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The genes encoding inducible AmpC beta-lactamases are chromosomally encoded i.e. part of the genome of the below organisms. This means a detection test is not necessary because identification of one of these organisms indicates that it is an AmpC producer and that it should be capable of producing AmpC inducibly.
Enterobacter spp.
Serratia spp.
Citrobacter freundii
Providencia spp.
Morganella morganii
Hafnia alvei
Pseudomonas aeruginosa
Aeromonas spp.
Occasional isolates of these organisms have a mutation in the induction aparatus. These are known as deprepressed mutants and produce AmpC constitutively at a high level. They are resistant to most beta-lactam antibiotics, whereas wildtypes are usually only resistant to beta-lactam antibiotics that are good inducers e.g. ampicillin and cefoxition.
Deprepressed mutants may emerge during therapy with certain beta-lactam antibiotics, especially 2nd and 3rd generation cephalosporins, to cause treatment failures. For this reason, the CLSI suggests that repeated testing of isolates that initially appear to be susceptible may be warranted. This is controversial. Some individuals advocate the more conservative approach of automatically reporting resistance to 1st, 2nd, and 3rd generation cephalosporins and aztreonam.
If you wish to demonstrate inducibility of the AmpC, this can be achieved with the disk approximation test: Sanders, C. C., and W. E. Sanders, Jr. 1979. Emergence of resistance to cefamandole: possible role of cefoxitin-inducible beta-lactamases. Antimicrobial Agents and Chemotherapy 15:792-797.
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