Background and aims:
Mass-sensitive quartz crystal microbalance with dissipation monitoring (QCM-D) is widely used in sensing bacteria binding. Studies indicate, that due to dissipative bacterial binding, mass loading cannot be directly calculated by the classical Sauerbrey equation. This study aimed at correlating the number of surface-bound bacteria with simultaneously detected frequency and dissipation signals to improve QCM-D data interpretation.
Materials and methods:
A QCM-D flow chamber device (qCell T μOpto; 3T analytik) with an optic window and a titanium (Ti) coated quartz sensor was combined with a fluorescent microscope. The early oral colonizer Streptococcus gordonii was cultured in Schaedler Medium, harvested by centrifugation, S. gordonii were stained with SYTO®9 and diluted with PBS to OD620 =0.1. After reaching stable signal baselines with PBS, the stained bacteria were pumped with 60 μl/min through the system for 3 hours, followed by 30 min PBS rinsing. Fluorescent microscopic images were made every 5 min. The bacterial number was counted with Image J and correlated with the frequency and dissipation signal through q viewer 18.0.
The 3-hour S. gordonii runs ended with a small increase of both frequency (ΔF=+37.7Hz) and dissipation (ΔD=+72.4Hz). The number of surface bound S. gordonii were found to be positively correlated with both frequency and dissipation with a better correlation for the dissipation.
A mass loss was detected by QCM-D during S. gordonii binding even though microscopically an increase of the number of attached bacteria was observed. This frequency increase can be interpreted here by the coupled oscillator model and indicated a soft, punctual binding of the bacteria. The QCM-D/microscopy set-up presented here can be used also for online testing of antimicrobial agents and surfaces.