Identification of CMY-190, a novel chromosomally encoded AmpC β-lactamase, and plasmid-encoded KPC-2 in a clinical isolate of Citrobacter youngae

Zeshi Liu ^1,2 Siquan Shen ^1,3 Xue Zhang ² Jing Lei ² Chengkang Tang ^1,3 SHI WU ^1,3 Ke Lei ² Jian Yin ² Yanping Zhang ² Yan Guo ^1,3 Yan Geng ^2* Fupin Hu ^1,3*

• ¹ Institute of Antibiotics, Huashan Hospital, Fudan University, Shanghai, Shanghai Municipality, China
• ² Department of Clinical Laboratory, The Second Affiliated Hospital of Xi'an Jiaotong University, Xi'an, China
• ³ Key Laboratory of Clinical Pharmacology of Antibiotics, Ministry of Health, Shanghai, China

This study aims to investigates the antibiotic resistance phenotype and genotype of Citrobacter youngae strain YS01, isolated from a peritoneal effusion sample, focusing on both chromosomale and plasmid-mediated resistance mechanisms to inform clinical antibiotic therapy. Our resultsfindings reveal the presence of the chromosomally encoded β-lactamase CMY-190 and the plasmid encoded carbapenemase KPC-2, which confer resistance to cephalosporins and carbapenems, respectively. CMY-190 exhibits substrate and inhibition profiles similar to AmpC β-lactamases and , sharesing 88.05% amino acid identity with the plasmid-encoded enzyme CFE-2 from Citrobacter freundii pJA99. DNA sequence analysis identified the ampR gene upstream of both blaCMY-190 and blaKPC-2. In additionFurthermore, genes were identified surrounding the ampR-ampC region in C. youngae, including ORF1, the fumarate operon (frdABCD), blc, and lolB, were identified, a DNA fragment not present in other Citrobacter species. The ampR-ampC gene was cloned into the PHSG398 vector and expressed in Escherichia coli DH5α, with the transformed strain showing partial resistance to cephalosporins. The blaKPC-2 was carried by Tn1721, previously identified mainlyprimarily in Asian strains of Klebsiella pneumoniae strains. The eExpression of KPC-2 was confirmed by conjugation of the donor bacterium C. youngae with E. coli J53, and by transformation of the plasmid containing the blaKPC-2 into E. coli DH5α, with all the transformed strains demonstrating resistancet to carbapenems and elevated carbapenem MICs. To our knowledge, this is the first report of a novel chromosomally encoded AmpC β-lactamase gene, blaCMY-190, and the emergence of blaKPC-2 in C. youngae.