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.
Highlights
Aquaculture is one of the fastest growing industries with~80 million tons of farmed fish and shellfish being produced annually
Flavobacterium johnsoniae-like isolates appear to be significant in the Southern African context as aquaculture pathogens (Flemming et al, 2007; Basson et al, 2008), being isolated predominantly from diseased cultured trout
Displaying susceptibility (Table 3).Concentrationdependent differences were observed in the antimicrobial effects of cinnamaldehyde and vanillin against the nine selected Flavobacterium spp. isolates and the 28 F. johnsoniae-like isolates (Tables 2 and 3)
Summary
Aquaculture is one of the fastest growing industries with~80 million tons of farmed fish and shellfish being produced annually. Over-crowding and increased stress levels in aquaculture systems predispose fish to develop flavobacterial infections, leading to significant economic losses (Flemming et al, 2007; Rattanchaikunsopon and Phumkhachorn, 2009; Schrader, 2008). Flavobacterium columnare, F. psychrophilum, F. branchiophilum, F. aquatile and F. johnsoniae have been associated with fish disease and have been detected in surrounding water in the presence of disease outbreaks. Flavobacterium johnsoniae-like isolates appear to be significant in the Southern African context as aquaculture pathogens (Flemming et al, 2007; Basson et al, 2008), being isolated predominantly from diseased cultured trout. Prevention of flavobacteria epizootics is Chenia and Singh hampered by the ubiquitous presence of Flavobacterium spp. in soils and waters and the identification of multidrug resistant strains (DeClerq et al, 2013), which makes control of flavobacterial disease outbreaks a significant challenge. F. johnsoniae isolates contain chromosomally located β-lactamase genes (Naas et al, 2003) and chloramphenicol-inducible resistance-nodulation-division family multidrug efflux pump system, FmeABC1 (Clark et al, 2009)
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