Should the size of antibiotic disc be included or to be subtracted from the result in measuring the zone of inhibition?
The zone is measured by the diameter (across the zone) and you do not consider the disc at all. If you have ony a partial zone due to overlapping zones by neighbouring disks, you can measure the radius from the middel (center) of the disc to the edge of the zone and multiply by two. Be sure to consider bacteriostatic vs bactericide measuring priniples according to what kind of antibiotic you are testing and what kind of bacteria.
A good link for more detailed explaination is to be found here: http://www.eucast.org/clinical_breakpoints/
Good luck
Measure it from the edge of the zone from one end to the next edge. the well should not be included.
Zone is measured from edge to edge of the clear area , which means disc will be inside and included in the measurement.
Dear Shabeer, as Jane said detailed explanations are in the eucast guidelines and its very self explanatory including all the relevant concentrations and breakpoints.
A few words as to the tool of measurement. This is important since 1-2 millimeters may change your result. You can use a ruler (but you may be off center). Using a digital caliper solves this problem. If you can justify the expense, there are optical image analyzers aimed at reading susceptibility test plates. They may, among other useful functions, give you the correct interpretation and feed the results directly to a computerized system. Most importantly, they take out the human factor, which may influence the previous two methods, out of reading the diameter.
I agree with Jane, but if u r using CDS method then its the edge of zone to edge of discthat is annual radius of zone
I just remind you a critical parameter in this assay, independently of what method you use to measure the diameter. The most important thing is to put exactly the same amount of media per plate (20-25 ml in 90 mm plates). If not, the diffusion is not homogeneous, because antibiotic diffuse in all directions, to the button of the plate too. By pouring the same volume, you ensure the same distance from top to button. To prepare the plates, I recommend a tested flat surface. For the measurement I recommend to do it by scanning the plate and use ImageJ (for free). Good luck!
The measurement of the diameter of the zone of inhibition includes the size of the disc. The EUCAST and CLSI are good guidelines for this.
I agree with Opeyemi Uwangbaoje LAWAL. Another aspect of the question is, however, how to measure it. We went through several steps during the years, starting with a ruler, followed by a digital caliper and ending up, currently, with an image analyzer that reads and interprets the results according to the chosen standard and feeds it into the LIMS program we use. The latter, although being the most expensive, increases reproducibility and minimizes human error, a significant factor when a difference of a few millimeters may mean the patient being treated with the wrong drug.
Has anyone used imageJ to measure extent of zoning on agar plates and what thresholding methods did you use?
many thanks.
My own experiences involve measuring with a transparent ruler from the back of the plate. You measure the diameter of the zone of inhibition including the diameter of the disc. The reason for including the disc especially where there is activity is that the disc itself sits in the centre of the zone of inhibition, so the area it occupies is part of the zone of inhibition.
I was wondering if we could use the "Area" measurement of Image J for antimicrobial activity. I haven't found any relative publication. If anybody could make a comment on this it would be very useful for me.
In teaching labs resources are limited. I agree to use a transparent ruler, same volume per dish, and include the antibiotic disk in the total diameter measured. if you have not exact circular shape of diameter (specially with learning students) one can measure large and small diameter and obtain an average mm of diameter.
The measurement of the diameter of the zone of inhibition includes the size of the disc.
Hello, if you use antibiotic disks in solid media plates so big diameter of zone of inhibition not always mean high antimicrobial activity. Besides antimicrobial activity, inhibition zone diameter really depends from penetration ability of antimicrobial substances trough agar layer
Article Methods of measuring zones of inhibition with the Bauer-Kirb...
So, if I don't see any zone of inhibition around my disc should my data value be 0.0 mm or the diameter of the disc (in this case my discs are 7.0 mm in diameter)?
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