Oxacillin-Supplemented Mueller-Hinton Agar for In Vitro Inhibition of Ambler Class C β-Lactamases in Enterobacterales

<b>Background</b>: The increasing incidence of infection with Gram-negative bacilli (GNB) producing broad-spectrum β-lactamases, such as extended-spectrum β-lactamases (ESBLs), cephalosporinases (AmpCs), and carbapenemases, has become a great clinical concern. AmpCs are found in many clinically relevant Enterobacterales, where they may compromise the effectiveness of most β-lactams, including carbapenems when associated with an impaired outer membrane. Detection and distinction between these resistance mechanisms are crucial for antimicrobial therapy and for implementation of proper infection control procedures to prevent further spread. <b>Methods</b>: The disk diffusion antibiogram using Mueller-Hinton agar (MHA) supplemented with cloxacillin (MHC), which inhibits AmpCs, was validated to identify AmpC-producing Enterobacterales (AmpC-PE). As cloxacillin is not available in several countries, we investigated the use of oxacillin as an alternative compound to inhibit AmpCs. The ability of MHA supplemented with oxacillin (MHO) to distinguish between carbapenem-resistant Enterobacterales (CREs) due to AmpC hyperproduction and the presence of a carbapenemase has particularly been investigated. <b>Results</b>: MHOs containing several concentrations of oxacillin were compared to MHA and MHC containing 250 mg/L cloxacillin (MHC250). A set of well-characterized Enterobacterales with different β-lactam resistance mechanisms were evaluated. MHO containing 300 mg/L of oxacillin (MHO300) gave similar results to MHC250. <b>Conclusions</b>: The use of MHO300 proved to be efficient in inhibiting AmpCs, allowing differentiation between AmpC hyperproducers and carbapenemase producers. In addition, the use of MHO300 allowed detection of resistance mechanisms hidden by AmpCs, such as ESBLs.