Rapid identification of Staphylococcus aureus directly from positive blood culture media using quantum dots as fluorescence probes

The subcultivation of positive blood cultures for organism identification is a time-consuming process. We explored the use of fluorescent quantum dots (QDs) as probes for the improved detection and identification of Staphylococcus aureus directly from positive blood culture media. Immunofluorescence probes were synthesized by linking biotin-conjugated QDs to streptavidin-conjugated IgG molecules. This QDs-IgG complex is capable of binding anti-SPA antibodies which specifically bind to a cell surface protein of S. aureus. The specificity of the method was confirmed using reference strains. Among 372 positive blood cultures, 73 containing gram-positive cocci in clusters were investigated. Relative to the standard culture method, the immunofluorescence assay showed 90.5% (19/21) sensitivity and 100% (52/52) specificity for S. aureus. These results suggest that this immunofluorescence assay allows the rapid identification of S. aureus directly from specimens.