Enhancing the antimicrobial efficacy of common herbs and spices through an optimized polyherbal approach

Abstract

Herbs and spices are used globally by most population groups across the world. In sub-Saharan Africa, they have medicinal significance in addition to their culinary uses. Although herbs and spices are often used in combination, there are few studies that report on their interactive antimicrobial effect. This study screened 17 culinary herbs and spices (crude extracts and essential oils) for antimicrobial activity, individually, followed by 1:1 combinations of the active extracts, to determine the outcome of combining these for antibacterial activity. The minimum inhibitory concentrations (MIC) were determined against six common foodborne pathogens using the broth microdilution assay. The design of experiments (DOE) approach was subsequently employed in MODDE 9.1® software, to optimise the experimental runs so as to identify the best interaction that would produce the best possible antimicrobial effect. Phytochemical profiling of the most active extracts was achieved using ultra-performance liquid chromatography coupled to mass spectrometry (UPLC-MS) analysis. The results demonstrated that combining Rosmarinus officinalis with either Syzygium aromaticum, or Salvia officinalis methanol extracts produced synergistic antimicrobial effects towards B. cereus with ΣFIC = 0.25 mg/ml and 0.31 mg/ml, respectively. The DOE predicted that a combination of higher ratios of R. officinalis (59.5 %), higher ratios of S. officinalis (40 %), and lower ratios of S. aromaticum (0.5 %) would produce the best antimicrobial effect with MIC = 0.17 mg/ml. This was experimentally confirmed and there was a strong correlation (r-value 0.73) between the predicted and experimental MIC values, leading to the identification of an optimal antimicrobial combination. The combination of R. officinalis (56 %) and S. officinalis (44 %) produced the antimicrobial effect (MIC = 0.19 mg/ml) which can be recommended for future studies.