## 1 Introduction  
Manganese (Mn) is principally found in its oxide form and is available as a major constituent in many naturally occurring minerals [1]. It is also an important enzyme cofactor and provides protection to bacteria cells against reactive oxygen species. Many bacteria require it to form symbiotic or pathogenic interactions with eukaryotic host cells [2]. Moreover, Mn is a trace element essential for proper metabolic activity of humans and other mammals [3]. This metal is also important to a variety of industries. Mn is present in chemical and metallurgic products, alloys of iron and steel, ceramic fertilizers, varnishes, and fungicide products in livestock feeding supplements [4]. The global utilization of Mn is above 1.5 million tons per year, and it is intended to rise [5]. As a result of elevated Mn demand, high-grade reserved Mn ore is gradually depleting, which could lead to higher prices of both Mn ore and alloys [6]. The extensive use of Mn leads to the generation of a substantial amount of waste, among which a considerable quantity does not possess any market value [7]. Some of this waste is discharged into the environment leading to its contamination [1]. The excess Mn in terrestrial and aquatic ecosystems is known to have many toxic effects on organisms. In plants, Mn can lead to thickened palisade tissue, a high abscisic acid concentration, and growth retardation. In humans, exposure to high levels of Mn causes neurological changes and abnormalities of the immune and reproductive systems [4; 8; 9]. The improving awareness of both the health risks linked with Mn waste and increasing demand of this metal, necessitating its recovery, make it desirable to seek an environmentally friendly and economic process to recycle Mn from contaminated ecosystems. Chemical reagents used in traditional mineral processing, including foam flotation, which require the use of brothers, depressants, collectors, and other compounds, can be toxic and dangerous to the environment [10; 11]. Due to the environmental risks posed by conventional ore enrichment methods, the use of biotechnological methods is gaining more interest. Great hopes are placed in the use of bacteria and their metabolic products for metal bioleaching, but also for bioremediation [12]. In recent years, bacteria that are able to recover Mn have been isolated, characterized, and used in biotechnology [5; 6; 13]. Depending on the microorganism, the mechanisms that they use for this process are biosorption and bioaccumulation [14]. Biosorption is usually not associated with cellular viability, whereas bioaccumulation is metabolically regulated. In the biosorption process, bacteria adsorb metals on the cellular surface through attachment/linkage onto many anionic functional groups [15]. During bioaccumulation, the intracellular uptake of metals takes place via precipitation linked with metabolic functions, chemical reactions, and/or ATP-driven active transport, after which metals are mainly accumulated inside the cells [16; 17]. The accumulation of manganese by bacteria is possible through possession of complex protein systems to maintain proper Mn levels in the cells; thus, avoiding its toxicity. These systems are composed of manganese transporters, manganese-dependent transcription regulators, and manganese efflux pumps like MntE, MntP, MntX, CtpC, and P-type ATPase [18; 19; 20]. Many studies have demonstrated that biosorption of metals might be applicable for treatment of metallurgical and mining wastewater. Bacteria are potential organic sorbents, which show cell surface sorption or/and intracellular accumulation as well as extracellular accumulation [21; 22]. The process can be metabolic-independent or metabolic-dependent. During the first process, a physical adsorption utilizes van der Waals' forces and it is usually very fast and can be reversible.