The aim of this study was to evaluate the antibacterial effect of the association between conventional
antibiotics and
essential oils (EOs) of endemic Moroccan thyme species, Thymus maroccanus and T. broussonetii, on
antibiotic-resistant bacteria involved in
nosocomial infections. Synergistic interactions between
antibiotics (
ciprofloxacin,
gentamicin,
pristinamycin, and
cefixime) and EOs, and between T. maroccanus and T. Broussonetii EOs were determined by the checkerboard test. Serial dilutions of two
antimicrobial agents were mixed together so that each row (and column) contained a fixed amount of the first agent and increasing amounts of the second one. The results indicate that the
oils had a high inhibitory activity against tested bacteria, except for Pseudomonas aeruginosa. In parallel with the increase of cellular killing, the release of 260nm-absorbing materials from bacterial cells, treated with EOs, increased in response to oil concentration. Out of 80 combinations tested between EOs and
antibiotics, 71% showed total synergism, 20% had partial synergistic interaction and 9% showed no effect. Combination with
carvacrol, the major constituent of T. maroccanus and T. broussonetii, showed also an interesting synergistic effect in combination with
ciprofloxacin. The effect on Gram-positive bacteria was more important than on Gram-negative bacteria. These findings are very promising since the use of these combinations for
nosocomial infections treatment is likely to reduce the minimum effective dose of the
antibiotics, thus minimizing their possible toxic side effects and treatment cost. However, further investigations are needed to assess the potential for therapeutic application.