Date of Award

2025

Document Type

Thesis

Degree Name

Doctor of Pharmaceutical Sciences - Pharmaceutics

Department

Pharmaceutics

Committee Chair and Members

Deborah Kayode, Chair

Abdullah Alzahrani

Ahmed Abu Fayyad

Keywords

Antimicrobial activity, Natural preservatives, Phenoxyethanol, Rosemary oil (Rosmarinus officinalis), Staphylococcus aureus, Turmeric (Curcuma longa)

Abstract

This study investigates the antimicrobial efficacy of turmeric (Curcuma longa) and rosemary essential oil (Rosmarinus officinalis) against Streptococcus aureus, with a comparative analysis to phenoxyethanol, a widely used synthetic preservative. The growing consumer demand for natural cosmetic formulations have intensified the search for plant-derived alternatives to synthetic preservatives. Turmeric and rosemary oil were selected based on their documented antimicrobial, antioxidant, and anti-inflammatory properties. The objective of this research was to evaluate whether these natural agents can serve as effective preservatives in topical gel formulations, either individually or in combination, and how their efficacy compares to phenoxyethanol.

Eight gel-based formulations were developed, incorporating turmeric, rosemary oil, and phenoxyethanol either independently or synergistically. Antimicrobial performance was assessed using the agar diffusion method and manual colony counting against a pure strain of Staphylococcus aureus. Results revealed that phenoxyethanol (F4) exhibited the strongest antimicrobial activity, indicated by the largest zone of inhibition and minimal bacterial colony formation. Turmeric (F2) demonstrated moderate activity with a smaller inhibition zone and visible colony growth. Rosemary oil (F3) produced a limited antimicrobial effect when used alone. However, the combination of turmeric and rosemary (F5) showed a synergistic interaction, enhancing the zone of inhibition but failing to reduce colony count significantly. Notably, the triple-combination formula (F8), containing turmeric, rosemary oil, and phenoxyethanol, exhibited improved surface-level diffusion and colony suppression, suggesting a promising hybrid preservation system. The study concludes that while turmeric and rosemary oil offer mild to moderate antimicrobial effects individually, their combined use can enhance surface inhibition. However, their overall efficacy remains inferior to phenoxyethanol unless used in synergy. These findings underscore the potential of integrating natural extracts into cosmetic preservation systems, particularly in combination with low doses of synthetic agents to reduce chemical load while maintaining microbial safety. Further research should investigate long-term stability, activity against a broader range of microorganisms, and compatibility in complex cosmetic matrices. This research contributes valuable insights into sustainable formulation strategies and supports the growing movement toward clean, nature-derived preservatives in modern cosmetic science.

Keywords: Antimicrobial activity, Turmeric (Curcuma longa), Rosemary oil (Rosmarinus officinalis), Phenoxyethanol, Cosmetic preservation, Natural preservatives, Antimicrobial resistance

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