Abstract

The development of antibiotic-resistant pathogens due to the indiscriminate use of antibiotics has led to advocacy for the use of natural products in the treatment of fishdiseases. The antimicrobial activity of methanolic and ethanolic extracts of walnut leaves and onion bulbs were evaluated against six pathogenic bacteria (Pseudomonas aeruginosa, Staphyloccocus aureus, Bacillus subtilis, Pseudomonas fluorescens,Escherichia coli, Samonella typhi) using the cup – plate method. Minimum Inhibitory Concentration (MIC) was determined using standard methods. Data were analysed using descriptive statistics. Onion bulbs and walnut leaves were also screened for secondary metabolites and this indicated the presence of saponins, tannin, alkaloids, cyanogenic glycosides and flavonoids; while anthraquinones were not detected in both plants. The zone of inhibition varied with the bacteria and type of extract. The average diameter of inhibition zones was 10 ± 0.00 and 9 ± 0.02 mm for methanolic and ethanolic extracts of walnut leaves and onion bulbs, respectively. Staphylococcus aureus, B. subtilis and P. aeruginosa were most sensitive to the extracts. However, S. aureus was more sensitive to the extracts of walnut leaves and S. typhi was the least sensitive. Bacillus subtilis wasmore sensitive to the extracts of onion compared to E. coli which was the least sensitive. Minimum inhibitory concentration of walnut leaves and onion bulbs extracts on the bacteria tested were both 500 µg/ml. The results indicated that walnut leaves and onion bulbs had antibacterial activity on the tested organisms and showed their prospects for their use in the treatment of fish diseases.

Key words: Antibacterial activity, onion bulb extract, walnut leaf extract, fish diseases.