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 wi th its specific antibody has previously been used as a marker of infection. This method has been modified to allow the binding activity between specie s to be followed using the frequency response of a quartz crystal microbala nce (QCM). The Bayston agglutination plate assay for Staphylococcus epiderm idis has been modified to allow the electrode of a QCM to act as a direct s ensor for the change in solution properties as agglutination occurs. Antibo dy and antigen were introduced to the crystal surface and the agglutination process was followed as a change in crystal resonant frequency. Serum, kno wn to be infected with the organism, gave a titre of 3.9 x 10(-2)% v/v (-11 8 Hz, +/-12 SD, N = 9) matching that given by triplicate plate assay. Uninf ected serum gave no frequency changes at this concentration, yielding a tit re of 2.5 x 10(-2)% v/v again matching the plate titre (N = 3). Infected se rum gave responses 40 times faster then those of the uninfected serum. The piezoelectric quartz crystal method gave a positive or negative diagnosis i n < 15 min compared with the 24 h required for the plate assay. (C) 1999 El sevier Science B.V. All rights reserved.