Abstract
Coinfection poses severe threats to poultry health, particularly due to the complexity and resilience of multispecies interactions, increasing the difficulty of treatment. Haemophilus spp., a heterotrophic bacterium, heavily relies on extracellular growth factors acquired from other organisms or its surrounding environment. Although coinfections by Avibacterium paragallinarum and Bacillus have been reported, the underlying mechanism of the cooperative interaction remains poorly understood. In this study, we characterized the growth-promoting properties and nicotinamide adenine dinucleotide production of some Bacillus species, including probiotic Bacillus, to evaluate the feasibility of A. paragallinarum coinfection in vitro. Meanwhile, we determined the minimum inhibitory concentration (MIC) and population dynamics of cocultured Bacillus and A. paragallinarum to assess the effect of bacterial interactions on antibiotic efficacy. Additionally, we demonstrated that B. cereus aggravates rhinitis symptoms in chickens infected with A. paragallinarum. Our findings reveal that Bacillus spp.-derived metabolites sustain A. paragallinarum growth and enhance its survival, thereby highlighting the infection risks associated with Bacillus colonization in the respiratory tract.