57 Development of an algorithm to discriminate between plasmid- and chromosomal-mediated AmpC Overall, in 172 E. coli strains, blaCMY-2 was the most prevalent (45.3%) resistance mechanism followed by hyperproducers (34.3%) (Figure 1). Figure 1. Clockplot showing the distribution of ampC genotypes in all 172 E. coli strains. The key is sorted in decreasing order of occurrence. Half a circle indicates 50%; each genotype fills part of the circle to indicate the percentage of each genotype. Etests and AmpC disc diffusion confirmation tests By combining the WGS results with the Etest results, we found higher median MICs of cefoxitin, ceftazidime and cefotaxime for strains that harbour a pampC gene (cefoxitin median=256 mg/L; ceftazidime median=10 mg/L; cefotaxime median=12 mg/L) compared with hyperproducers (cefoxitin median=48 mg/L; ceftazidime median=2 mg/L; cefotaxime median=1.5 mg/L) and negatives (cefoxitin median=32 mg/L; ceftazidime median=0.38 mg/L; cefotaxime median=0.38 mg/L) (Figure S2). Furthermore, zone inhibition differences found with the AmpC Confirm Kit showed higher zone inhibition differences in the pampC strains (ceftazidime + cloxacillin versus ceftazidime median=12 mm; cefotaxime + cloxacillin versus cefotaxime median=8 mm) compared with negative strains (ceftazidime + cloxacillin versus ceftazidime median=3 mm; cefotaxime + cloxacillin versus cefotaxime median=1 mm). However, pampC-positive strains showed more overlap with the hyperproducer group (ceftazidime + cloxacillin versus ceftazidime median=8 mm; cefotaxime + cloxacillin versus cefotaxime 4
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