Antibacterial activity of leaf essential oils and their constituents from Cinnamomum osmophloeum

Abstract

The antibacterial activities of the essential oils from leaves of two Cinnamomum osmophloeum clones (A and B) and their chemical constituents were investigated in this study. The nine strains of bacteria, including Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis, methicillin-resistant Staphylococcus aureus (MRSA), Klebsiella pneumoniae, Salmonella sp., and Vibrio parahemolyticus, were used in the antibacterial tests. Results from the antibacterial tests demonstrated that the indigenous cinnamon B leaf essential oils had an excellent inhibitory effect. The MICs (minimum inhibitory concentrations) of the B leaf oil were 500 μg/ml against both K. pneumoniae and Salmonella sp. and 250 μg/ml against the other seven strains of bacteria. Cinnamaldehyde possessed the strongest antibacterial activity compared to the other constituents of the essential oils. The MICs of cinnamaldehyde against the E. coli, P. aeruginosa, E. faecalis, S. aureus, S. epidermidis, MRSA, K. pneumoniae, Salmonella sp., and V. parahemolyticus were 500, 1000, 250, 250, 250, 250, 1000, 500, and 250 μg/ml, respectively. These results suggest that C. osmophloeum leaf essential oil and cinnamaldehyde are beneficial to human health, having the potential to be used for medical purposes and to be utilized as anti-bacterial additives in making paper products.

Introduction

Plant-derived essential oils have long been used as flavoring agents in food and beverages and, due to the presence of antimicrobial compounds, they have potential as natural agents for food preservation (Helander et al., 1998). Cinnamon oil is commonly used in the food industry because of its special aroma. In addition, its antimicrobial activity has also attracted great attention from many researchers. Cinnamomum cassia bark oil is generally used in food and beverages and is very valuable in commerce. The main constituents of C. cassia bark oil are cinnamaldehyde and coumarin (Hu et al., 1985). These two compounds have also been used as food additives. It has also been demonstrated that Cinnamomum zeylanicum oil has an inhibitory effect against meat spoilage organisms (Ouattara et al., 1997). Singh and his co-workers (Singh et al., 1995) have demonstrated that C. zeylanicum bark oil has fungitoxic properties against fungi involved in respiratory tract mycoses, such as Aspergillus niger, A. fumigatus, A. nidulans, and A. flavus. In addition, C. zeylanicum oil has been tested for maize kernel protection against A. flavus (Montes-Belmont and Carvajal, 1998). The results have revealed that it can effectively inhibit the growth of A. flavus and has no phytotoxic effect on germination and corn growth. The important chemical constituents of C. zeylanicum oil are cinnamaldehyde and eugenol (Ross, 1976). These two compounds have also been demonstrated to have inhibitory properties against A. flavus (Montes-Belmont and Carvajal, 1998).

Cinnamomum osmophloeum Kaneh. (Lauraceae) is an endemic tree in Taiwan. It has been of interest to researchers because the chemical constituents of its leaf essential oils were similar to those of C. cassia bark oil. In order to explore the potential usefulness of the leaf essential oils of C. osmophloeum, it is important to know their chemical constituents. Therefore, the chemical constituents of leaf essential oils from various C. osmophloeum clones found in different regions of Taiwan have been studied by many researchers. Hu and his coworkers analyzed the composition of the essential oils of C. osmophloeum leaves collected randomly from 21 provenances in central, southern, and eastern parts of Taiwan. It was found that cinnamaldehyde was the major constituent in the leaf essential oils of some C. osmophloeum clones, and that eugenol was present in the leaf essential oils of the other C. osmophloeum clones. In addition, based upon the chemical composition of the different leaf essential oils, they classified C. osmophloeum into nine types such as cassia type, cinnamaldehyde type, coumarin type, linalool type, eugenol type, camphor type, 4-terpinenol type, linalool-terpinenol type, and mixed type (Hu et al., 1985).

Although the chemical constituents of leaf essential oils of various C. osmophloeum clones have been studied, the potential antimicrobial activities of their leaf essential oils and their constituents has not yet been evaluated. In this study, the essential oils of leaves collected from C. osmophloeum clones (A and B) were extracted and their chemical compositions analyzed, then the antibacterial activity of the essential oils and their chemical constituents were investigated. In addition, the antibacterial activity of cinnamaldehyde congeners was examined to help understand the effect of chemical structure on the antibacterial activity.