Evaluation of antagonistic activity and antibiotic susceptibility of Bacillus mojavensis strains isolated from the intestines of broiler chickens

Relevance. The growing challenge of antibiotic resistance in veterinary medicine necessitates the development of safe therapeutic alternatives, among which probiotic preparations based on Bacillus spp. represent a promising strategy. The aim of this study was to evaluate the antagonistic properties and antibiotic susceptibility profiles of 15 Bacillus mojavensis strains isolated from the intestines of broiler chickens, and to select candidate strains for a probiotic formulation.

Methods. Antagonistic activity was assessed using the perpendicular streak method against 15 indicator cultures, including Gram-positive and Gram-negative bacteria, as well as micromycetes and yeasts. Susceptibility to 17 antibacterial agents was determined by the disk diffusion method, followed by hierarchical cluster analysis (Ward’s method, Euclidean distance) and strain ranking based on integrated parameters.

Results. Substantial inter-strain variability was observed in both antibiotic susceptibility and antagonistic activity. Three strains (No. 7, No. 10, and No. 12) exhibited the highest cumulative susceptibility and were classified as highly susceptible. Strain No. 10 demonstrated the highest integral antagonistic activity (total inhibition zone diameter — 402.7 mm), significantly exceeding the other isolates, along with a pronounced fungistatic effect against Aspergillus niger and other filamentous fungi. Strains No. 8 and No. 15 displayed a multidrug-resistant phenotype and were consequently excluded from the candidate pool. Based on comprehensive ranking analysis, strain No. 10 is recommended as a primary component for the development of a broad-spectrum probiotic preparation. The proposed methodological approach enables objective strain differentiation according to biosafety and functional efficacy criteria.

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