Abstract

Phytochemicals are being explored as therapeutic alternatives in aquaculture since they have de-stressing, growth-promoting, appetite-increasing, immune-stimulating, and antimicrobial properties.  The susceptibility of 28 Flavobacterium johnsoniae-like isolates and nine selectedFlavobacterium spp. isolates to three phytochemicals, viz.: cinnamaldehyde (10 - 250 µg/ml), vanillin (5 - 500 µg/ml) and four crude Kigelia africana extracts (4 – 10 mg/ml ethyl acetate, dichloromethane, methanol and hexane), were assessed using disk diffusion assays and compared to standard antimicrobial agents, ampicillin and tetracycline using activity indices.  Cinnamaldehyde (250 µg/ml) was more effective than 250 µg/ml vanillin, which was ineffective even at higher concentrations.  K. africana extract (4 mg/ml) antibacterial efficacy decreased in the following order: Ethyl acetate, methanol, dichloromethane and hexane.  The 10 mg/ml methanolic K. africana extract was most effective, with 100% of isolates displaying susceptibility, irrespective of the isolation source.  Methanolic extract (10 mg/ml) activity indices ≥ 1 were obtained for 67.9 and 71.4% of isolates, respectively, relative to AMP10 and TE30.  Cinnamaldehyde and the K. africana methanol extract are promising candidates to be tested for their efficacy in the treatment of Flavobacterium-associated fish infections.  These phytochemicals might be environmentally-friendly, cost-effective alternatives to antimicrobial agent use in aquaculture, with a lesser potential of resistance development.

Key words: Aquaculture, Flavobacterium, phytotherapy.