Heme-dependent siderophore utilization promotes iron-restricted growth of the Staphylococcus aureus hemB small colony variant
Authors: Batko Izabela Z. , Flannagan Ronald S. , Guariglia-Oropeza Veronica , Sheldon Jessica R. , Heinrichs David E. 
Published: June 2021

The ability to acquire iron is essential for Staphylococcus aureus to cause infection. Respiration deficient S. aureus small colony variants (SCVs) frequently cause persistent infections, which necessitates they too acquire iron. How SCVs obtain iron remains unknown and so here we addressed this outstanding question by creating a stable hemB mutant in S. aureus USA300 strain LAC. The mutant, auxotrophic for hemin, was assessed for its ability to grow under iron- restriction and with various iron sources. The hemB SCV utilizes exogenously supplied heme but was attenuated for growth under conditions of iron starvation. RNA-seq analyses showed that both WT S. aureus and the hemB mutant sense and respond to iron starvation, however, growth assays show that the hemB mutant is defective for siderophore-mediated iron acquisition. Indeed, the hemB SCV demonstrates limited utilization of endogenous staphyloferrin B or exogenously provided staphyloferrin A, Desferal, and epinephrine, which enabled the SCV to sustain only minimal growth in iron deplete media. Direct measurement of intracellular ATP in hemB and WT S. aureus revealed that both strains can generate comparable levels of ATP during exponential growth suggesting defects in ATP production cannot account for the inability to efficiently utilize siderophores. Defective siderophore utilization by hemB bacteria was also evident in vivo. Indeed, the administration of Desferal failed to promote hemB bacterial growth in vivo, in contrast to WT, in every organ analyzed except for the murine kidney where growth was enhanced. In support of the hypothesis that S. aureus accesses heme in kidney abscesses, in vitro analyses revealed that increased heme availability enables hemB bacteria to utilize siderophores for growth when iron availability is restricted. Taken together, our data support the conclusion that heme is not only used as an iron source itself, but as a nutrient that promotes utilization of siderophore-iron complexes.

Author summary

S. aureus small colony variants (SCVs) represent a difficult to treat subpopulation of bacteria that are associated with chronic recurrent infection and worsened clinical outcome. Indeed, SCVs persist within the host despite appropriate administration of antibiotics. This study yields insight into the mechanisms by which S. aureus SCVs acquire iron which, during infection of a host, is a difficult-to-acquire metal nutrient. Under heme limited conditions, hemB S. aureus is severely impaired for siderophore-dependent growth and, in agreement, murine infection indicates that hemin-deficient SCVs meet their nutritional requirement for iron through utilization of heme in vivo. Importantly, we demonstrate that hemB SCVs rely upon heme as a nutrient to promote siderophore utilization. Therefore, perturbation of heme biosynthesis and/or utilization represents a viable to strategy to mitigate the ability of SCV bacteria to acquire siderophore- bound iron during infection.