrie dish, and not placing the ruler directly on the agar suface + bacteria. Hold the plate up to the light so you can see the clear areas through it, and place a ruler over the back of the plate to make your measurement. We use this exact same process in the clinical microbiology laboratory to measure zones of inhibition from disks impregnated with antibiotics.

Hope this helps..

-L. Bry, MadSci Microbiologist

Reference: Diagnostic Microbiology by Koneman (sections concerning "Kirby-Bauer" antibiotic susceptibility testing).

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