Deleting fis downregulates virulence and effectively protects Pasteurella multocida infection in mice.

Pasteurella multocida (P. multocida) is an important pathogen causing various diseases in both domestic and wild animals. The factor for inversion stimulation (Fis) is a nucleoid-associated protein with diverse functions in various bacteria, which positively regulate the transcription of capsular glycosaminoglycan genes in P. multocida. However, the precise mechanistic understanding of how the fis regulate virulence of P. multocida remains largely unknown. In this study, we discovered that fis transcript levels of P. multocida CQ2, serotype A (PmCQ2) were significantly increased in vivo, and showed a positive correlation with the capsule and virulence of P. multocida. To further understand how the fis regulated P. multocida pathogenesis, a homologous recombination strategy was used to generate fis-deleted strain. Then, the growth velocity, virulence characteristics, immune/inflammatory responses, and the survival rates of challenged mice were determined. The findings revealed that the presence of fis promoted the growth, regulated synthesis of capsule and biofilm of PmCQ2, and helped to resist phagocytosis by macrophages. Notably, we firstly demonstrated that Fis determined whether P. multocida can use bound iron ion for its survival. Consequently, the loss of fis greatly restricted P. multocida pathogenicity, as evidenced by reducing tissue bacterial loads as well as inflammatory factors levels. Moreover, the fis deletion strain was endowed with strong cross immunoprotected properties against P. multocida serotype A and B. Thus, these results suggested the pathogenic role of fis in P. multocida and proposed that fis deletion strain could be used as an attenuated vaccine candidate against P. multocida of serotype A and B.