Bacillus spp. Potentiate the Virulence and Intracellular Invasion of A. paragallinarum in Chickens

Coinfection poses severe threats to poultry health, particularly due to the complexity and resilience of multispecies interactions, increasing the difficulty of treatment. <i>Haemophilus</i> spp., a heterotrophic bacterium, heavily relies on extracellular growth factors acquired from other organisms or its surrounding environment. Although coinfections by <i>Avibacterium paragallinarum</i> and <i>Bacillus</i> 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 <i>Bacillus</i> species, including probiotic <i>Bacillus</i>, to evaluate the feasibility of <i>A. paragallinarum</i> coinfection in vitro. Meanwhile, we determined the minimum inhibitory concentration (MIC) and population dynamics of cocultured <i>Bacillus</i> and <i>A. paragallinarum</i> to assess the effect of bacterial interactions on antibiotic efficacy. Additionally, we demonstrated that <i>B. cereus</i> aggravates rhinitis symptoms in chickens infected with <i>A. paragallinarum</i>. Our findings reveal that <i>Bacillus</i> spp.-derived metabolites sustain <i>A. paragallinarum</i> growth and enhance its survival, thereby highlighting the infection risks associated with <i>Bacillus</i> colonization in the respiratory tract.