I did lawn on an E. coli strain onto a plate of Mueller-Hinton agar. For other plates I got clear zones of inhibition, but for few particular antibiotics (CRO, OFX and CIP) against this strain some colony growth is observed. So is this strain resistant against these antibiotics?
Hello Shahidur Shahed
The Clinical Laboratory Standards Institute (CLSI) has established interpretation of antimicrobial sensitivity testing as sensitive, intermediate, or resistant, based on either the diameter of the zone of inhibition or the Minimum Inhibitory Concentration (MIC).
The area around the antibiotic disk that has no bacterial growth is known as the zone of inhibition. The zone of inhibition is a uniformly circular zone of no bacterial growth around the antibiotic disk. The larger this zone is, the more sensitive the bacteria is to that antibiotic. The smaller the zone is, the more resistant (and, thus, less sensitive) the bacteria is.
But as you move farther away from the antibiotic disk, the concentration of the antibiotic will decrease. At a certain distance from the disk, you'll start to see bacterial growth again because the antibiotic concentration is too low to affect the bacteria.
You may have to measure the diameter of the zone of inhibition by using a ruler or caliper. Measure each zone with unaided eye while viewing the back of the petri dish. Hold the plate a few inches above a black, non-reflecting surface illuminated with reflected light.
The zone diameter can be determined by measuring the radius of the zone. Measure from the center of the antibiotic disk to a point on the circumference of the zone where a distinct edge is present. Multiply this measurement by 2 to determine the diameter of the zone of inhibition. You may compare to that of a standard strain and the results may be interpreted as sensitive, intermediate resistant or resistant, based on CLSI guidelines.
I have attached the protocol below that will help you to decide the results for CRO, OFX and CIP. For measuring the zone of inhibition and zone size you may refer to Fig 8, 9 and 10 of the attached protocol.
https://asm.org/getattachment/2594ce26-bd44-47f6-8287-0657aa9185ad/Kirby-Bauer-Disk-Diffusion-Susceptibility-Test-Protocol-pdf.pdf
Best.
What you are seeing is zone of inhibition but then resistant (or tolerant) colonies are growing as an outer ring. To measure the zone just ignore the colonies and measure to the edge of the clearing.
You might see cleaner results if you incubated your plates for a shorter period of time and you may reduce the resistant colonies.
Thank you very much @Micheal. I will ignore the colonies and measure the initial zone of inhibition.
- Badges
- Science topic
- Similar topics
- Biological Science
- Biota
- Eukaryota
- Fungi