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. 2016 May 3;6(2):20.
doi: 10.3390/bios6020020.

Real-Time Detection of Staphylococcus Aureus Using Whispering Gallery Mode Optical Microdisks

Hala Ghali  1 , Hicham Chibli  2 , Jay L Nadeau  3 , Pablo Bianucci  4 , Yves-Alain Peter  5
Affiliations

Real-Time Detection of Staphylococcus Aureus Using Whispering Gallery Mode Optical Microdisks

Hala Ghali et al. Biosensors (Basel). .

Abstract

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.

Keywords: bacterial sensing; optical microdisk; specific binding; surface functionalization.

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Figures

Figure 1
Figure 1
Scanning electron micrograph of an optical microdisk.
Figure 2
Figure 2
Functionalization of silica microdisks. (a) the silicon dioxide surface is hydroxylated with oxygen plasma; (b) the disk is immersed in triethoxysilane-PEG-NH2; (c) the free amines of the PEG-silane are covalently coupled to LysK using carbodiimide coupling.
Figure 3
Figure 3
Optical characterization setup. The light from a tunable laser at 630 nm propagates through a polarization controller and is coupled inside the microdisk using a tapered fiber. The signal conveyed from the photodetector to the oscilloscope yields the transmission spectrum. Resonant modes are observed as a series of dips.
Figure 4
Figure 4
Wavelength shift vs. time after S. aureus and E. coli binding at a cell concentration of OD600 = 0.4 (5 × 109 CFU/mL). After 15 min, a maximum shift is obtained and is about 0.22 nm for the Staph., whereas no shift is observed for E. coli detection. Error bars are standard deviations as discussed in Section 3.2 of the text.
Figure 5
Figure 5
Wavelength shifts after S. aureus binding for four different concentrations. Error bars are standard deviations as discussed in Section 3.2 of the text.
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