88 Chapter 5 Discussion Four different screening agars were evaluated on their ability to identify AmpC producing E. coli. Cefotaxime- and ceftazidime containing agar showed similar results in sensitivity and specificity for AmpC detection in the tested panel of E. coli isolates. The addition of cefoxitin increased specificity for AmpC detection, but did not influence sensitivity. Although there are no studies on AmpC selective agars, these results are in line with studies showing that cefoxitin might be a useful screening additive for AmpC production (Ingram et al. 2011; Polsfuss et al. 2011; Peter-Getzlaff et al. 2011). Polsfuss et al. analysed the use of cefoxitin disc diffusion compared to cefotetan for different Enterobacteriales (Polsfuss et al. 2011). The study included 211 potential AmpC producers based on the basis of cefoxitin inhibition zone diameters of ≤18 mm, cefotetan inhibition zone diameters of ≤16 mm, and/or positive ESBL screening diameters according to CLSI guidelines. The potential AmpC producers were compared to 94 isolates that tested negative in the three mentioned screening criteria. All isolates were subjected to an AmpC multiplex PCR and promoter/attenuator regions of E. coli isolates were sequenced. The detection of a pAmpC gene and/or known promoter/attenuator mutations leading to AmpC hyperproduction were considered as gold standard. The majority of acquired AmpC beta-lactamases were of the CMY-group and DHA-group, none of the strains possessed beta-lactamases of the ACC, FOX, MOX or ACT/MIR groups. Cefoxitin showed a sensitivity of 97,4% and a specificity of 78,7%. The use of cefotetan, another type of cephamycin, was tested as well, but although this method showed a better specificity (99,3%), sensitivity was much lower (52,6%). As already described, Reuland et al. compared different screening methods based upon reduced 3rd generation cephalosporin susceptibility for plasmid-encoded AmpC in Enterobacteriales (E. Ascelijn Reuland et al. 2014). The study collection consisted out of 356 Enterobacteriales resistant to a third generation cephalosporins (cefotaxime and/or ceftazidime) and/or cefoxitin, of which 68.8% was determined as E. coli. A total of 34 pAmpC containing isolates (28 E. coli) were detected containing genes from the CMYgroup n=29 ; DHA-group n=4 and ACC-group n=1. No analysis on chromosomal AmpC hyperproducers was performed. The strategy using reduced susceptibility to cefotaxime and/or ceftazidime together with reduced susceptibility to cefoxitin showed a sensitivity of 97% and a specificity of 90%. The only isolate not detected contained a blaACC-gene. In our study one isolate containing an ACC-gene did not grow on the cefoxitin containing agars. Although most acquired AmpC beta-lactamases are inhibited by
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