Whispering Gallery Mode (WGM) microresonators have recently been studied as a means to achieve real-time label-free detection of biological targets such as virus particles, specific DNA sequences, or proteins. Due to their high quality (Q) factors, WGM resonators can be highly sensitive. A biosensor also needs to be selective, requiring proper functionalization of its surface with the appropriate ligand that will attach the biomolecule of interest. In this paper, WGM microdisks are used as biosensors for detection of Staphylococcus aureus. The microdisks are functionalized with LysK, a phage protein specific for staphylococci at the genus level. A binding event on the surface shifts the resonance peak of the microdisk resonator towards longer wavelengths. This reactive shift can be used to estimate the surface density of bacteria that bind to the surface of the resonator. The limit of detection of a microdisk with a Q-factor around 104 is on the order of 5 pg/mL, corresponding to 20 cells. No binding of Escherichia coli to the resonators is seen, supporting the specificity of the functionalization scheme.
Rapid, specific detection of pathogenic bacteria remains a major challenge in infectious disease diagnostics. Bacteriophages can show genus- or even species-level specificity and have been developed for biosensing purposes, but the possibility of using individual phage proteins for detection has not been fully explored. This work exploits the ability of specific phage proteins, the endolysins LysK and Φ11, and the bacteriocin lysostaphin, fixed on silicon wafers to bind staphylococci. The proteins show activity against eight tested clinical isolates of S. aureus and to S. epidermidis, but no binding to Escherichia coli and limited binding to Micrococcus. Binding was quantified by clearing assays in solution and by functionalization of silicon wafers followed by light microscopy. Bacterial binding densities on functionalized surfaces were ~3 cells/100 μm(2). The small size of the proteins makes the system robust and easy to handle, and the principle is generalizable to many different biosensor platforms, including label-free systems such as optical microresonators.
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