Application of the quartz crystal microbalance to the monitoring of Staphylococcus epidermidis antigen–antibody agglutination

The change in solution properties due to the agglutination of an antigen with its specific antibody has previously been used as a marker of infection. This method has been modified to allow the binding activity between species to be followed using the frequency response of a quartz crystal microbalance (QCM). The Bayston agglutination plate assay for Staphylococcus epidermidis has been modified to allow the electrode of a QCM to act as a direct sensor for the change in solution properties as agglutination occurs. Antibody and antigen were introduced to the crystal surface and the agglutination process was followed as a change in crystal resonant frequency. Serum, known to be infected with the organism, gave a titre of 3.9x10-2% v/v (-118 Hz, ±12 SD, N=9) matching that given by triplicate plate assay. Uninfected serum gave no frequency changes at this concentration, yielding a titre of 2.5x10-2% v/v again matching the plate titre (N=3). Infected serum gave responses 40 times faster then those of the uninfected serum. The piezoelectric quartz crystal method gave a positive or negative diagnosis in <15 min compared with the 24 h required for the plate assay.