Application of Optical and Acoustic Methods for Detection of Bacterial Pathogens Using DNA Aptamers as Receptors

Bacterial contamination in food presents real and valid danger for human health. Every year tons of food need to be thrown out and it is estimated billions of people get sick from food poisoning, leading to deaths in some hundreds of thousand cases (mostly in children). Most of the bacterial contaminations can be traced to about 20-30 pathogenic bacteria. In our work we focused on the detection of Escherichia coli and Listeria monocytogenes with optical and acoustic methods. In both methods we used specific DNA aptamers as receptors. Aptamers are single stranded DNA or RNA that in correct environment folds into structures specifically binding to a target on bacteria with K_d around 1-10 nM. For optical method it is possible to modify gold nanoparticles (AuNp) with aptamer (if it is modified with thiol group for example) and it is then possible to see the interaction of AuNp with bacteria on change of the absorbance spectrum. Aptamers also electrostatically interact with the AuNp without specific binding and it is possible to increase the stability of AuNp to increase of ionic strength by salt with aptamer being present. The incubation of such solution with bacteria then removes the protecting aptamer and after addition of salts to a solution of gold nanoparticles it is possible to measure their rate of aggregation as a function of bacterial concentration present in a sample. Another way to exploit optical system to measure the binding of bacteria to aptamers is to use white light reflectometry to measure change in thickness on a silicon chip modified with aminylated aptamer through silica chemistry. Lastly it is possible to modify the gold electrode of quartz TSM crystal with thiolated or aminylated aptamer and measure frequency change and dissipation change after bacteria binding. It seems that the interaction of bacteria with the surface is not strictly mass based, and it is therefore advantageous to perform analysis of change of viscoelastic properties on the surface. We will present the comparative analysis of the sensitivity of optical and acoustics methods for detection of bacterial pathogens.