Objective In order to establish a scientific foundation for the prevention and management of diseases in largemouth bass (Micropterus salmoides) culture, the objective of this study is to examine the in vitro antibacterial effects of 59 Chinese medicines and their combinations on the pathogenic bacteria of largemouth bass and to identify the Chinese medicines that have effective bacteriostatic effects.

Method The agar plate diffusion method and Oxford cup perforation method were used to screen the single Chinese medicines with bacteriostatic effect in a preliminary manner. In order to screen the single and compounded Chinese medicines with better bacteriostatic effect, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the single and compound Chinese medicines against the pathogenic bacteria from largemouth bass were determined using the two-fold dilution method.

Result Both Edwardsiella tarda and Aeromonas hydrophila were shown to be extremely sensitive to eight Chinese medicines, including Caesalpinia sappan, Terminalia chebula, Scutellaria baicalensis, Galla chinensis, Syzygium aromaticum, Euphorbia humifusa, Sanguisorba officinalis and Punica granatum. Of these, the diameters of inhibition circles of G. chinensis and S. aromaticum against A. hydrophila were 21.20 mm and 26.19 mm, respectively, with MICs of 7.81 mg/mL and MBCs of 15.63 mg/mL; the inhibition circle of G. chinensis against E. tarda reached 30.33 mm in diameter, with MICs of 3.91 mg/mL and MBCs of 7.81 mg/mL, respectively; the two pathogens were highly sensitive to M. officinalis, P. vulgaris, A. argyi, and V. yedoensis with diameters ranging from 15.84 mm to 19.47 mm respectively. The most pronounced bacteriostatic effect on A. hydrophila was recorded with C. sappan+G.chinensis and S. aromaticum+T. chebula exhibiting MIC and MBC of 3.91 mg/mL. Additionally, C. sappan+G. chinensis, S. aromaticum+S. baicalensis, C. sappan+T. chebula, G. chinensis+S. baicalensis, and G. chinensis+P. granatum demonstrated efficacy against E. tarda, with MIC and MBC of 3.91 mg/mL. C. sappan+T. chebula, S. aromaticum+T. chebula, and S. aromaticum+S. baicalensis exhibited additive effects on the two pathogenic bacteria. The inhibitory effects among the combinations of C. sappan+C. chinensis, S. aromaticum+P. granatum, and C. sappan+P. granatum demonstrated unrelated inhibitory effects on the two pathogens. Conversely, the combinations of S. aromaticum+G. chinensis, G. chinensis+C. chinensis exhibited antagonistic effects on the two pathogenic bacteria.

Conclusion In conclusion, six single Chinese medicines including C. sappan, T. chebula, S. baicalensis, G. chinensis, S. aromaticum and P. granatum, exhibit superior antibacterial properties against A. hydrophila and E. Tarda. Specifically, C. sappan+G. chinensis had the strongest inhibitory effects on the two pathogens.