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Name: Tahir
Status: other
Grade: 12+
Location: Outside U.S.
Country: Pakistan
Date: Spring 2014

Question: I'm assigned to measure the size of bacterial colonies on MacConkey's agar plate but I couldn't find any method to do so.

Hi Tahir,

Typically we would measure from the bottom of the plate in centimeters (or more likely, in millimeters) across the widest point of the colony. If the colonies are irregularly-shaped, you'll just have to do your best at measuring them as fairly as possible (maybe give a vertical and horizontal measurement to better indicate the clonal size). Regardless, make sure you also record observations about colony shape, size, color, texture (bumpy and rough vs. smooth and round, etc.) as these qualitative measurements can still yield valuable information. Finally, if your colonies are too small to measure accurately, you could always let them incubate a little longer, allowing them more time to grow.

- Rick

I would suggest he check the web for MacConkey. It is an important medium in microbiology especially for sewage type dangerous bacteria and cautions handling any media and the bacteria streaked on it must be handled under a supervised protocol with a bonafied microbiologist.

"MacConkey agar is a selective and differential media used for the isolation and differentiation of non-fastidious gram-negative rods, particularly members of the family Enterobacteriaceae and the genus Pseudomonas."

There are better ways using to measure size of colonies a microscope or a laser (confocal microscope would be excellent).

Stephen R. Dunn Ass't Professor of Medicine (ret.) Dept. Medicine/ Div. Nephrology & Cancer Genomics Kimmel Cancer Center Thomas Jefferson University

Dear Tahir,

You could try to photocopy the plate and see if you can measure the spots.

Jim Tokuhis

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