Isolation, Molecular Characterization, and Antibacterial Activity of Actinomycetes Associated With Horsemint (Mentha longifolia).

INTRODUCTION: Actinomycetes, renowned for their antibiotic-producing capabilities, represent a promising avenue for the discovery of novel antimicrobial agents, especially amid the global challenge of antibiotic resistance. This study explored the endophytic actinomycetes associated with horsemint (Mentha longifolia), a medicinal plant known for its antimicrobial properties, to uncover potential sources of novel antibiotics. MATERIALS & METHODS: Actinomycetes were isolated from horsemint samples collected in Ilam Province, Iran. Forty isolates were identified based on morphological and molecular analyses, including 16S rRNA gene amplification. Antibacterial activity was evaluated against clinically relevant pathogens, including ESKAPE bacteria. Six isolates exhibited significant inhibitory effects, particularly against Pseudomonas aeruginosa and Klebsiella pneumoniae, with some also active against Staphylococcus aureus. RESULTS: Further analysis revealed the presence of biosynthetic gene clusters (BGCs), including non-ribosomal peptide synthetases (NRPS) and polyketide synthases (PKS-I and PKS-II), in several isolates. Notably, strains such as T35 and T37 harbored all three gene types and demonstrated broad-spectrum antibacterial activity. Strain B22, containing the NRPS gene, showed significant inhibition of both drug-resistant and drug-sensitive pathogens. However, some isolates with BGCs exhibited no antibacterial activity, suggesting that gene expression and metabolite production are influenced by regulatory or environmental factors. CONCLUSION: This study highlights the untapped potential of M. longifolia-associated Actinomycetes as a source of bioactive compounds. The discovery of strains with robust antibacterial activity underscores their value in addressing the urgent need for new antimicrobial agents, especially in combating antibiotic resistance. These findings also emphasize the importance of plant-microbe interactions in natural product biosynthesis. Future work should focus on optimizing conditions for activating silent gene clusters and further characterizing the therapeutic potential of these bioactive compounds.