Gleb Pishchany

The pathogenesis of human infections caused by the gram-positive microbe Staphylococcus aureus has been previously shown to be reliant on the acquisition of iron from host hemoproteins. The iron-regulated surface determinant system (Isd) encodes a heme transport apparatus containing three cell wall-anchored proteins (IsdA, IsdB, and IsdH) that are exposed on the staphylococcal surface and hence have the potential to interact with human hemoproteins. Here we report that S. aureus can utilize the host hemoproteins hemoglobin and myoglobin, but not hemopexin, as iron sources for bacterial growth. We demonstrate that staphylococci capture hemoglobin on the bacterial surface via IsdB and that inactivation of isdB, but not isdA or isdH, significantly decreases hemoglobin binding to the staphylococcal cell wall and impairs the ability of S. aureus to utilize hemoglobin as an iron source. Stable-isotope-tracking experiments revealed removal of heme iron from hemoglobin and transport of this compound into staphylococci. Importantly, mutants lacking isdB, but not isdH, display a reduction in virulence in a murine model of abscess formation. Thus, IsdB-mediated scavenging of iron from hemoglobin represents an important virulence strategy for S. aureus replication in host tissues and for the establishment of persistent staphylococcal infections.

Staphylococcus aureus pathogenesis is significantly influenced by the iron status of the host. However, the regulatory impact of host iron sources on S. aureus gene expression remains unknown. In this study, we combine multivariable difference gel electrophoresis and mass spectrometry with multivariate statistical analyses to systematically cluster cellular protein response across distinct iron-exposure conditions. Quadruplicate samples were simultaneously analyzed for alterations in protein abundance and/or post-translational modification state in response to environmental (iron chelation, hemin treatment) or genetic (Deltafur) alterations in bacterial iron exposure. We identified 120 proteins representing several coordinated biochemical pathways that are affected by changes in iron-exposure status. Highlighted in these experiments is the identification of the heme-regulated transport system (HrtAB), a novel transport system which plays a critical role in staphylococcal heme metabolism. Further, we show that regulated overproduction of acidic end-products brought on by iron starvation decreases local pH resulting in the release of iron from the host iron-sequestering protein transferrin. These findings reveal novel strategies used by S. aureus to acquire scarce nutrients in the hostile host environment and begin to define the iron and heme-dependent regulons of S. aureus.

The larval stage of the stingless bee Scaptotrigona depilis must consume a specific brood cell fungus in order to continue development. Here we show that this fungus is a member of the genus Zygosaccharomyces and provides essential steroid precursors to the developing bee. Insect pupation requires ecdysteroid hormones, and as insects cannot synthesize sterols de novo, they must obtain steroids in their diet. Larval in vitro culturing assays demonstrated that consuming ergosterol recapitulates the developmental effects on S. depilis as ingestion of Zygosaccharomyces sp. cells. Thus, we determined the molecular underpinning of this intimate mutualistic symbiosis. Phylogenetic analyses showed that similar cases of bee-Zygosaccharomyces symbiosis may exist. This unprecedented case of bee-fungus symbiosis driven by steroid requirement brings new perspectives regarding pollinator-microbiota interaction and preservation.