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Antibacterial Activity of Plant Extracts

Tom McCloud mccloud-tom at worldnet.att.net
Wed Dec 13 20:13:50 EST 2000

       A method you can use which is do-able as a science fair project
is simply by disc diffusion.    5mm paper discs containing a known
amount of antibiotic can be purchased.    These discs are your
standards.   You should always get, for example, a 30mm clear zone, or
zone of inhibition around the disc.  You put several onto the 'lawn'
of microbes growing in the Petri dish.     If the clear zone is
greater or less than the 'standard' then you can normalize your
results i.e. if the clear zone is only 27mm for the standards then add
3 to the measured zone of all your test discs.  
      As for your extracts, you apply them to 5mm paper discs, dry to
remove solvent and lay onto the microbial lawn.   After 24 to 48hrs
incubation under appropriate conditions you read the clear zone using
a simple micrometer, or even a metric ruler.  Do your assays in
triplicate.   For rather typical plant extracts you should be applying
at least  one-half milligram to each 5mm disc.     There are common
plant materials around your house you might want to test, like a tea
bag, coffee beans, or from the spice rack, mace or nutmeg.   As known
antimicrobial controls (as opposed to standards)  you might want to
use some household disinfectants,  OTC antimicrobials,  and phenol, if
you can get it.    As a rule of thumb, the greater the diameter of the
clear zone, then the more potent the antimicrobial activity.
          Admittedly a too short and overly simplified explanation,
but try it, and you'll find that it works.
          Tom McCloud       SAIC/Frederick Cancer Research

On 13 Dec 2000 17:04:22 -0000, PDOGJPS at aol.com wrote:

>My name is Jeffrey Siegal and I am doing a science fair project on the 
>effectiveness of plant extracts (eucalyptus, garlic, and grapefruit seed 
>extract) in inhibiting the growth of e coli and s aureus. I want to compare 
>their effectiveness to Penicillin, Streptomycin and Erythromycin.
>Is there a test to compare antibacterial effectiveness of these herbal 
>extracts to the drug. I was planning to use the Kirby-Bauer method but there 
>is not, to my knowledge, a chart or table to identify the effectiveness.
>Can someone help?
>Thank you, jeffrey
>PDOGJPS at aol.com

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