The national public health laboratories within the Food‐ and Waterborne Diseases and Zoonoses (FWD) network has agreed on a panel of priority antimicrobials and optional antimicrobials to test for and report to ECDC (ECDC, 2016 , 2021 ). Compared with earlier recommendations, a second beta‐lactam (ceftazidime) and a carbapenem (meropenem) were added. For 2021, all MS reported results on meropenem and all but four for ceftazidime. Three last‐line antimicrobials – azithromycin, colistin and tigecycline – are also included in the priority list. For colistin, however, the methodology is complicated due to chemical properties of the substance and a joint EUCAST and Clinical and Laboratory Standards Institute (CLSI) subcommittee confirmed that broth microdilution is so far the only valid method for colistin susceptibility testing (CLSI and ECDC, 2016 ). Disk diffusion does not work because of poor diffusion of the large colistin molecule in the agar and tested gradient strips also underestimate colistin MIC values, again most likely due to poor diffusion in the agar (Matuschek et al., 2018 (link)). Therefore, only countries performing broth microdilution (or those predicting resistance from WGS) should report on colistin resistance. Nine MSs reported AST results for azithromycin, tigecycline and colistin for 2021.
Due to the problems in detecting low‐level fluoroquinolone resistance in Salmonella spp. using disk diffusion, nalidixic acid was, for a long time, used as a marker for fluoroquinolone resistance. After the discovery that plasmid‐mediated fluoroquinolone resistance is often not detected using nalidixic acid, EUCAST studied alternative disks and concluded that pefloxacin was an excellent surrogate marker (except for isolates having the aac(6′)‐Ib‐cr gene as the only resistance determinant) (Skov and Monnet, 2016 (link)). Since 2014, EUCAST has recommend this agent for screening of low‐level fluoroquinolone resistance in Salmonella with disk diffusion (EUCAST, 2014 ) and, since June 2016, this is also reflected in the EU protocol. In 2021, all countries reporting measured values for disk diffusion tested with pefloxacin instead of ciprofloxacin, except for Latvia where this information is unknown. Eleven countries reported the combination drug co‐trimoxazole (trimethoprim–sulfamethoxazole) in addition to, or instead of, testing the substances separately, partly because this combination is used for clinical treatment and partly because no EUCAST interpretive criterion exists for sulfamethoxazole for Salmonella.
Due to the problems in detecting low‐level fluoroquinolone resistance in Salmonella spp. using disk diffusion, nalidixic acid was, for a long time, used as a marker for fluoroquinolone resistance. After the discovery that plasmid‐mediated fluoroquinolone resistance is often not detected using nalidixic acid, EUCAST studied alternative disks and concluded that pefloxacin was an excellent surrogate marker (except for isolates having the aac(6′)‐Ib‐cr gene as the only resistance determinant) (Skov and Monnet, 2016 (link)). Since 2014, EUCAST has recommend this agent for screening of low‐level fluoroquinolone resistance in Salmonella with disk diffusion (EUCAST, 2014 ) and, since June 2016, this is also reflected in the EU protocol. In 2021, all countries reporting measured values for disk diffusion tested with pefloxacin instead of ciprofloxacin, except for Latvia where this information is unknown. Eleven countries reported the combination drug co‐trimoxazole (trimethoprim–sulfamethoxazole) in addition to, or instead of, testing the substances separately, partly because this combination is used for clinical treatment and partly because no EUCAST interpretive criterion exists for sulfamethoxazole for Salmonella.
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