Attenuation mechanisms and vaccine potential of the serial passage–derived Pasteurella multocida strain PMZ8 in ducks

Fowl cholera, caused by Pasteurella multocida, causes major losses in poultry worldwide, but its molecular pathogenesis remains poorly understood, limiting rational control. This study characterized an attenuated strain, PMZ8, obtained after 50 rounds of serial passaging from a highly virulent duck-derived isolate. Long-read whole-genome sequencing identified three fixed nonsynonymous mutations, in which Fis (R86S) and WaaF (G7R) contribute to virulence attenuation. PMZ8 exhibited an acapsular phenotype, a truncated LPS core, enhanced biofilm formation, defective envelope integrity, and increased intracellular ROS levels. Compared with the WT strain, PMZ8 presented at least a 10⁶-fold increase in the LD₅₀ via both intramuscular and oral routes. High-dose inoculation was well tolerated, indicating a favorable safety profile in ducklings. Vaccinated ducks developed significantly elevated anti-P. multocida IgG with strong bactericidal activity and serum IFN-γ production. Upon lethal homologous challenge, oral or intramuscular immunization with PMZ8 markedly reduced bacterial burdens and tissue lesions and conferred 85% or 80% protection, respectively. Collectively, our findings elucidate the capsule- and LPS-associated mechanisms underlying PMZ8 attenuation and support its further development as a live attenuated vaccine for the control of fowl cholera in ducks.