Ultra-sensitive chemiluminescent detection of Staphylococcus aureus based on competitive binding of Staphylococcus protein A-modified magnetic beads to immunoglobulin G

Staphylococcus protein A (SPA) is a surface protein only expressed naturally in the cell walls of Staphylococcus aureus (S. aureus) and binds specifically to the Fc region of immunoglobulin G (IgG). This fact can be utilized for the detection of S. aureus. Specifically, SPA-modified magnetic beads, compete with S. aureus pathogens for binding to rabbit IgG that previously was labeled with horseradish peroxidase (HRP). The beads were then magnetically separated, and chemiluminescence (CL) was generated by adding the reagents luminol and H2O2. Under optimal conditions, the intensity of CL decreases with increasing concentration of S. aureus over a very wide linear range (10 to 1.0 x 10(9) cfu center dot mL(-1)), with a limit of detection of 6.0 cfu center dot mL(-1) at an S/N ratio of 3. The assay (including binding reaction, magnetic separation, washing of beads and detection) is completed within 50 min which is faster than many reported methods. It can well distinguish S. aureus from other Gram-positive and Gram-negative bacteria. The magnetic beads have the beneficial effect of eliminating undesired matrix effects and of concentrating the sample. The method was applied to the analysis of urine, apple juice and glucose injection samples spiked with S. aureus, and recoveries ranged from 85 to 107 %.