Non-transferrin-bound iron in platelet concentrates promotes the growth of Staphylococcus epidermidis

Staphylococcus epidermidis, the most common organism implicated in bacterial contamination of platelet (PLT) concentrates (PCs), does not grow in serum unless transferrin is fully saturated and there is non-transferrin-bound iron (NTBI) available. Here, the occurrence and origin of NTBI in PCs has been studied.

NTBI in PC supernatants was determined by a chelation method and by the bleomycin-detectable iron assay. Iron binding by transferrin was determined by spectrophotometry, and transferrin iron forms, by urea gel electrophoresis. The growth of inoculated S. epidermidis in PC supernatants was monitored by optical density and determination of viable counts.

PCs contained approximately 0.14 micromol per L redox-active iron measured by the bleomycin assay and approximately 0.7 micromol per L NTBI by the chelation method. As a further indication of the presence of NTBI, the growth of S. epidermidis in the PC supernatants was inhibited by iron chelation with deferoxamine. Transferrin in the PC medium was only partially saturated with iron, and the reason for the presence of NTBI was found to be impaired iron binding by transferrin. Iron was displaced from transferrin by citrate at molar ratios to transferrin that occur in citrated plasma and in PLT additive solution (AS). Citrated plasma supported the growth of S. epidermidis whereas serum did not.

PCs stored in plasma or AS contain a low level of NTBI because of the displacement of iron from plasma-derived transferrin by citrate. NTBI in the PC medium supports the growth of S. epidermidis.