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This is the zone of inhibition. In general, larger zones of inhibition correlate with lower minimum inhibitory concentrations (MICs) of antibiotic or extract for that bacterial strain. [1] An exception to this is when molecules of the antibiotic or extract are large or hydrophobic because these diffuse through the agar slowly. [6]
If the antibiotic inhibits microbial growth, a clear ring, or zone of inhibition, is seen around the disc. The bacteria are classified as sensitive, intermediate, or resistant to an antibiotic by comparing the diameter of the zone of inhibition to defined thresholds which correlate with MICs. [14] [16]
A better diffusion leads to a truer zone of inhibition. Mueller–Hinton agar was co-developed by a microbiologist John Howard Mueller and a veterinary scientist Jane Hinton at Harvard University as a culture for gonococcus and meningococcus. They co-published the method in 1941. [3]
The growth of bacteria that are optochin-sensitive will show a zone of inhibition around an optochin disc, while the growth of bacteria that are resistant to optochin will not be affected. In vitro, a minimum inhibitory concentration (MIC) of 1:10,000,000 will inhibit the growth of pneumococci, and 1:500,000 is bactericidal.
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This is called a D-zone test, or D test. If a 'D' shape is formed around the clindamycin disk (distinguished from a circular zone of inhibition) then the isolate is reported as resistant to clindamycin. [5] This occurs due to erythromycin inducing the bacteria's erm gene, and thus making it resistant to clindamycin (MLS-B phenotype). [6] [7]
[citation needed] The inhibition zone sizes from disk test results were compared to MIC values based on the reference agar dilution procedure.The correlation between zone sizes and MIC values was then assessed using regression analysis and regression lines were used for extrapolating zone interpretive limits that corresponded to the MIC ...
After the required incubation period, when an even lawn of growth is distinctly visible, the MIC value is read where the pointed end of the inhibition ellipse intersects the side of the strip. Etests can be used as an alternative method to determine the minimum inhibitory concentrations of a wide range of antimicrobial agents against different ...