The role of efflux in bacterial resistance to soft metals and metalloids.

Bacteria have evolved various types of resistance mechanism to toxic soft metals and metalloids, including cadmium/zinc, copper/silver and arsenic/antimony. Active efflux of the metal is a frequently utilized stratagem to produce resistance by lowering the intracellular concentration to subtoxic levels. Reduction to a less-toxic form or to a form recognized by an efflux system also occurs. Pumps utilized for resistance may have evolved from normal cellular systems. For example, plasmid-mediated cadmium resistances may have evolved from a common ancestor of the pump involved in zinc homoeostasis. Pumps are more efficient than carriers and may have evolved by developing carriers that associate with ATPase subunits.