Citrus essential oils (EOs) have shown significant antibacterial activity. The present study was undertaken to evaluate the antibacterial activity of the peel oils of Citrus microcarpa and C. x amblycarpa against Escherichia coli. The minimum inhibition concentration (MIC) was determined by using the broth microdilution assay. The checkerboard method was used to identify synergistic effects of the EOs with tetracycline, while bacteriolysis was assessed by calculating the optical density of the bacterial supernatant, crystal violet assay was used to assess their antibiofilm. Ethidium bromide accumulation test was employed to assess efflux pump inhibition. Electron microscope analysis was performed to observe its morphological changes. The EOs of C. microcarpa and C. x amblycarpa were found to contain D-limonene major compound at 55.78% and 46.7%, respectively. Citrus microcarpa EOs exhibited moderate antibacterial against E. coli with a MIC value of 200 μg/mL. The combination of C. microcarpa oil (7.8 μg/mL) and tetracycline (62.5 μg/mL) exhibited a synergy with FICI of 0.5. This combination inhibited biofilm formation and disrupt bacterial cell membranes. Citrus microcarpa EOs blocked the efflux pumps in E. coli. Citrus microcarpa EOs demonstrated promising antibacterial activity, which can be further explored for the development of drugs to combat E. coli.
Zingiber and Alpinia species (family: Zingiberaceae) are popularly used in food as spices and flavoring agents and in ethnomedicine to heal numerous diseases, including immune-related disorders. However, their ethnomedicinal uses have not been sufficiently supported by scientific investigations. Numerous studies on the modulating effects of plants and their bioactive compounds on the different steps of the immune system have been documented. This review aimed to highlight up-to-date research findings and critically analyze the modulatory effects and mechanisms of the extracts and secondary compounds of several Zingiber and Alpinia species, namely, Zingiber officinale Roscoe, Z. cassumunar Roxb., Z. zerumbet (L.) Roscoe ex Sm., Alpinia galanga Linn., A. conchigera Griff, A. katsumadai Hayata, A. oxyphylla Miq., A. officinarum Hance, A. zerumbet (Pers.) Burtt. et Smith, and A. purpurata (Viell.) K. Schum. on the immune system, particularly via the inflammation-related signaling pathways. The immunomodulating activities of the crude extracts of the plants have been reported, but the constituents contributing to the activities have mostly not been identified. Among the extracts, Z. officinale extracts were the most investigated for their in vitro, in vivo, and clinical effects on the immune system. Among the bioactive metabolites, 6-, 8-, and 10-gingerols, 6-shogaol, and zerumbone from Zingiber species and cardamomin, 1'-acetoxychavicol acetate, yakuchinone, rutin, 1,8-cineole, and lectin from Alpinia species have demonstrated strong immunomodulating effects. More experimental studies using cell and animal models of immune-related disorders are necessary to further understand the underlying mechanisms, together with elaborate preclinical pharmacokinetics, pharmacodynamics, bioavailability, and toxicity studies. Many of these extracts and secondary metabolites are potential candidates for clinical development in immunomodulating agents or functional foods to prevent and treat chronic inflammatory disorders.