Evaluation of Streptomyces species from historically sewage sludge contaminated soils for tolerance to copper(II) toxicity

Pollution of the environment by copper(II) exerts numerous adverse impacts on soils, microorganisms, plants, animals and humans. Assessing these impacts on microorganisms is essential as they are potential agents of bioremediation to purify the ecosystem. The aim of this study was to investigate the tolerance of Streptomyces species obtained from historically contaminated soils to copper (II) toxicity. Soil samples were collected from farms around sewage sludge dump sites (tests) and where there were no dumps (control). Standard microbiological methods were used to isolate, characterize and identify the three best copper tolerant species. Also, the impact on Streptomyces growth characteristics such as radial growth (R2), index of growth (GI) and percentage inhibition of radial growth (PIRG) by copper (II) was examined. Results obtained indicated that; the minimum inhibitory concentration of copper (II) for some of the Streptomyces isolates was 600 mg/l. However, three isolates; SW2B, SW7A and SW7B had MIC of 3000 mg/l and were selected for further investigations. Increasing metal concentration from 100 mg/l to 2000 mg/l reduced R2 from 40.2 to 4.0 and GI from 0.77 to 0.08, corresponding to 69.48 % increase in PIRG for SW2B. For isolate SW7A, R2 and GI decreased from 33.7 to 3.8 and from 0.67 to 0.08 respectively as copper (II) concentration increased from 100 mg/l to 2000 mg/l, giving 59.33 % increase in PIRG. The highest increase in PIRG (88.04 %) was obtained for SW7B, corresponding to a decrease of R2 and GI from 39.2 to 5.2 and 0.77 to 0.10 respectively. Molecular method was used to identify the isolates as Streptomyces chartreusis (SW2B), Streptomyces aureoverticillatus (SW7A) and Streptomyces subrutilus (SW7B).