bla SED-1 beta-lactamase-producing Citrobacter sedlakii isolated from horses and genomic comparison with human-derived isolates.

Aims: We aim to detect beta-lactamase-producing Citrobacter sedlakii from horses and compare the genomic characteristics with isolates from humans.

Methods and result: We characterized phenotypically and genotypically nine C. sedlakii isolates from the feces of horses and then compared them to human-derived isolates using whole genome sequencing and phylogenomic methods. Seven isolates (7/9) were ampicillin-resistant, while at least one isolate was resistant to ceftriaxone, gentamicin, meropenem, and streptomycin. All nine isolates were carriers of the chromosomal-mediated blaSED-1  beta-lactamase gene, which confers resistance to ampicillin. One isolate was positive for the mcr-9 gene that confers resistance to colistin, and another isolate had the aac(6')-lid gene that confers resistance to aminoglycosides. Seven isolates (7/9) were carriers of genes that confer metal resistance to copper, silver, and arsenic. Phylogenetically, two horse-derived isolates clustered together with two human-derived isolates from the NDARO database.

Conclusion: The results from our study provide insight into the antimicrobial susceptibility of C. sedlakii in horses, which was previously lacking, and the specific beta-lactamase gene mediating resistance.