Synergistic Antibacterial Effect of Eugenol and Biogenic Silver Nanoparticles on Staphylococcus pseudintermedius Isolated from Canine Keratoconjunctivitis Sicca

Plants are a valuable source of bioactive compounds with therapeutic potential. Antibacterials of natural origin represent a promising and sustainable alternative in the fight against bacterial infections. In addition to being effective against bacterial growth, these natural agents may have lower toxicity and fewer side effects, which reinforces their value in the development of new therapeutic strategies. This study reports on the antibacterial effect of eugenol (EUG) and biogenic silver nanoparticles (bioAgNPs) synthesized using the aqueous extract of <i>Trichilia catigua</i> A. Juss. bark, alone or in combination, against planktonic and sessile cells of multidrug-resistant <i>Staphylococcus pseudintermedius</i>, one of the main opportunistic pathogens in dogs. EUG and bioAgNPs showed a dose- and time-dependent bactericidal effect on planktonic cells, interfering with cell membrane integrity. The interaction between EUG and bioAgNPs was classified as synergistic or indifferent for planktonic cells. Except for one isolate, the combination exhibited a synergistic effect for biofilms previously formed on abiotic surfaces for 24 h. Both bioactive compounds promoted morphological and ultrastructural changes in <i>S. pseudintermedius</i> biofilms. All concentrations of EUG and bioAgNPs in synergistic or indifferent combinations showed reduced toxicity to mammalian cells. These findings suggest that the EUG and bioAgNP combination could be a promising strategy for controlling <i>S. pseudintermedius</i> infections.