Plant Growth-Promoting Rhizobacteria with Antimicrobial and Auxin-Producing Potential: Toward Eco-Friendly Agricultural Solutions

Strains of the genus Streptomyces and the species Bacillus subtilis can produce secondary metabolites such as antimicrobials and phytohormones like auxins. Therefore, different Streptomyces spp. and B. subtilis strains were evaluated for their antimicrobial activity against phytopathogenic microorganisms. The highest inhibitions against Clavibacter michiganensis by each genus was 100 % comparable to various antibiotics, while inhibition against Fusarium oxysporum was 39 and 20 % for Streptomyces and B. subtilis respectively. One of the strains from each genus with the best results was subsequently selected to evaluate their phytohormone production in submerged culture. The selected Streptomyces strain showed a maximum accumulation of 2.2 mg/L in Gause medium on day 12, and B. subtilis produced 1.6 mg/L at hour 72 in LB medium. These results suggest that the selected strains can be used as biocontrol agents due to their antimicrobial activity and their capacity to produce phytohormones that promote growth and crop protection.