Hi, if isolate is cefoxitin resistant this means it is AmpC B-lactamase-produccer

(not ESBL-producer), and thus you have to look for genes responsible for production of AmpC B-lactamase like MIR-1, DHA-1, CMY-2, etc.....

These days, whole genome sequencing is typically done.

is there any authentic molecular marker?

There are many different types of beta-lactamases, which is why whole genome sequencing is actually an efficient way to see what genes are present in a given strain. If you just want to know whether there is any beta-lactamase enzyme activity in the strain, you could test an extract for the ability to hydrolyze the colorimetric substrate nitrocefin.

Hi, if isolate is cefoxitin resistant this means it is AmpC B-lactamase-produccer

(not ESBL-producer), and thus you have to look for genes responsible for production of AmpC B-lactamase like MIR-1, DHA-1, CMY-2, etc.....