Antimicrobial Susceptibility Testing of Ornithobacterium rhinotracheale isolates

Ornithobacterium rhinotracheale (ORT) is a major respiratory pathogen of poultry, yet its antimicrobial resistance (AMR) mechanisms remain poorly understood. Using phenotypic susceptibility testing and whole genome sequencing of 28 U.S. field isolates, we observed universal multidrug resistance, consistent genotype-phenotype discordance, and no known virulence or plasmid determinants. These results indicate gaps in current AMR gene catalogs and suggest novel or uncharacterized mechanisms underlying resistance in ORT. Phylogenomics shows regionally persistent but globally connected lineages, consistent with long term circulation in commercial flocks. This study supports the need for systematic surveillance, functional validation of resistance determinants, and expanded database curation to guide treatment, stewardship, and One Health risk mitigation