Emergence of linezolid-resistant Enterococcus cecorum clade F harboring optrA/fexA causing enterococcal spondylitis in commercial broilers in China.

Background: Enterococcus cecorum, long regarded as a commensal bacterium in the avian gut, has recently emerged as a significant pathogen causing enterococcal spondylitis. The widespread use of antibiotics has driven the emergence of multidrug resistance in Enterococcus, particularly linezolid-resistant strains, posing risks to poultry farming and public health. Here, we report the first isolation and characterization of a linezolid-resistant E. cecorum strain (2023EC-GS-SDAU-1) harboring the optrA/fexA resistance gene cluster from Chinese commercial broilers, and analyze its genomic profile, resistance mechanisms, and pathogenicity.

Results: Whole-genome sequencing identified 15 resistance genes, including optrA, fexA, catA8, SAT-4, AAC(6')-Ie-APH(2'')-Ia, and aad(6)-all newly identified in E. cecorum. The optrA/fexA/ermA cluster colocalized on chromosomal genomic island GI12 is flanked by codirectional insertion sequences (ISEfa5), forming a mobile transposon-like unit. Phylogenetic analysis placed the strain within clade F, closely related to pathogenic isolates from the U.S. and France. Antimicrobial susceptibility testing confirmed multidrug resistance to aminoglycosides, macrolides, florfenicol, and linezolid. Additionally, 31 virulence genes were identified, and experimental infections in broilers recapitulated spondylitis lesions, validating pathogenicity.

Conclusion: This study is the first to characterize a linezolid-resistant E. cecorum strain harboring the optrA/fexA cluster in poultry in China, implicating ISEfa5-mediated mobilization in resistance dissemination. The findings underscore the poultry production chain as a critical reservoir for oxazolidinone resistance (optrA) and highlight urgent needs for enhanced surveillance to mitigate zoonotic risks.