25 AmpC beta-lactamases: epidemiology, infection control and treatment using only phenotypic assays (Coolen et al. 2019). Although most chromosomal AmpC hyperproducing E. coli appear to have lower MICs for third-generation cephalosporins, there is evidence that changes in the AmpC beta-lactamase or changes in membrane permeability can lead to an increased cephalosporin resistance (Coolen et al. 2019; Nordmann, Poirel, and Nordmann 2007). Co-expression of ESBL and pampC genes in the same isolate can be even more challenging to detect using phenotypic assays because this detection is dependent on genotypic confirmation (J.A.J.W Kluytmans et al. 2021; Martinez and Simonsen 2017). Plasmid-based resistance is considered a greater threat regarding infection control than clonal transmission of chromosome-encoded resistance genes. Therefore, the focus within infection control is mainly on pAmpC producing Enterobacterales and less on the AmpC hyperproducing Enterobacterales. Certain types of plasmids are associated with specific pampC genes. Common plasmid families related to blaCMY-2 are IncA /C-, IncB /O/K and IncI (Alessandra Carattoli et al. 2018; Pietsch et al. 2018). In particular, IncI plasmid families are increasingly found in combination with blaCMY-2. IncI-ST12 is one of the most common plasmid sequence types (Pietsch et al. 2018; Roer et al. 2019). Strikingly, the IncI plasmids harbouring blaCMY-2 are found in several domains, such as human clinical samples, rectal carrier samples from travellers, veterinary samples and pet samples (Lorme et al. 2018; Hansen et al. 2016; Roer et al. 2019; E. P. M. Den Drijver et al. 2020). It is not yet known whether the elevated prevalence of these IncI plasmids is due to the presence of a common clone or more efficient transfer mechanisms among bacteria. Detection of Plasmid AmpC Detection of plasmid encoded AmpC is based on phenotypic and genotypic testing. Due to the coexistence of both chromosome and plasmid encoded ampC genes, it can be difficult to distinguish them. The presence of ESBL can make detection even more difficult. The recently updated NVMM guideline for BRMO detection recommends screening and confirmation of plasmid AmpC at Enterobacterales (J.A.J.W Kluytmans et al. 2021). The high pAmpC resistance phenotype to cephamycins is often used as a first screening criterion. A cefoxitin MIC >8 mg/L in combination with elevated MICs for cefotaxime, ceftriaxone or ceftazidime (MIC >1 mg/L) may indicate the presence of AmpC production (J.A.J.W Kluytmans et al. 2021). The confirmation tests are based on the aforementioned inhibitory capacity of cloxacillin or boric acid. Various disc diffusion tests and gradient strips are available, which confirm the presence of AmpC production based on zone difference between e.g., a third-generation cephalosporin with or without an inhibitor (J.A.J.W Kluytmans 2
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