Antimicrobial Activity and Effect of Selected Essential Oil Components on Cell Membrane Lipids

The objectives of this study were to evaluate the antimicrobial activity of essential oil components (EOC) against Salmonella Typhimurium and Listeria monocytogenes and evaluate changes in bacterial membrane composition by observing changes in the fatty acids of Salmonella associated with exposure and adaptation to (or growth in the presence of) cinnamaldehyde (CIN) and carvacrol (CRV) EOC. Ten EOC were tested for efficacy using the broth microdilution and macrodilution methods. Beta-caryophyllene, limonene, alpha-pinene, and thymol were not lethal to Salmonella and Listeria at 2800mg/L. Dose-response models showed that the most effective EOC against Salmonella were CIN and CRV (50% probability of lethality with avg. of 280mg/L and 1080mg/L, respectively) regardless of method. CIN (790mg/L) and RHO (1810mg/L) were the most lethal against Listeria. CIN- or CRV-adapted or non-adapted Salmonella were inoculated into broth containing 250mg/L CIN, 871mg/L CRV, or no EOC. At 2 and 24h, the saturated fatty acid (SFA) to unsaturated fatty acid (UFA) ratio in the membrane of CIN- or CRV-adapted cells treated with CIN or CRV was numerically higher than in the non-adapted cells treated with CIN or CRV. Significant (p\u3c0.05) changes were observed with CIN or CRV exposure. After 2h of exposure to CIN, the non-adapted CIN-treated cells had lower total phospholipid fatty acid (PLFA), lower C16:1w7c, and higher C18:0 and SFA than the non-adapted control. After 24h of exposure to CIN, the non-adapted CIN-treated Salmonella had lower Cy17:0 and Cy19:0 than the non-adapted control and higher C16:0 and SFA. Compared to the non-adapted control at 2h, the non-adapted CRV-treated Salmonella had lower PLFA, and lower C16:1w7c, Cy17:0, C18:1w9c, C18:1w7c, and UFA and higher C14:0, C18:0, SFA, and SFA/UFA ratio. At 24h, the non-adapted CRV-treated cells had lower PLFA, Cy17:0, C18:1w7c, and UFA and higher C16:1w7c and SFA than the non-adapted control. Antimicrobial treatment appeared to decrease the fluidity of the Salmonella membrane by increasing SFA. This decreased fluidity may prevent additional CIN and CRV from permeating. Growth in the presence of the antimicrobial had a much smaller effect on the fatty acid composition. Additional measurement of the membrane transition temperature from gel-to-liquid-crystalline phase would indicate fluidity