Aims: This study investigated the antibacterial activity of five phytochemicals (carvacrol, citral, eugenol, linalool and thymol) alone or in combination with florfenicol or oxytetracycline against bacteria isolated from silver catfish (Rhamdia quelen). We also analysed the potential of these compounds to inhibit biofilm formation and haemolysis caused by the bacteria.
Methods and results: Bacteria were tested with antimicrobials to calculate the multiple antibiotic resistances. The checkerboard assay was used to evaluate a putative synergy between five phytochemicals and antimicrobials against the strains isolated. The biofilm formation inhibition assay was performed with phytochemicals and antimicrobials, and the haemolysis inhibition assay was performed with the phytochemicals. Carvacrol, eugenol and thymol were the most effective phytochemicals. Three combinations (linalool with florfenicol or oxytetracycline against Aeromonas hydrophila and citral with oxytetracycline against Citrobacter freundii) demonstrated synergy in the checkerboard assay. All phytochemicals inhibited biofilm formation and haemolysis activity.
Conclusion: The tested phytochemicals showed satisfactory activity against fish pathogenic bacteria.
Significance and impact of the study: The phytochemicals did not present antagonistic interactions with the antimicrobials, allowing their combined use, which may contribute to a decrease in the use of conventional drugs and their residues in aquatic environment.
Keywords: antimicrobials; aquaculture; biofilms; fish (live); resistance.
© 2018 The Society for Applied Microbiology.