Antibiotic potentiating effects of Salix twig extracts against Bacillus cereus

Plant extracts and compounds are known to possess antibiotic potentiating effects (Abreau et al., 2017).
The use of plant extracts and compounds as antibiotic adjuvants could be a way to repurpose antibiotics
that have lost their effects against resistant bacteria. Moreover, in the combinations it is possible to use
the antibiotics in extremely small concentrations, thus reducing their toxic effects. In addition, the risk of
development of resistance is reduced for a combination. Only a few Salix species, such as the African S.
ledermannii, have so far been evaluated for their combination effects with antibiotics (Demgne et al., 2022).
Thus, we have chosen to study Salix starkeana, S. myrsinifolia and S. aurita that grow naturally in Finland.
In our studies, methanol extracts of the winter twigs of these Salix species gave growth inhibitory effects
against B. cereus with MIC values of 625, 1250 and 2500 μg/ml, respectively. In addition, two antibiotics,
tetracycline with MIC of 0.488 μg/ml and ciprofloxacin hydrochloride with MIC of 0.06 μg/ml against B.
cereus were chosen for the combination studies. A 96 well microplate checkerboard method was used to test
the combination effects (Mordmuang et al., 2019). To evaluate the results of the combinations, we calculated
the FIC indexes (SFIC or FICI) for each combination producing 90% or more of growth inhibition. The FICI
was calculated as ΣFIC = FICA + FICB = (CA/MICA) + (CB/MICB), where MICA and MICB are the MICs
of drugs A and B alone, respectively, and CA and CB are the concentrations of the drugs in combination,
respectively. Several combinations of all tested Salix extracts with tetracycline hydrochloride resulted in
synergistic effects. Moreover, our combination tests showed that even at concentrations of 0.03 ́ MIC (0.061
μg/ml), tetracycline produced synergistic effects when combined with a Salix aurita methanol twig extract
at 625 μg/ml