Dear Suzan,

It will be more appropriate if the zones were distributed such that the distances between them are identical. I noticed that the zones are not paced in the circumference of the plate's circle which might be the reason for the phenomenon that you observe.

Hoping this will be helpful,

Rafik

Another option could be that the culture wasn't pure and that you have tested a mix of two bacteria. Maybe one same strain on the plate but one with and another without a plasmid encoding a resistant phenotype. With as consequence one susceptible and one not.

This is typical for a mixed culture (can be a mixture of the same strain with a susceptible and a resistant subpopulation, so-called heteroresistance, or a mixture of two strains of the same or even different species). If you pick a colony from inside the inhibition zone for identification, you may be able to clarify, what’s going on.

Hi:

Its clear that you have mixed culture.

Have you applied the guidelines of recent CLSI document, if you stick to these guidelines, every things will be fine.

Regards