ABSTRACT

This study was undertaken to screen agricultural soils for potential antimicrobial metabolites. A total of nine different species of soil microorganisms were isolated from ninety (90) soil samples in Umuahia and screened for their ability to produce antimicrobial substances using the paper disc diffusion technique against five human pathogens: Staphylococcus aureus, Escherichia coil, Pseudomonas aeruginosa, Salmonella enterica SerovarTyphi and Candida albicans. Soil samples from each site was processed using the serial dilution technique and spread plated onto Starch Casein agar, Tryptone soy agar and Sabouraud Dextrose agar plates. Each isolate was tested for optimum antimicrobial metabolite production by impregnating a 5mm paper disc with 20.tl of each isolate suspension. The isolates were identified following a series of biochemical testing as Pseudomonas sp., Bacillus sp., Enterobacter clocae, Lysinibacillus sp. and Streptomyces sp. 16sRNA sequenicing of the isolates revealed that three of the most promising isolates showed 93%, 99% and 99% similarity with Lysinibacillusfusformis, Enterobacter clocae and Aspergillus niger respectively. The antimicrobial spectrum of the metabolites produced by Lysinibacillusfusformis showed activity against at least two of the test organisms, with the growth of Salmonella enterica SerovarTyphi, Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coil with clear zones of 9.3mm, 7.6mm, 8.3mm and 11.6mm respectively. Similarly, recommendable antagonism was recorded for Penicillium sp against E. coil, S. aureus and Salmonella enteric SerovarTyphi with zone sizes of 11.3mm, 10.3mm and 9.6mm respectively. Optimization of growth conditions of the promising isolates for enhanced activity showed that the antimicrobial activity of the metabolites from each isolate increased generally from 24h to 48h of incubation. Maximum zone of inhibition was observed after 48h incubation for all isolates. The optimum pH for Lysinibacillusfusiformis and Enterobacter cloacae was 7.0 whereas Aspergillus niger was at pH 6.0. The use of Starch and glycerol as a carbon source for Enterobacter cioacae produced metabolites that were effective against Candida aibicans and E. coil with inhibition zone of 14mm and 12mm while Aspergillus niger utilized these carbon sources only partially. This study suggests that agricultural soils harbor microorganisms with potentials to produce antimicrobial substances which can be up scaled or optimized for use against drug-resistant strains.