Almost one third of proteins in nature depend on a particular metal for their diverse functions [8] and divalent metal cations are essential for bacteria [9] . For instance, manganese plays a primary antioxidant role in bacteria [10] and affects bacterial pathogenesis [11] . Zinc is one of the most abundant metals in bacteria and is an essential co-factor of many metabolic enzymes and transcription factors [12] . The Mn 2+ ion is characterized as hard metal and tends to prefer hard ligands, while zinc prefers soft ligands [13] . To obtain appropriate cellular concentrations of transition metal ions, bacteria have evolved elaborate machineries (such as ABCtransporters and ion channels) to transport these ions across biological membranes.