Essential Oils of Zingiber Species from Vietnam: Chemical Compositions and Biological Activities

Mosquito-borne diseases are a large problem in Vietnam as elsewhere. Due to environmental concerns regarding the use of synthetic insecticides as well as developing insecticidal resistance, there is a need for environmentally-benign alternative mosquito control agents. In addition, resistance of pathogenic microorganisms to antibiotics is an increasing problem. As part of a program to identify essential oils as alternative larvicidal and antimicrobial agents, the leaf, stem, and rhizome essential oils of several Zingiber species, obtained from wild-growing specimens in northern Vietnam, were acquired by hydrodistillation and investigated using gas chromatography. The mosquito larvicidal activities of the essential oils were assessed against Culex quinquefasciatus, Aedes albopictus, and Ae. aegypti, and for antibacterial activity against a selection of Gram-positive and Gram-negative bacteria, and for activity against Candida albicans. Zingiber essential oils rich in α-pinene and β-pinene showed the best larvicidal activity. Zingiber nudicarpum rhizome essential oil showed excellent antibacterial activity against Enterococcus faecalis, Staphylococcus aureus, and Bacillus cereus, with minimum inhibitory concentrations (MIC) of 2, 8, and 1 μg/mL, respectively. However, the major components, α-pinene and β-pinene, cannot explain the antibacterial activities obtained.


Introduction
Vietnam is located in the tropics of Southeast Asia, and several mosquito-borne diseases are endemic, including Japanese encephalitis [1], dengue fever [2], and Zika [3]. Culex species are considered to be important vectors of Japanese encephalitis virus (JEV), including Culex quinquefasciatus
Zingiber vuquangensis leaf and rhizome essential oils were both rich in α-pinene (11.3% and 9.8%, respectively) and β-pinene (38.5% and 45.0%, respectively). The sesquiterpene hydrocarbons trans-β-elemene (5.9% and 10.0%), and (E)-caryophyllene (12.2% and 14.4%) were major components in the leaf and stem essential oils, respectively. The leaf, stem, and rhizome essential oils from Z. vuquangensis from Vu Quang National Park, Ha Tinh Province, Vietnam, have been previously published [46]. A comparison of the major components is summarized in Table 4. Although there are qualitative similarities in the essential oil compositions from these two collections (α-pinene, β-pinene, and (E)-caryophyllene are major components), there are some notable differences. Bornyl acetate and zerumbone were major components in the rhizome essential oil from the Vu Quang collection, but were not observed in the Pù Hoạt sample; trans-β-elemene was observed in relatively small concentrations in the sample from Vu Quang, but was a major component in the leaf and stem essential oils from Pù Hoạt. The differences in chemical composition can be attributed to the geographical locations of the two collections and/or the season when the samples were collected. The Vu Quang sample was collected in August, 2014 (rainy season), while the Pù Hoạt sample was collected in April, 2019 (dry season).

Mosquito Larvicidal Activity
Several of the Zingiber essential oils (depending on availability) were assayed for insecticidal activity against larvae of Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus mosquitoes. The 24and 48-h larvicidal activities are presented in Table 5.

Antimicrobial Activity
Several of the essential oils from Zingiber species were tested for antibacterial activity against a panel of Gram-positive (Enterococcus faecalis, Staphylococcus aureus, and Bacillus cereus), and Gram-negative (Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica) bacteria, and for anticandidal activity against Candida albicans ( Table 6). The essential oils generally showed good to excellent activity against the Gram-positive organisms compared to Gram-negative. It has frequently been noted that Gram-positive bacteria demonstrate a higher susceptibility to essential oils than do Gram-negative organisms [64][65][66]. This phenomenon has been attributed to the existence of cell wall lipopolysaccharides in the Gram-negative bacteria, which can inhibit the hydrophobic essential oil constituents from diffusing into the cells [67,68]. Candida albicans was also relatively sensitive to the Zingiber essential oils.
The essential oil with the best overall antimicrobial activity was Z. nudicarpum rhizome essential oil from Pù Hoạt Nature Reserve with MIC < 10 µg/mL against all three Gram-positive organisms and MIC = 16 µg/mL against P. aeruginosa and C. albicans. It is difficult to correlate essential oil composition with antimicrobial activity, however. The rhizome essential oil of Z. nudicarpum was rich in α-pinene (18.7%) and β-pinene (58.3%). The antimicrobial activities of α-pinene and β-pinene have ranged from excellent to inactive against E. faecalis, S. aureus, B. cereus, or C. albicans [69][70][71]. However, the presence of these two compounds as major components is not enough to impart good antimicrobial activity. The leaf essential oil of Z. recurvatum and the leaf and stem essential oils of Z. cornubracteatum from Bến En National Park were also rich in α-pinene (16.3%, 10.1%, and 9.9%, respectively) and β-pinene (71.6%, 67.3%, and 66.8%), but these essential oils showed significantly lower antimicrobial activity. There are likely synergistic and/or antagonistic effects of minor components responsible for the activities.