Antimicrobial Potential of Rhizosphere-Associated Actinomycetes Isolated from Salvia rosmarinus and Mentha spicata
Antimicrobial Rhizosphere Actinomycete from Salvia & Mentha
DOI:
https://doi.org/10.66224/BiotechIntellect.3.1.59Keywords:
Rhizosphere, Salvia rosmarinus, Mentha spicata, Antimicrobial activity, StreptomycesAbstract
Background and Objective: Rhizosphere-associated actinomycetes are promising sources of microorganisms with antimicrobial potential. This study aimed to isolate actinomycetes from the rhizospheres of rosemary (Salvia rosmarinus) and spearmint (Mentha spicata) and evaluate their antagonistic activity against selected bacterial and fungal pathogens.
Materials and Methods: Rhizosphere soil samples were collected from rosemary and spearmint plants, and actinomycetes were isolated using culture-based methods involving sample pretreatment, serial dilution, and cultivation on selective media. All isolates were screened for antagonistic activity using the cross-streak method on International Streptomyces Project medium 2 (ISP2) and tryptic soy agar (TSA). The target microorganisms included Escherichia coli, Pseudomonas aeruginosa, methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant S. aureus (MRSA), Enterococcus faecalis, Listeria monocytogenes, and Candida albicans. Nine isolates were selected for further investigation. The isolates were cultivated in tryptic soy broth, and cell-free culture supernatants (CFS) were prepared by centrifugation and membrane filtration. The antimicrobial activity of the CFS was evaluated using the agar-well diffusion method. Four selected isolates were subjected to 16S rRNA gene sequencing for molecular identification.
Results and Conclusion: Of the 36 isolates screened by the cross-streak method, 15 exhibited antimicrobial activity, with inhibition profiles varying according to the isolate, test organism, and culture medium. RR1 showed the broadest antagonistic activity in this assay. In the agar-well diffusion assay, the CFS of RR1 inhibited all seven test organisms, with inhibition zones of 12–19 mm, whereas MR4 produced 20-mm inhibition zones against both MSSA and MRSA but showed no detectable inhibition of the other test organisms. The activity profiles differed between the two assays for some isolates. Based on 16S rRNA gene sequence comparisons, RR1, RR13, and RR29 showed 100% sequence identity with Streptomyces amritsarensis, while MR4 showed 100% identity with Streptomyces antibioticus. These results identify RR1 as a promising candidate for further investigation of broad-spectrum antimicrobial activity and MR4 for studies of anti-staphylococcal activity. However, further characterization, including minimum inhibitory concentration determination and chemical analysis of active compounds, is required to establish the potency and nature of their antimicrobial activity.
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Copyright (c) 2026 Mahsa Khooshehgir, Shaghayegh Nasr (Author); Nayyereh Alimadadi
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