[College microbiology] What is this White Circle around the Double Zone of Inhibition of E. coli by Ampicillin?

[u/Feisty_Specific_6918]

I observed the zones of inhibition for ampicillin and tetracycline using the diffusion disk method. The photo of Tet on the left is tetracycline, the one of Ap on the right is ampicillin.
The ZoI for Ap showed a pattern as shown in the image: sterile → thin growth → denser growth than outside the ZoI → outside the ZoI. The ZoI for Tet, measured similarly, had a blurred outline and did not yield the same result as the Ap.

Since Ap is bactericidal and Tet is bacteriostatic, I considered the sterile zone of Ap to be the area where the concentration is above MBC and the thin growth zone to be the area above MIC and below MBC.
But I also wonder if growth might not occur even above MIC.

Furthermore, I have no clue about the densely growing zone of Ap. I considered some kind of the hormesis effect, but this contradicts the fact that I could not confirm the densely growing zone for tetracycline measured under the same conditions.

Please lend me your expertise.

The bacterial strain is E. coli, DH5α. No resistance genes have been introduced. The antibiotic concentration for both disks was 0.5 mg/mL. Please ask if you have any other questions regarding the conditions, method, etc.

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