Exploration of the Antibacterial Mechanism of the Aqueous Extract of Bidens pilosa L. Against the Avian Pathogen Escherichia coli

Bidens pilosa L. extract (BPE), a traditional medicine known for its antimicrobial properties, has not been thoroughly investigated for its potential against avian pathogenic Escherichia coli (APEC), a major pathogen responsible for severe economic losses and high mortality in poultry. This study aimed to comprehensively assess the antibacterial activity of BPE against APEC through both in vivo and in vitro experiments and to explore its underlying mechanisms. In a chicken infection model, BPE treatment led to an 80% cure rate and 20% mortality, in contrast to the 90% diarrhea and 70% mortality observed in the untreated model group. BPE also significantly alleviated intestinal tissue damage and reduced serum levels of inflammatory cytokines IL-6 and IL-1β (p < 0.01). In vitro analyses revealed a minimum inhibitory concentration (MIC) of 625 mg/mL. BPE dose-dependently suppressed bacterial motility, swarming, and biofilm formation (p < 0.01) and markedly increased membrane permeability, indicated by elevated release of nucleic acids, proteins, and alkaline phosphatase (p < 0.001). Moreover, PCR results showed that treatment with BPE at 1/2 MIC for 24 h significantly downregulated multiple virulence-associated genes, including aatA, papC, ibeB, vat, ompA, iss, fyuA, and irp2 (p < 0.01). These results demonstrate that BPE exerts its anti-APEC effects by damaging cell membrane integrity, inhibiting biofilm formation and motility, and suppressing virulence gene expression. Our findings support the potential of BPE as a natural alternative for controlling APEC infections and contribute a scientific basis for the use of traditional herbal medicine in combating bacterial diseases.