90 Chapter 5 clinical samples is comparable to an in vitro setting. Our strategy using the combination of pre-enrichment and selective screening agars was applied in a prevalence study on AmpC in the Amphia hospital (E. Den Drijver et al. 2018). Yield of AmpC E. coli was comparable to other Dutch prevalence studies (E. Ascelijn Reuland et al. 2015; Van Hoek et al. 2015). However, a more elaborate evaluation of clinical samples is needed to confirm our results on sensitivity and specificity of our screening strategy. In this study the use of screening agars was only assessed on E. coli, as this is one of the most prevalent Enterobacteriales with acquired beta-lactamase genes. We expect the agar to be useful for other AmpC producing Enterobacteriales, however sensitivity and specificity for other species may differ. For example, K. pneumoniae without pAmpCs can be cefoxitin-resistant due to loss of porin expression (Martínez-Martínez 2008). Furthermore, not all existing beta-lactamase groups were included. No E. coli isolates containing AmpC FOX and ACT/MIR groups nor the ESBL SHV and TEM or other ESBL groups nor carbapenemases were tested. Co-expression of AmpC and ESBL in E. coli isolates lacked as well. Although, beta-lactamases from the CTX-M, CMY and DHA groups seem to be most prevalent in the European setting (David M. Livermore et al. 2007; Jacoby 2009), future analysis is needed to see if results are applicable to other Enterobacteriales, beta-lactamase gene groups and beta-lactamase gene combinations as well. The AmpC screening agars are not able to differentiate pAmpC vs cAmpC in E. coli. Both mechanisms cause elevated 3rd generation cephalosporin MICs, and MIC distributions of cAmpC and pAmpC tend to overlap (Edquist et al. 2013; Aarestrup et al. 2010).This makes it difficult to create a specific pAmpC or cAmpC screening agar. EUCAST guidelines advise a PCR to distinguish both mechanisms. A screening agar on AmpC may limit the amount of strains for molecular confirmation. Conclusions Cefotaxime or ceftazidime had a similar sensitivity and specificity for AmpC in screenings agar in our isolate panel. Addition of cefoxitin to create a more AmpC selective agar had little influence on sensitivity, but increased specificity. Our results apply mainly for blaCMY-type producing E. coli and further studies are needed to evaluate an agar containing cefotaxime or ceftazidime with cefoxitin in a clinical setting. We expect the screening agars to be feasible to screen for AmpC rectal carriage, when using pre-enrichment similar to current ESBL screening agar strategies. So, agars containing
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