Outbreak of High-Risk Clone ST323 Resistant to Ceftazidime-Avibactam Due to Acquisition of and to Cefiderocol Due to Mutated Gene.

The incidence of Ceftazidime/Avibactam (CZA)-resistant isolate co-producing carbapenemase 2 (KPC-2) and Vietnamese extended-spectrum β-lactamase 25 (VEB-25) has been on the rise in Greece over the past five years. This study investigates the isolation of ST323 isolates co-resistant to CZA and cefiderocol (FDC) from colonized and infected patients in a single hospital in Athens. CZA-resistant strains were isolated from 5 ICU patients from 27 December 2023 to 22 January 2024. Antimicrobial susceptibility was tested against a panel of agents. Whole-genome sequencing of the isolates was carried out to identify the acquired resistance genes and mutations that were associated with CZA and FDC resistance. The isolates belonged to ST323 and harbored and . The isolates had a minimum inhibitory concentration (MIC) of >256 mg/L for CZA and 32 mg/L for FDC, due to the disrupted catecholate siderophore receptor Fiu. was located on an IncC non-conjugative plasmid and on a ~14 kb multidrug resistance (MDR) region comprising 15 further acquired resistance genes. Transformation studies showed that the -carrying plasmid provided resistance to most of the β-lactams tested, including CZA. The isolates remained susceptible to carbapenems, imipenem/relebactam, and meropenem/vaborbactam. The plasmid harbored the citrate-dependent iron (III) uptake system (), which increased the MIC of FDC from ≤0.08 mg/L to 2 mg/L. The gene was associated with IncC plasmids which are important contributors to the spread of key antibiotic resistance genes. Strict infection control measures must be elaborated upon to prevent the spread of extensively drug-resistant organisms such as those described here.