Fig. 1.

Proposed mechanisms of bacteria in resisting metal ion and produce NPs simultaneously. Biosorption occurs on the bacterial cell wall, which involves the binding of metal cations to the negatively charged functional groups on the bacteria such as carboxyl, phosphate, and hydroxyl [22]. EPS secreted by bacteria also act as biosorption site in the form of biofilm, to tolerate metal ions by trapping them within the EPS matrix and reducing them to the less toxic metal [22]. Precipitation is one of the mechanisms that lower the metal ion toxicity through the reaction between anions such as hydroxyl ion and metal ions (cation) that occurs either intra- or extracellularly. Unlike biosorption, bioaccumulation is an active metabolic process that requires energy and intracellular metal ions binding, with the help of low molecular weight proteins such as metallothioneins to facilitate the process [22, 26]