Abstract
Background:
Multidrug-resistant (MDR) bacteria in poultry pose a serious health threat. Enterococcus spp. and Pasteurella spp. are common in broiler chickens and can resist antibiotics. Cobalt nanoparticles (CoNPs) offer a new, natural antimicrobial alternative.

Aim:
To assess the antibacterial and antioxidant properties of cobalt nanoparticles synthesized using Matricaria chamomilla extract. The focus was on their effect against MDR Enterococcus and Pasteurella isolates from broiler chickens.

Methods:
One hundred samples were collected from the lungs and ceca of diseased broilers. The isolates were identified by biochemical tests and polymerase chain reaction. Cobalt nanoparticles were synthesized and characterized using UV-Vis , scanning electron microscopy, FTIR, and X-ray diffraction. The antibacterial activity was tested via agar well diffusion, minimum inhibitory concentrations (MICs), and minimum bactericidal concentrations (MBCs) assays. The antioxidant activity was assessed using the DPPH assay.

Results:
CoNPs showed clear inhibition zones against both pathogens. For Pasteurella, inhibition increased significantly at 40–80 mg/ml (p < 0.05). Enterococcus showed weaker but still significant responses. The MIC and MBC for Pasteurella were 27.08 μg/ml and 66.66 μg/ml, respectively. The values for Enterococcus were higher (80.33 μg/ml and 116.6 μg/ml). Gene expression assays revealed significant downregulation of virulence genes in both the aforementioned organisms (p < 0.05). The antioxidant activity of CoNPs increased with concentration, reaching 59.48% at 160 μg/ml.

Conclusion:
Co nanoparticles exhibited strong antibacterial and moderate antioxidant effects. They offer a promising alternative for managing MDR infections in poultry. Further in vivo trials are needed to confirm the efficacy and safety of the drug.

Key words: Antioxidant; Broiler; Cobalt; Enterococcus; Pasteurella.