The Interactive Antimicrobial Activity of Withania somnifera (L.) Dunal Root Extracts and Conventional Antibiotics Against some Bacterial Triggers of Autoimmune Inflammatory Diseases

Background: Withania somnifera (L.) Dunal roots are known for their anti-inflammatory and rejuvenative properties. This study focuses on the growth inhibitory activity of W. somnifera root extracts against some bacte- rial triggers of autoimmune inflammatory diseases, as well as the extracts in combination with conventional antibiotics. Methods: W. somnifera root powder was extracted with solvents of varying polarity and screened for inhibition of bacterial growth. Inhibition on agar was assessed by disc diffu- sion techniques, whilst the minimum inhibitory concentrations (MICs) were quantified by liquid dilution assays. To screen for combinatorial effects, the W. somnifera root extracts were combined with a range of conventional antibiotics and tested against each bacterial strain using liquid dilution assays. Extract toxicities was examined using Artemia nauplii bioassays, while their phytochemical profiles were determined using standard meth- odologies. Results: W. somnifera root extracts were unable to inhibit the growth of P. mirabilis, K. pneumonia, A. baylyi and Pseudomonas aeruginosa. However, the methanolic and aqueous extracts were good inhibitors of Y. enterocolitica, with MICs of 696 and 358 μg/mL respectively. These extracts also inhibited S. pyogenes growth, albeit with substantially higher MIC values indicative of only mild inhibition. Only small zones of inhibition were observed on agar for these extracts against both strains. Combina- tions of the W. somnifera extracts and conventional antibiotics generally produced additive or indifferent interactions, indicating that they are safe to use concomitantly without compromising the efficacy of either compo- nent. A single antagonistic combination (aqueous extract and tetracycline) against Y. enterocolitica was observed. All extracts were determined to be non-toxic as assessed in Artemia nauplii bioassays. Conclusions: Meth- anolic and aqueous W. somnifera extracts were strong inhibitors of Y. enterocolitica growth and mild inhibitors of S. pyogenes. These extracts were also non-toxic in the Artemia nauplii bioassay, indicating their potential for the prevention and treatment of Hashimoto’s disease.