Abstract 1283 Antibacterial and Synergistic Effects of Antibiotics, Essential Oils, and Essential Oil Components against E. coli and S. epidermidis

Antibiotics have changed the world, as they are one of the most important medical discoveries of the 20th century with the ability to kill bacterial infections, previously the leading cause of death. However, with overuse in agriculture, overprescription by doctors, and misuse by patients, public health experts have identified an increase in bacterial resistance, which is slowly leading to a return of the pre-antibiotic era. In an effort to combat this increased resistance and explore other antimicrobial avenues, this study investigates the antimicrobial and synergistic potential of antibiotics, essential oils, and essential oil components. We selected seven antibiotics (kanamycin, ampicillin, neomycin, gentamicin, tobramycin, ciprofloxacin HCl, and sulfamethoxazole), three components of essential oils (thymol, menthol, and carvacrol), and five essential oils (thyme, pink grapefruit, tea tree, oregano, and clove), and used two species (Escherichia coli ATCC 25922 and Staphylococcus epidermidis ATCC 14990). To assess antibacterial properties, we determined the minimum inhibitory concentration (MIC) values and we analyzed synergy using the fractional inhibitory concentration (FIC) values. We observed that MIC values tended to be greater for the Gram-negative species compared to the Gram-positive species, and synergy occurred between the sulfamethoxazole-trimethoprim antibiotic combination as well as between menthol and all aminoglycoside antibiotics. It is possible that a lower concentration of the antibacterial is needed for Gram-positive organisms because they lack an outer membrane typical of Gram-negative organisms, and that the mechanisms of action of the menthol essential oil and aminoglycoside antibiotics are similar.