82 Chapter 5 A study by Reuland et al compared three different screening strategies of pAmpC Enterobacteriales based on susceptibility patterns(E. Ascelijn Reuland et al. 2014). These included: reduced susceptibility to cefotaxime and/or ceftazidime (>1 mg/L), reduced susceptibility to cefoxitin (> 8mg/L), or a combination of cefotaxime and/or ceftazidime (>1 mg/L). Using just cefoxitin during a screening resulted in high sensitivity (97%), but was not specific for AmpC (72%) as a porin deficiency can cause cefoxitin resistance as well. Combining reduced cefotaxime and/or ceftazidime susceptibility with reduced cefoxitin susceptibility increased the specificity to 90%, without significant loss of sensitivity (97%). Reuland et al. did not use a screening agar to differentiate pAmpC Enterobacteriales. We hypothesize that an agar which combines cefoxitin with either cefotaxime or ceftazidime may result in an AmpC specific screening agar, which could be of additional value to the use of an ESBL-agar with cloxacillin. The first aim of this study was to determine the performance of both cefotaxime and ceftazidime containing agars on the specificity and sensitivity for AmpC-producing E. coli compared to ESBL-producing and AmpC/ESBL negative E. coli. Second, we evaluated the influence of adding cefoxitin to these agars for detection of AmpC producing E. coli. Methods Collection of isolates A panel of 159 E. coli isolates was used for the evaluation of the AmpC screening agar. This panel consisted of 40 E. coli isolates harbouring pAmpC, 40 E. coli isolates with cAmpC mutations known to lead to hyperproduction (referred in this study as “cAmpC positive”), 40 ESBL-producing E. coli isolates and 39 E. coli without signs of ESBL, pAmpC or cAmpC hyperproduction based on genotype (referred in this study as “AmpC/ ESBL negative”). The ESBL-producing E. coli and AmpC/ESBL negative E. coli isolates were confirmed to lack any known alterations in the promoter/attenuator region of the AmpC gene leading to hyperproduction of the beta-lactamase, as reported by Tracz et al (Tracz et al. 2007). Isolates were recovered at different study sites or from various clinical specimens, and were isolated from humans (n=141) and animals (n=18) (E. Den Drijver et al. 2018; Hordijk, Wagenaar, van de Giessen, et al. 2013; C. M. Dierikx et al. 2012; Marjolein F Q Kluytmans-Van Den Bergh et al. 2016; C. Dierikx et al. 2013; Ferdous M, Friedrich AW, Grundmann H, de Boer RF, Croughs PD, Islam MA, Kluytmans-van den Bergh MF, Kooistra-Smid AM 2016). More detailed information on
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