Bacterial diseases occur in fishes due to environmental pollution and culture system at high density. Antibiotics have been effective in treating bacterial diseases, but the reduction of their efficacy from resistance has become a serious problem. Essential oils and flavonoid are known as antifungal, antiviral, and antibacterial substances and it was also reported that some of these substances increase the antimicrobial susceptibility to antibiotics. With this back ground as the basis, the purpose of this study is to explore the possibility that antibiotics can perform better efficacy in the presence of essential oils and flavonoid against Edwardsiella tarda and Streptococcus iniae.
Before tasting combination effects, minimum inhibitory concentration (MIC) were obtained for 20 aquaculturally important antibiotics which are classified into 7 groups (β-lactam, tetracycline, aminoglycoside, macrolide chloramphenicol, quinolone, sulfonamide) against E. tarda and S. iniae. Then, it was tasted whether MIC changes in the presence of essential oils (thymol, carvacrol, eugenol, cinnamaldehyde), and flavonoid (quercetin). For this, concentration of natural substances was 1 ㎍/㎖. Essential oils and flavonoid increased antibacterial efficacy of oxytetracycline and flumequine against E. tarda by 2 folds. Essential oils and flavonoid increased antibacterial efficacy of nalidixic acid against E. tarda by 4 folds. Flavonoid increased antibacterial efficacy of josamycin against S. iniae by 4 folds. The results obtained in this study indicate that essential oils and flavonoid can increase antimicrobial activity of nalidixic acid and josamycin against E. tarda and S. iniae in fish. Natural compounds examined in this study will also be useful in decreasing the overuse of antibiotics in fish.

A thesis submitted to the Committee of the Graduate School of Kunsan National University in partial fulfillment of the requirements for degree of Master of Science in February 2014.