Bacterial Transition Metal Homeostasis

Abstract

Since details on metal cation transport proteins and on the allocation mechanisms for transition metals are provided elsewhere in this book, I will present aspects of transition metal homeostasis in a hopefully novel overview. We will start with a microbial look at the transition metal Periodic Table, cation speciation, and availability in the environment. This information provides rules that might govern microbial metal cation homeostasis from the outside of the cell. The fate of metal cations inside the cell is influenced by redox potentials and affinities to ligands in complex compounds. Understanding this topic requires study of interactions between metal cations and the consequences thereof. External availability and internal binding equilibria are connected by transport reactions. These lead to metal cation concentrations in cellular compartments, which are in flow equilibrium of import and export reactions. Thus, cellular cation homeostasis may be described as an interplay of transport flow backbone and competitive binding reactions. Both together provide an energy landscape for each metal cation and cellular compartment. As a recent part of the transport flow backbone in Gram-negative bacteria, efflux across the outer membrane from the periplasm to the outside has been identified. Active outer membrane efflux might indeed be taking place in Gram-negative bacteria. Thus, the periplasm is important in bacterial metal cation homeostasis.