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Name: Alexis
Status: educator
Grade: 9-12
Location: IL
Country: USA
Date: Spring 2013

What does the disk-diffusion technique in microbiology generally measure?

There are a few uses of this technique, but in general it is a way to test the sensitivity of a bacterium to an antibiotic. The paper discs are impregnated with a certain concentration of an antibiotic. the bacteria are inoculated onto an agar plate to completely cover it. Then discs with different antibiotics are dropped onto the plate. After 24 hours of growth, if the bacteria is sensitive to the antibiotic, it will be prevented from growing around the disc and there will be a zone of inhibition: an area around the disc where there is no bacterial growth. The diameter of the circle is an indication of how sensitive the bacteria is to that particular antibiotic. A large zone means its very sensitive and would be a good choice. If there is growth all the way up to the disc it is resistant to that antibiotic and it shouldn't be used. Different bacteria have different sensitivities.


The disk diffusion technique is a simple way to test the antibiotic resistance of a bacterial strain. Briefly, bacteria are plated onto a petri dish, after which disks impregnated with an antibiotic are placed on the dish at different points. Antibiotic will diffusion out of the disk in a circle, creating a 'zone of inhibition' around the disk (if, indeed, the bacteria are susceptible to the antibiotic). By measuring the existence and thickness of the zone of inhibition, we can get an idea of how potent the antibiotic is at limiting the growth of this bacterial strain.

S. Unterman Ph.D.

Hi Alexis,

Thanks for the question. The disk-diffusion method is a method used to determine whether an antibiotic will inhibit the growth of a certain strain of bacteria. In the disk-diffusion method, a drop of the antibiotic solution is placed onto an agar disk containing bacteria. The antibiotic diffuses through the agar. If there is a ring or disk, then that indicates the antibiotic prevents the bacteria from growing. If no disk or ring is observed, then the antibiotic cannot prevent the bacteria from growing.

I hope this helps. Please let me know if you have more questions. Thanks Jeff

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