Structural Organization and Energization of Iron Acquisition Systems in Staphylococcal Membranes

Abstract

New antimicrobial strategies to prevent bacterial infections are urgently needed due to the increase in antibiotic resistances in pathogens. Iron is an essential trace metal that needs to be acquired by bacteria for proliferation and infection. Thus, targeting bacterial iron acquisition systems might represent a promising antibacterial strategy. In the human body free iron is actively limited, which is referred to as “nutritional immunity”. To overcome these limitations, staphylococci have several iron uptake systems. The staphylococcal pathogens Staphylococcus aureus and Staphylococcus lugdunensis acquire hemoglobin-derived heme using their iron-regulated surface determinant (Isd) system. The molecular mechanism of heme extraction and its guidance over the staphylococcal cell well is well understood. In contrast, heme transport across the membrane is less well understood and was subject of this thesis. Using protein interaction studies and growth analyses, we demonstrated that the ATPase FhuC energizes the Isd membrane transporter in S. aureus. Interestingly, FhuC is already known to energize the siderophore uptake transporters Fhu, Hts and Sir in S. aureus. In contrast, we found that S. lugdunensis encodes the ATPase isdL in its isd locus and does not depend on FhuC for heme transport. In addition, S. lugdunensis encodes an ABC transporter from the energy coupling factor type (ECF), and we show the system to accept heme from numerous hemoproteins independently from the Isd system. Moreover, we could show that S. lugdunensis uses the same transport systems as S. aureus for siderophore uptake, namely Hts, Sir, Fhu and Sst, all energized by FhuC. Furthermore, we could show that appropriate formation of functional membrane microdomains (FMM) and the scaffolding protein FloA are necessary for the function of the Isd system in both S. aureus and S. lugdunensis. Using co-immunoprecipitation, we detected direct interaction between FloA and the permease IsdF and deletion mutants of floA showed a growth deficit with hemoglobin as sole iron source. Our experiments show novel links between membrane structuring processes and nutrient acquisition in bacterial pathogens. Thus, FMM/FloA inhibition might represent a new strategy to treat staphylococcal infections by sensitizing the pathogens to host nutritional immunity mechanisms.