[Microbiology] ESBL revertants

[A. Seoane]

Dear members of the list:
After performing an Etest-ESBL confirmatory test
(positive result), I noticed some colonies growing
inside the inhibition ellipses. After subculturing of
these colonies and retesting, the image is the same:
Clearly positive result but with some colonies inside
the ellipses that would render the test negative if
taken into account. I remember having seen a paper
maybe 2-3 years ago describing an effect like this and
concluding that colonies inside ellipses were ESBL
revertants. Does anybody know this paper? Does anybody
have any experience on this subject?

Thank you for your help!!

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