Synergistic action of Thymus capitatus or Syzygium aromaticum essential oils and antibiotics combinations against multi-resistant Salmonella strains

Antibiotic-resistant foodborne Salmonella is considered among the pathogen bacteria that threaten public health seeing that this bacterium has become more virulent and has led to an in-creased risk rate among infected patients. This study aimed to evaluate the antibacterial efficacy of Thymus capitatus and Syzygium aromaticum essential oils (EO) against Salmonella strains, sepa-rately and in combination with antibiotics (gentamicin and ampicillin). The antimicrobial activ-ity was evaluated against eight strains of foodborne Salmonella which were isolated from poultry and shellfish, through the use of the agar well diffusion and the broth microdilution techniques. The combined effect of EOs and antibiotics was determined by the use of the checkerboard assay. The results of the antibacterial activity showed that the both EOs were active on all tested strains, in which the inhibition zones varied from 13.6 & PLUSMN; 2.8 mm to 43.6 & PLUSMN; 4.04 mm, and MIC values ranged from 0.125% to 0.5%. From the 36 tested combinations, three types of interactions were obtained between S. aromaticum EO and gentamicin, in which 94% were totally synergistic, 3% had a partial synergy against S. Typhimurium and 3% had no interaction against S. Kentucky. The best antibacterial activity was obtained in the combination of T. capitatus and ampicillin, in which the fractional inhibitory concentration index (FICi) ranged from 0.018 to 0.5. To the best of our knowledge, so far no study has been published before examining the combined effect of EOs with antibiotics against multi-resistant strains of Salmonella in Morocco. Thus, these results could be a starting point for further investigations in order to control infections that are caused by Salmonella.