Evaluation of four phenotypic methods to detect plasmid-mediated AmpC β-lactamases in clinical isolates

Four phenotypic methods (three dimensional test, AmpC test, cloxacillin synergy test and cefotetan/cefotetan-cloxacillin E-test) to detect plasmid-mediated AmpC β-lactamases (pAmpC) were compared in 125 clinical Enterobacteriaceae isolates with AmpC profile: 74 E. coli (bla (CMY-2): 70; bla (DHA-1): 4), five K. pneumoniae (bla (CMY-2): 2; bla (DHA-1): 3), six P. mirabilis (bla (CMY-2): 6) and 40 negative isolates for pAmpC β-lactamases. All evaluated methods showed a good sensitivity (>95%) but low values of specificity (<60%) in E. coli, explained by an increase of AmpC expression caused by chromosomal ampC promoter/attenuator mutations (-42, -18, -1, +58, predominantly). The cefotetan/cefotetan-cloxacillin or cloxacillin synergy test may be advocated as phenotypic screening test, and the AmpC test as confirmatory test for detection of pAmpC in isolates that lack or minimally express chromosomally encoded AmpC β-lactamases. In the case of E. coli, the phenotypic evaluated tests were not able to differentiate between chromosomal ampC overexpression or acquisition of plasmid-encoded ampC genes.