Isolation of soil actinomycetes from different sites and screening for their antimicrobial activity

Abstract

Screening of several microbes for bioactive compounds is an open avenue for discovery of novel bioactive compounds with varied functions as agricultural chemicals. The work done resulted in isolation of seventeen actinomycete isolates: 8 from a composite mixture of sheep pen soil, garden soil and pastureland within Chiromo area, 4 from Embu soils and 5 from a cowshed in Kabete. Of these, 4 isolates were antagonistic to the test organisms namely Alternaria so/ani, Fusarium oxysporum fsp phaseoli and Colletotrichum kahawae as shown by formation of inhibition zones in solid medium. The bioactive compound from the four actinomycete isolates code named EW1P18, EW3P43,EW3P41 and C did not show any antibacterial activity to Pseudomonas syringae, Ralstonia solanacearum, Escherichia coli and Staphylococcus aureas also used as test orgamsms. The antibiotic activity of live cultures, culture filtrates and the freeze dried samples of the bioactive compounds from the four antagonists were determined by paper disc method. This showed marked growth inhibition '!..,f the fungal test pathogens. The primary test pathogen, Colletotrichum kahawae, showed growth reductions by 64.76%, 47.62%, 41.20% and 63.33% by isolates EW3P43, EW3P41;;,EWIP18and C respectively. The crude culture filtrates from the same antagonists also gave.Jl mean inhibitory zone size of 14.5mm, ] 3.75mm, 13.35mm and 14.65mm fot isolates EW1P18, EW3P41, EW3P43 and respectively. The number of antibiotic substances present in the culture filtrate of each isolate could not be established by bioautography in this experiment since there were no clear zones observed on the chromatography paper. This could have been attributed to the actinomycete isolates being poor fermenters, the solvent systems used for extraction may not have been suitable for them or the compound may not have been extracted properly by the solvents used.