Targeting hemoglobin receptors IsdH and IsdB of Staphylococcus aureus with a single VHH antibody inhibits bacterial growth.

Methicillin-resistant Staphylococcus aureus, or MRSA, is one of the major causative agents of hospital-acquired infections worldwide. Novel antimicrobial strategies efficient against antibiotic-resistant strains are necessary and not only against S. aureus. Among those, strategies that aim at blocking or dismantling proteins involved in the acquisition of essential nutrients, helping the bacteria to colonize the host, are intensively studied. A major route for S. aureus to acquire iron from the host organism is the Isd (iron surface determinant) system. In particular, the hemoglobin receptors IsdH and IsdB located on the surface of the bacterium are necessary to acquire the heme moiety containing iron, making them a plausible antibacterial target. Herein, we obtained an antibody of camelid origin that blocked heme acquisition. We determined that the antibody recognized the heme-binding pocket of both IsdH and IsdB with nanomolar order affinity through its second and third complementary-determining regions. The mechanism explaining the inhibition of acquisition of heme in vitro could be described as a competitive process in which the complementary-determining region 3 from the antibody blocked the acquisition of heme by the bacterial receptor. Moreover, this antibody markedly reduced the growth of three different pathogenic strains of MRSA. Collectively, our results highlight a mechanism for inhibiting nutrient uptake as an antibacterial strategy against MRSA.

Structure and role of the linker domain of the iron surface-determinant protein IsdH in heme transportation in Staphylococcus aureus.

Staphylococcus aureus is a major cause of deadly nosocomial infections, a severe problem fueled by the steady increase of resistant bacteria. The iron surface determinant (Isd) system is a family of proteins that acquire nutritional iron from the host organism, helping the bacterium to proliferate during infection, and therefore represents a promising antibacterial target. In particular, the surface protein IsdH captures hemoglobin (Hb) and acquires the heme moiety containing the iron atom. Structurally, IsdH comprises three distinctive NEAr-iron Transporter (NEAT) domains connected by linker domains. The objective of this study was to characterize the linker region between NEAT2 and NEAT3 from various biophysical viewpoints and thereby advance our understanding of its role in the molecular mechanism of heme extraction. We demonstrate the linker region contributes to the stability of the bound protein, likely influencing the flexibility and orientation of the NEAT3 domain in its interaction with Hb, but only exerts a modest contribution to the affinity of IsdH for heme. Based on these data, we suggest that the flexible nature of the linker facilitates the precise positioning of NEAT3 to acquire heme. In addition, we also found that residues His45 and His89 of Hb located in the heme transfer route toward IsdH do not play a critical role in the transfer rate-determining step. In conclusion, this study clarifies key elements of the mechanism of heme extraction of human Hb by IsdH, providing key insights into the Isd system and other protein systems containing NEAT domains.