First report on the biological characteristics of carbapenem-resistant Salmonella Thompson with blaCMY-2 production combined with OmpC and OmpF outer membrane protein deficiency in bloodstream infection

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Objectives To investigate the antimicrobial resistance profile and molecular characteristics of the first carbapenem-resistant Salmonella Thompson ( S . Thompson) strain isolated from a bloodstream infection case. Methods A strain of S. Thompson was isolated from blood culture sample and identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Serotyping was performed via the serum agglutination test. Antimicrobial susceptibility testing was conducted using the broth microdilution method. Whole-genome sequencing (WGS) was employed to detect antimicrobial resistance (AMR) genes, plasmid replicons, and virulence genes. The efflux pump activity against carbapenem antibiotics was evaluated using the efflux pump inhibitor carbonyl cyanide 3-chlorophenylhydrazone (CCCP) inhibition assay. SDS-PAGE analysis of outer membrane proteins was used to analyze the alterations in outer membrane proteins of the strain. Results Antimicrobial susceptibility testing revealed that the strain exhibited resistance to penicillins, cephalosporins, quinolones and carbapenem antibiotics. WGS analysis demonstrated that this isolate belonged to ST26 type and carried multiple resistance genes, including bla CMY-2 , qnrS1 , mph(A) , sul1 , sul2 , and aph(6)-Id . Additionally, two plasmid replicons (IncC and IncHI2/IncHI2A) and a variety of virulence genes were identified in the isolate. The CCCP efflux pump inhibition test yielded negative results, indicating no detectable efflux pump-mediated carbapenem resistance. SDS-PAGE analysis showed the absence of protein bands corresponding to the outer membrane proteins OmpC and OmpF. Conclusion The carbapenem resistance mechanism of this S . Thompson strain is attributed to two factors: the production of the β-lactamase encoded by bla CMY-2 and the deficiency of porins OmpC and OmpF. This study provides critical evidence for the rational use of antibiotics in clinical practice and offers a scientific basis for the development of effective strategies to prevent the transmission of carbapenem-resistant S . Thompson.

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