Influence of cell surface appendages on the bacterium-substratum interface measured real-time using QCM-D

Quartz crystal microbalance with dissipation (QCM-D) utilizes an oscillating quartz crystal to register adsorption of rigid masses through a decrease in its resonance frequency f. In addition, QCM-D has the ability to measure the dissipative nature of nonrigid masses adhering to the crystal surface in the form of oscillation amplitude decay time. Although QCM has been applied to register bacterial adhesion to the crystal surface, full interpretation of the frequency change and dissipation signal has hitherto been impossible due to the complex interactions within the distance of 250 nm between the substratum and the bacterial cell surface. Here, we study adhesion of a series of Streptococcus salivarius mutants, possessing various surface appendages of known lengths, as a function of time using QCM-D. In addition, the number of bacteria adhering to the crystal surface was determined. The results show that adhesion of a "bald" bacterium, completely devoid of surface appendages, is registered as a frequency decrease. Adhesion of bacteria possessing surface appendages yields either a much smaller decrease or an increase in frequency, despite the fact they adhere in higher numbers. Furthermore, the magnitude of frequency and dissipation shifts was found to be influenced by the distance at which the cell body was held from the sensor surface by its surface appendages.