Identification of the Avian Pasteurella multocida phoP Gene and Evaluation of the Effects of phoP Deletion on Virulence and Immunogenicity

Pasteurella multocida (P. multocida) is an animal pathogen of worldwide economic significance that causes fowl cholera in poultry and wild birds. Global gene regulators, including PhoP are important in regulating bacterial virulence and are good targets for developing attenuated vaccines against many pathogenic bacteria. However, the biological significance of phoP gene has not been identified in P. multocida. Here, we identified the phoP gene in P. multocida, and we evaluated the roles of phoP in P. multocida by deleting the phoP gene. The P. multocida phoP mutant exhibited similar growth curves and lipopolysaccharide and outer membrane protein profiles but displayed defective polymyxin resistance in vitro compared with the parent strain. Additionally, the phoP deletion resulted in decreased virulence. The LD50 of the ΔphoP mutant was 32- and 154-fold higher than the parent strain via the oral and intranasal routes, respectively. Transcriptome sequencing analysis showed that 161 genes were up-regulated and 173 genes were down-regulated in the absence of the phoP gene. Finally, the immunogenicity and protective efficacy of the ΔphoP mutant were evaluated. Immunized ducks produced significantly higher levels of serum IgY and bile IgA compared to the control ducks, and immunization with the ΔphoP mutant conferred 54.5% protection efficiency against challenge with the virulent P. multocida. This work provides a platform to dissect the function of phoP and develop a new vaccine against P. multocida.