Article Bioremediation Potential of Streptomyces sp. MOE6 for Toxic Metals and Oil Marwa O. Elnahas 1 , Liyuan Hou 2 , Judy D. Wall 3 and Erica L.-W. Majumder 2, *   Citation: Elnahas, M.O.; Hou, L.; Wall, J.D.; Majumder, E.L.-W. Bioremediation Potential of Streptomyces sp. MOE6 for Toxic Metals and Oil. Polysaccharides 2021, 2, 47–68. https://doi.org/10.3390/ polysaccharides2010004 Received: 14 December 2020 Accepted: 19 January 2021 Published: 25 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemistry of Natural and Microbial Products, Pharmaceutical Industries Research Division, National Research Centre, Cairo 12622, Egypt; marwaomar2008@yahoo.com 2 Department of Chemistry, SUNY College of Environmental Science and Forestry, Syracuse, NY 13210, USA; lhou02@esf.edu 3 Department of Biochemistry, University of Missouri, Columbia, MO 65211, USA; wallj@missouri.edu * Correspondence: emajumder@wisc.edu; Tel.: +1-315-470-6854; Fax: +1-315-470-6856 Abstract: Toxic metal contamination has serious effects on human health. Crude oil that may contain toxic metals and oil spills can further contaminate the environment and lead to increased exposure. This being the case, we chose to study the bio-production of inexpensive, environmentally safe materials for remediation. Streptomyces sp. MOE6 is a Gram-positive, filamentous bacterium from soil that produces an extracellular polysaccharide (MOE6-EPS). A one-factor-at-a-time experiments showed that the maximum production of MOE6-EPS was achieved at 35 ◦ C, pH 6, after nine days of incubation with soluble starch and yeast extract as carbon sources and the latter as the nitrogen source. We demonstrated that MOE6-EPS has the capacity to remove toxic metals such as Co(II), Cr(VI), Cu(II) and U(VI) and from solution either by chelation and/or reduction. Additionally, the bacterium was found to produce siderophores, which contribute to the removal of metals, specifically Fe(III). Additionally, purified MOE6-EPS showed emulsifying activities against various hydrophobic substances, including olive oil, corn oil, benzene, toluene and engine oil. These results indicate that EPS from Streptomyces sp. MOE6 may be useful to sequester toxic metals and oil in contaminated environments. Keywords: extracellular polysaccharide; heavy metal removal; Streptomyces sp.; bioremediation; microbially enhanced oil recovery 1. Introduction Metals, such as Zn 2+ , Cr 3+ , Hg 2+ , Fe 2+ , Fe 3+ , and Cu 2+ , are essential nutrients (with the exception of Hg 2+ ) as they are required in trace amounts for many biological reactions. However, in high concentrations, they show serious toxicological effects in humans such as organ damage, nervous system damage, and cancer [1]. Metals and heavy metals are also used in large quantities in many production industries, including pharmaceuticals, oil refining, plastics, rubbers, and organic chemicals [2]. Improper disposal of industrial wastes and leachates often causes contamination of nearby environments and can lead to the bioaccumulation of heavy metals throughout the food chain [3]. The amount of contamination varies by site, but concentrations over 500 mg of metal ion per kg of soil or sludge have been reported for chromate [4,5]. Methods currently used to remove the toxic metals from contaminated soil, wastewater, or mixed metal-petroleum, include chemical precipitation, ion exchange, carbon adsorption, reverse osmosis, and ultra-filtration [2,6]. Disadvantages of such methods are high cost, high energy, high reagent requirements, potential for by-products to cause additional pollution, and the possible incomplete re- moval of the metal ions [1,7]. Thus, it is crucial to develop cost-effective and eco-friendly remediation methods for toxic metals and hydrocarbons. Two biological methods of metal remediation are biosorption and bioaccumulation [8]. Microbial biomass from bacteria, fungi or algae has been shown to remove heavy metals Polysaccharides 2021, 2, 47–68. https://doi.org/10.3390/polysaccharides2010004 https://www.mdpi.com/journal/polysaccharides