Abstract: ObjectiveThe antibacterial effects of protocatechuic acid and chlorogenic acid combined with antibiotic on fish-source Streptococcus were studied to provide reference for the comprehensive prevention and treatment of fish-source Streptococcus disease.MethodThe minimum inhibitory concentrations(MIC)of protocatechuic acid,chlorogenic acid and antibiotics against fish-source Streptococcus were tested through double microdilution method,and fish-source Streptococcus were induced to subculture by inhibitory concentration(1/2MIC)of protocatechuic acid or chlorogenic acid combined with antibiotics respectively.Then the fractional inhibitory concentration indexes(FICI)of protocatechuic acid or chlorogenic acid combined with antibiotics respectively were determined by microdilution checkerboard method.Re-sultThe MICs of protocatechuic acid and chlorogenic acid against fish-source Streptococcus were 2500 μg/mL and 6000 μg/mL.After fish-source Streptococcus being induced to subculture by 1250 μg/mL protocatechuic acid and 3000 μg/mL chlorogenic acid respectively combined with mequindox,amikacin,colistin,sulfamonomethoxine,clinafloxacin,and fosfomycin,the MICs of the antibiotics greattly decreased except sulfamonomethoxine. When protocatechuic acid was combined with the antibiotics mentioned above,the FICIs were 0.500,0.750,0.516,1.125,0.140 and 0.516 respective-ly.When chlorogenic acid was combined with the antibiotics mentioned above,the FICIs were 0.501,0.625,0.501,>2.000, 0.140 and 0.501.According to the criteria for FICI,there existed synergistic effects when protocatechuic acid was com-bined with mequindox and clinafloxacin;there existed additive effects when protocatechuic acid was combined with ami-kacin,colistin and fosfomycin,but there were indifferent effects when protocatechuic acid was mixed with sulfamono-methoxine.There were synergistic effects when chlorogenic acid was combined with amikacin and clinafloxacin;there existed additive effects after chlorogenic acid was mixed with mequindox,colistin and fosfomycin,but there were indif-ferent effects when chlorogenic acid was combined with sulfamonomethoxine.ConclusionThe combination of protocate-chuic acid or chlorogenic acid with antibiotics can enhance the antibacterial activity of most tested antibiotics against fish-source Streptococcus and presents synergistic effects or additive effects.Therefore,it can be promoted in aquaculture.