Salix aurita and Salix myrsinifolia Twig Extracts Show Synergistic or Additive Effects with Conventional Antibiotics Against Bacillus cereus

Bacillus cereus is a gram-positive foodborne enterotoxigenic bacterium that can form endospores and biofilms to resist heating and the effects of antibiotics. The emergence of multidrug-resistant B . cereus strains which are typically resistant to β-lactam antibiotics have made it difficult to treat foodborne bacterial infections. In this study, we investigated the antibiotic potentiating effects of methanol extracts of Salix aurita and Salix myrsinifolia twigs on tetracycline and ciprofloxacin hydrochloride against B. cereus . A two-dimensional checkerboard method was used, and the fractional inhibitory concentration (FIC) index was used to ascertain the synergy. The FIC index values ranged between 0.133–2.0 in all the tested concentration ratios of the antibiotic combinations. Combinations containing low concentrations of both plant species and tetracycline as well as S. aurita and ciprofloxacin gave strong synergistic interactions when compared with combinations containing a high concentration of both the antibiotic and the plant extract. The most significant synergistic effects for tetracycline in combination with the Salix extracts were obtained with the combinations containing 0.061 µg/ml of tetracycline with 19.56 µg/ml of the S. aurita extract and 0.061 µg/ml of tetracycline with 9.77 µg/ml of the S. myrsinifolia extract, both showing a FIC index of 0.133 and resulting in an 8-fold reduction of the MIC of tetracycline alone (MIC 0.488 µg/ml). Besides, a combination containing tetracycline at 0.015 µg/ml and the S. aurita extract at 625 µg/ml gave a synergistic effect resulting in a 32-fold reduction of the MIC of tetracycline. The best combination obtained with ciprofloxacin was observed for a combination containing 15 ng/ml of ciprofloxacin with 313 µg/ml of the S. aurita extract (FIC index 0.250). S. aurita and S. myrsinifolia extracts greatly enhanced the antibacterial efficacy of tetracycline and could be promising antibiotic adjuvants.