The streptococcal iron uptake (Siu) transporter is required for iron uptake and virulence In a zebrafish infection model

A limited understanding of iron uptake mechanisms is available for Streptococcus pyogenes, a haemolytic human pathogen capable of using a variety of haemoproteins in addition to ferric and ferrous iron. This study characterizes a transporter named siu (for streptococcal iron uptake), which consists of an ATP-binding protein (SiuA), a substrate-binding protein (SiuD), and two membrane permease subunits (SiuBG). An siuG mutant was constructed and characterized. The mutant demonstrated growth reduction in comparison to the parent strain when grown in complex medium containing iron in the form of blood, haemoglobin or serum. Only a small reduction in the growth of the siuG mutant was observed in medium containing ferric iron. However, in iron uptake assays the siuG mutant showed a decrease of approximately 30 % in Fe3+ incorporation. Addition of 6 mu M haem to the medium inhibited Fe3+ uptake by the wild-type by 76 %, while addition of protoporphyrin IX did not, suggesting that utilization of haem as an iron source is responsible for the inhibition of Fe3+ uptake. Inactivation of siuG moderately reduced the ability of haem to inhibit Fe3+ incorporation by the cells. Inactivation of siaB (encoding a Membrane permease of a second iron transporter) had a similar outcome, and inactivation of both transporters had a cumulative effect. These observations implicate both the siu and sia transporters in haem utilization by Strep. pyogenes. Studies in a zebrafish infection model revealed that the siuG mutant was attenuated in both intramuscular and intraperitoneal routes of infection. Together these observations show that the siu system is an iron acquisition pathway in Strep. pyogenes that is important both in vitro and in vivo.