Enhancing the antimicrobial efficacy of culinary herbs and spices by experimental design

Herbs and spices are widely used across different populations, serving both culinary and medicinal purposes, particularly in sub-Saharan Africa [ 1 ]. Although herbs and spices are often used in combination, limited studies have explored their interactive antimicrobial effect [ 2 ] [ 3 ]. This study investigated the antimicrobial activity of 17 culinary herbs and spices (crude extracts and essential oils), individually, followed by 1:1 combinations of active extracts against six common foodborne pathogens. The minimum inhibitory concentrations (MIC) were determined using the broth microdilution assay. The design of experiments (DOE) approach was subsequently employed in MODDE 9.1 ®​ software, to optimize the experimental runs so as to identify the most effective antimicrobial interaction. 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 Bacillus 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 prediction was experimentally confirmed, with a strong correlation (r=0.73) [ 4 ] between 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. Publication History Article published online: 05 March 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany