No, usually not. In general, the MIC is defined as "the lowest concentration (...) that will inhibit the visible growth of a microorganism after overnight incubation" (see link).

Be aware, that MIC determination is a highly standardized procedure, and that labs doing MIC determination are advised to adhere to the respective guidelines. CLSI in its M07-A9 document gives an incubation time of 16 to 20 hrs, except for some fastidious organisms or difficult-to-detect resistance mechanism. Specifically, for Staph, the MIC should be read after 16 - 20 hrs (except Vanco and Oxacillin, which require 24 hrs of incubation), as recommended in the CLSI M100 guideline.

If you incubate longer, you are running into problems with growth of the fraction of bugs, which was not initially inhibited and of those bacteria which regrow after post-antibiotic effects fade out.

http://jac.oxfordjournals.org/content/48/suppl_1/5.full.pdf+html

It's Staphylococcus aureus the bacteria I'm testing

No, usually not. In general, the MIC is defined as "the lowest concentration (...) that will inhibit the visible growth of a microorganism after overnight incubation" (see link).

Be aware, that MIC determination is a highly standardized procedure, and that labs doing MIC determination are advised to adhere to the respective guidelines. CLSI in its M07-A9 document gives an incubation time of 16 to 20 hrs, except for some fastidious organisms or difficult-to-detect resistance mechanism. Specifically, for Staph, the MIC should be read after 16 - 20 hrs (except Vanco and Oxacillin, which require 24 hrs of incubation), as recommended in the CLSI M100 guideline.

If you incubate longer, you are running into problems with growth of the fraction of bugs, which was not initially inhibited and of those bacteria which regrow after post-antibiotic effects fade out.

http://jac.oxfordjournals.org/content/48/suppl_1/5.full.pdf+html

Hola Nacho,

tal como te comentan en las respuestas anteriores, una de las condiciones fundamentales de los estudios de sensibilidad es ceñirte estrictamente a los protocolos de estudio (CLSI, EUCAST...) para que los resultados sean fiables y reproducibles. En este sentido, la CIM a 48 h es un parámetro atípico, que te puede falsear las CIMs al alza, ya que en 48 horas es posible que se produzca una hidrólisis del antibiótico y un sobrecrecimiento del microorganismo.

Thank you all for your early & clarifying responses

Hello,

as already mentioned, you have to follow exactly the protocols for the standard method you are using (either EUCAST or CLSI). There are some differences between the two protocols, so take a look to all details. Both have good agreement, but I work with fungi, so the procedure is a litlle bit different.

Good luck for your experiments and also, always use  (of course) reference strains as controls for all experiments in all plates you perform the method.

Best regards.