The Intricate Balance of Metal Trafficking in Bacteria: Import of Iron in Bacillus anthracis and Export of Excess Copper in Escherichia coli

Bacterial organisms continuously maintain homeostasis even in changing environments. This ability to maintain homeostasis is especially critical for pathogenic and opportunistic bacteria, which must adapt to both abiotic and biotic host environments. Both types of environments present unique limitations and conditions. Transition metal homeostasis under these varying conditions is important for bacterial survival. Transition metals such as zinc, cobalt, iron and copper are essential for cell survival, but become toxic if in excess. The host organism often takes advantage of this requirement by greatly limiting access to transition metals to limit infections, but in other environments, toxic levels of metal may be present. Bacterial organisms have developed many mechanisms to maintain transition metal homeostasis. This study focuses on two bacterial systems that are utilized to maintain metal balance; the heme-acquiring iron surface determinant (Isd) system of Bacillus anthracis and the copper and silver export Cus system of Escherichia coli. Host organisms use many proteins and systems to limit iron access from pathogenic bacteria, known as nutrient immunity. B. anthracis must acquire iron from the host organism upon infection and so has evolved multiple iron acquisition systems. The Isd system employs two extracellular proteins, IsdX1 and IsdX2, to remove heme from hemoglobin to use as an iron source. Once bound to heme, these hemophores transfer heme to a cell surface attached protein, IsdC, which further relays the molecule to be transferred into the cell for iron use. This study focused on the kinetics of heme transfer to better understand how acquisition occurs. This study determined that the oxidation state of the iron-heme molecule plays a significant role in the kinetics of heme acquisition by IsdX1 and subsequent transfer to IsdC. This work clarifies and further establishes the mechanism of iron acquisition by B. anthracis during infection. Copper and silver are used in many settings as antimicrobial agents, including as an alternative to antibiotic drugs. Pathogenic and opportunistic bacteria, such as E. coli, experience stress upon contact with copper and silver surfaces and materials. Copper is an essential transition metal, while silver is not biologically used, but both become toxic when in excess due to redox properties and disruption of biological molecules. E. coli utilizes several systems to remove excess copper and silver to resist toxicity. The Cus system, consisting of the soluble CusF and tripartite pump CusCBA, specifically exports copper and silver from the periplasm. Several roles of CusF have been suggested from in vitro data. The components CusAB were hypothesized to be the essential proteins of the CusCBA pump, while the outer membrane unit may not contribute specificity or be necessary for export. This study focused on the role and importance of CusF and outer membrane channel CusC during copper stress in vivo. An in vivo interaction between CusF and CusB was identified during copper stress. The data from this work indicate that cusF and cusC directly affect intracellular copper accumulation. Furthermore, this study revealed that SdsP may play in a secondary role to CusC to complement CusC to maintain copper resistance. This works establishes the importance of CusC as the main outer membrane component during copper export in E. coli