Transition metals, such as iron, copper, zinc, and manganese play an important role in many bacterial biological processes that add to an overall evolutional fitness of bacteria. They are often involved in regulation of bacterial virulence as a mechanism of host invasion. However, the same transition metals are known to play an important role in host-defense mechanisms against bacteria through Fenton chemistry evoked toxicity as an example. Copper and zinc are used as a mechanism to poison bacteria whereas other metals, such as, iron and manganese are withheld by the predator to prevent reconstruction of Fe-S clusters and the use of Mn as a protectant against reactive oxygen species. Therefore, tight regulation of transition metal distribution in bacteria and hosts is a vital part of host-pathogen interactions.