Phytochemical composition and synergistic antimicrobial effects of <i>Rosmarinus officinalis</i> essential oils during flowering in an arid mediterranean region

Imane Abbad; Bouchra Soulaimani; Imane El Hakioui; Soraia El Baz; Elena Maria Varoni; Marcello Iriti; Imane Abbad; Bouchra Soulaimani; Imane El Hakioui; Soraia El Baz; Elena Maria Varoni; Marcello Iriti

doi:10.3934/microbiol.2025033

AIMS Microbiology

2025, Volume 11, Issue 4: 769-785. doi: 10.3934/microbiol.2025033

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Research article

Phytochemical composition and synergistic antimicrobial effects of Rosmarinus officinalis essential oils during flowering in an arid mediterranean region

1.
Laboratory of Water Sciences, Microbial Biotechnologies, and Natural Resources Sustainability (AQUABIOTECH), Unit of Microbial Biotechnologies, Agrosciences, and Environment (BIOMAGE)-CNRST Labeled Research Unit No.4, Faculty of Sciences-Semlalia, University Cadi Ayyad, Marrakech, Morocco
2.
Agrosciences Program, College of Agriculture and Environmental Science, Mohammed VI Polytechnic, Ben Guerir 43150, Morocco
3.
Department of Biomedical, Surgical and Dental Sciences, University of Milan, Milan, Italy

Received: 03 August 2025 Revised: 07 October 2025 Accepted: 17 October 2025 Published: 30 October 2025

Rosmarinus officinalis has attracted significant attention due to its broad-spectrum antimicrobial activity, largely attributed to its bioactive essential oils (EOs). Several studies indicate that the flowering period is crucial for harvesting rosemary's aerial parts for optimal EO extraction. However, its prolonged flowering period complicates the determination of an optimal harvest time, potentially affecting yield, chemical composition, and efficacy. This study provides, for the first time, a systematic month-by-month evaluation of EO yield, chemical composition, and synergistic antimicrobial potential of rosemary cultivated under arid Mediterranean conditions during its flowering period (September to March). EO samples were analyzed by GC-MS and assessed for antimicrobial activity against clinically relevant pathogens, including S. aureus, E. coli, S. enterica, and four Candida species. The synergistic potential was further evaluated with two conventional antimicrobials (streptomycin and amphotericin B). The results showed that the EO yields ranged from 1.73% to 2.75%, with a clear peak in autumn. GC-MS analysis identified 31 compounds, dominated by 1,8-cineole (27.57 ± 0.76%–36.28 ± 0.26%), α-pinene (15.36 ± 0.23%–28.97 ± 0.10%), and camphor (7.12 ± 0.00%–15.37 ± 0.12%), confirming the prevalence of the 1,8-cineole/α-pinene/camphor chemotype. Antimicrobial assays demonstrated stronger activity against fungal strains, particularly C. krusei and C. albicans, with enhanced efficacy observed in EOs collected in October-November. Synergy assays showed significant potentiation of streptomycin activity, particularly against E. coli in autumn, with up to a 32-fold increase in efficacy. In contrast, only limited synergistic effects were observed with amphotericin B. Overall, our findings emphasize the clinical relevance of optimizing harvest timing, as autumn-harvested rosemary EOs exhibit the greatest potential as natural antibiotic adjuvants against multidrug-resistant pathogens. Nevertheless, the partial antagonism with amphotericin B highlights the need for strain-specific compatibility assessments to avoid compromising drug efficacy in combinatory therapies.
- Rosmarinus officinalis,
- flowering period,
- essential oil,
- chemical composition,
- autumn period,
- antimicrobial activity,
- synergistic effects
Citation: Imane Abbad, Bouchra Soulaimani, Imane El Hakioui, Soraia El Baz, Elena Maria Varoni, Marcello Iriti. Phytochemical composition and synergistic antimicrobial effects of Rosmarinus officinalis essential oils during flowering in an arid mediterranean region[J]. AIMS Microbiology, 2025, 11(4): 769-785. doi: 10.3934/microbiol.2025033

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Abstract

Rosmarinus officinalis has attracted significant attention due to its broad-spectrum antimicrobial activity, largely attributed to its bioactive essential oils (EOs). Several studies indicate that the flowering period is crucial for harvesting rosemary's aerial parts for optimal EO extraction. However, its prolonged flowering period complicates the determination of an optimal harvest time, potentially affecting yield, chemical composition, and efficacy. This study provides, for the first time, a systematic month-by-month evaluation of EO yield, chemical composition, and synergistic antimicrobial potential of rosemary cultivated under arid Mediterranean conditions during its flowering period (September to March). EO samples were analyzed by GC-MS and assessed for antimicrobial activity against clinically relevant pathogens, including S. aureus, E. coli, S. enterica, and four Candida species. The synergistic potential was further evaluated with two conventional antimicrobials (streptomycin and amphotericin B). The results showed that the EO yields ranged from 1.73% to 2.75%, with a clear peak in autumn. GC-MS analysis identified 31 compounds, dominated by 1,8-cineole (27.57 ± 0.76%–36.28 ± 0.26%), α-pinene (15.36 ± 0.23%–28.97 ± 0.10%), and camphor (7.12 ± 0.00%–15.37 ± 0.12%), confirming the prevalence of the 1,8-cineole/α-pinene/camphor chemotype. Antimicrobial assays demonstrated stronger activity against fungal strains, particularly C. krusei and C. albicans, with enhanced efficacy observed in EOs collected in October-November. Synergy assays showed significant potentiation of streptomycin activity, particularly against E. coli in autumn, with up to a 32-fold increase in efficacy. In contrast, only limited synergistic effects were observed with amphotericin B. Overall, our findings emphasize the clinical relevance of optimizing harvest timing, as autumn-harvested rosemary EOs exhibit the greatest potential as natural antibiotic adjuvants against multidrug-resistant pathogens. Nevertheless, the partial antagonism with amphotericin B highlights the need for strain-specific compatibility assessments to avoid compromising drug efficacy in combinatory therapies.

Acknowledgments

We gratefully acknowledge all members of the investigator's laboratory for their valuable discussions. We also extend our thanks to Cadi Ayyad University and its technical staff for their assistance in the cultivation of Rosmarinus officinalis.

Conflict of interest

The authors declare no conflicts of interest.

Author contributions

Conceptualization, I.A. and B.S.; methodology, I.A., B.S. and I.H.; formal analysis, I.A. and I.H.; investigation, I.A. and I.H.; data curation, I.A. and I.H.; writing-original draft preparation, I.A. and I.H.; writing-review and editing, I.A. and I.H.; supervision, M.I., E.M.V. and S.E.B.; project administration, M.I., E.M.V. and S.E.B.; funding acquisition, M.I. and E.M.V.

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