3.2 Comparison of antibacterial capacity of different CEOs
Different CEOs have different antibacterial effects due to their differences in chemical composition and structure, and their antibacterial abilities are related to their different active ingredients and interactions between active ingredients. Kim's research shows that different chemical components in CEOs have different antibacterial abilities. Among the active components, phenols showed the strongest antibacterial activity, followed by aldehydes, alcohols, ketones, esters and hydrocarbons. In addition, a large number of studies have shown that hydroxyl (-- OH), methoxyl (-- OCH3), olefins bond and lipophile chemical structure play an important role in improving the antibacterial ability of essential oils. In the antibacterial evaluation system, the minimum inhibitory concentration (MIC) is one of the commonly used indicators to measure the antibacterial effect. The smaller the MIC value is, the better the antibacterial effect is. Table 1 lists MIC values of several common CEOs, comparing their antibacterial capabilities. It can be seen from the table that different CEOs have different antibacterial powers against the same bacteria, and the same CEOs have different antibacterial powers against different bacteria. In general, bergamot essential oil and lemon oil showed better antimicrobial effectiveness for the five common CEOs listed in Table 1.
3.3 Selective antibacterial activity of CEOs
CEOs have selective antibacterial activity. Studies have reported CEOs who can inhibit the activity of pathogenic bacteria, such as Listeria, Salmonella, EScherichia coli, Staphylococcus aureus, and Bacillus cereus, while stimulating beneficial microbes, such as lactobacillus in pigs. CEOs have high antibacterial activity against EScherichia coli, but little or no effect against Lactobacillus, and the inhibitory effect of CEOs is dose-dependent. In addition, studies have found that when CEOs act on bacteria, lactobacilli have mics more than twice as high and minimum bactericidal concentration (MBC) values more than four times as high as E. coli. These studies have shown that CEOs have selective antibacterial activity against both harmful and beneficial bacteria.
The selective antibacterial activity of CEOs may be related to the structure of the bacterial cell membrane. The selective antibacterial activity of CEOs against Escherichia coli and Lactobacillus has been reported to some extent. Is commonly believed that plant essential oil of gram-positive pathogens inhibition effect is higher than that of gram-negative pathogens, differences in these two bacterial susceptibility to plant essential oil is believed to be differences in cell wall structure, because of gram positive bacteria lack of gram-negative bacteria of outer membrane, the outer membrane containing polarity and across the membrane channel at the end of the lipopolysaccharide (LPS), It allows the passage of hydrophilic solutes, making it difficult for hydrophobic compounds such as plant essential oil components to diffuse into cells. This would therefore make gram-negative bacteria more resistant to essential oils. However, contrary results were obtained in the study of Ambrosio et al. E. coli was gram-negative bacteria and Lactobacillus was Gram-positive bacteria, but E. coli was more sensitive to CEOs than Lactobacillus because the antibacterial spectrum of plant essential oils depended on the specificity of functional groups of job essential oils to single or multiple targets. Some essential oil compounds can break down the outer membrane of E. coli, releasing substances associated with this membrane and penetrating the cell, thus causing damage and bacteriostatic effects. Compounds in CEOs may also have this ability, making CEOs more sensitive to E. coli. In addition, studies by Viuda-Martos et al. revealed that CEOs can inhibit lactobacillus only at high concentrations, which to some extent proves the selective antibacterial activity of CEOs against EScherichia coli and Lactobacillus. However, the mechanisms of selective antibacterial activity between CEOs need to be further studied.
