Multichannel QCM-Based System for Continuous Monitoring of Bacterial Biofilm Growth

Abstract

Quartz crystal microbalance (QCM) sensors are becoming a good alternative to analytical methods for the measurement of bacterial growth in liquid media culture. For this purpose, two essential resonance parameters allow monitoring of biofilm formation: the series resonance frequency shift and the change of the resistance at this frequency. Nevertheless, several problems arise in determining these parameters, as their relative variation is very small. This means that an accurate procedure must be implemented for the measurement of the QCM resonance parameters, including the automatic calibration of the frequency response effects of the measurement circuits and the automatic compensation of the static electrical capacitance of the QCM.In this paper, a novel multichannel system for on-line monitoring of biofilm formation based on QCM sensors is proposed. QCM resonance parameters are determined from the electrical impedance analysis by means of an autobalanced impedance bridge. This configuration has allowed the implementation of an affordable multichannel measurement instrument. Obtained results, based on binary mixtures of water-glycerol measurements and real microorganism experiments, are in good agreement with the theoretical behaviour. These results show the great potential of this instrument to be used for monitoring microbial growth and biofilm formation. IntroductionMicrobiology requires systems capable of measuring, controlling and monitoring bacterial growth. For this purpose, classical methods are based on the observation of bacterial growth within liquid media cultures [1]. This is usually carried out using destructive and discontinuous techniques, which result in the breakage of the biofilm complex structure. This fact has strongly limited the study of the factors that affect the formation and control of bacteria film that take part in the experiments. For this reason, in recent years novel methods and electronic instrumentation have been devised in order to become good alternatives to these classical techniques [2]. The most relevant approaches are optical methods [3]-[6], electrical impedance measurement and spectroscopy [7]-[9], microfluidic devices [10],[11], and piezoelectric devices based systems. With respect to the latter, tuning forks [12],[13], surface acoustic wave devices [14],[15], and quartz crystal microbalances (QCM) [16]-[23], they all have demonstrated great potential in the study of biofilm growth.In this paper, the analysis of the mechanical resonance of a QCM, on whose surface the microorganisms adhere and the biofilm grows, is used. The usefulness of QCM is that they permit the direct in situ real-time observation of the overall process, from the initial bacteria adhesion to the biofilm formation and subsequent growth, when they are compared with other popular techniques such as direct cell counting methods or the determination of dry mass. Also QCM techniques provide an easy way to mechanize and automate the monitoring procedure. Two essential resonance parameters reflect the chang...

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