Acoustic sensing of the bacterium–substratum interface using QCM-D and the influence of extracellular polymeric substances

It is commonly assumed that bacterial presence on a QCM sensor-surface is associated with a negative frequency shift according to conventional mass-loading theory. Here, we demonstrate that bacteria adhering to QCM sensor-surface may yield positive frequency shifts up to 1.9 × 10−6 Hz per bacterium according to a coupled-oscillator theory. Furthermore, it is demonstrated that the excretion of extracellular polymeric substances (EPS) by adhering bacteria can change the frequency shift in the negative direction by 1.7 × 10−6 Hz per bacterium, according to conventional mass-loading theory. The difference in frequency shifts between an EPS-producing and a non-EPS producing staphylococcal strain correlated with the excretion of 3 × 10−14 g EPS per bacterium, representing only a few percent of the weight of a bacterium. Thus an adsorbed molecular mass as low as a few percent of the mass of an adhering bacterium significantly alters the QCM -signal. Since adhesion of many different bacterial strains is accompanied by molecular adsorption of EPS, with potentially opposite effects on the QCM -signal, a combination of the coupled-oscillator and normal mass-loading theory has to be applied for proper interpretation of QCM -frequency shifts in bacterial detection.