Investigation of antibiotic and phenolic acid combinations effective on Neisseria gonorrhoeae

Ceftriaxone, which is widely used against Neisseria gonorrhoeae, has become less effective in recent years due to the development of resistance mechanisms by bacteria. In this study, the antibacterial activity of phenolic acids in combination with some antibiotics against Neisseria gonorrhoeae was studied using various methods. Of the phenolic acids used, gallic acid and caffeic acid, in particular, showed significant activity against Neisseria gonorrhoeae. According to microdilution results, MIC values of 1.171 μg/mL for gentamicin, 5.625 μg/mL for ceftriaxone, 12.5 μg/mL for caffeic acid and finally 6.25 μg/mL for gallic acid were found. Synergistic activity was evaluated using the checkerboard analysis technique. Gallic acid showed a synergistic effect with ceftriaxone 0.38 (FIK: ≤ 0.5) and a synergistic effect with caffeic acid and ceftriaxone 0.49 (FIK: ≤ 0.5). The combination of gentamicin and gallic acid showed an additive effect of 0.93 (FIK < 1) and an indifference effect with caffeic acid of 1.72 (FIK>1.0-4.0). In the flow cytometry results, the combination of gentamicin + gallic acid caused apoptosis at a rate of 85.8%. In the future, gallic acid will have a significant potential in the treatment of Neisseria gonorrhoeae infections as an alternative to antibiotic treatments such as ceftriaxone or gentamicin.