Y-014. Development of a Photonic Biosensor Assay to Diagnose Francisella tularensis Infection

T. J. Inzana, K. Cooper, A. B. Bandara, A. Wang;
Virginia Polytechnic Institute & State Univ., Blacksburg, VA.

The threat of an intentional release of select agents prompts the need for culture-free diagnostic tests that are both highly sensitive and specific and that can be completed rapidly under field or laboratory conditions. The photonic biosensor is a portable sensing method that permits real-time monitoring of the direct binding of antigen to antibody, or DNA hybridization to a probe, without requiring labeled receptor molecules. We studied the use of a photonic biosensor assay to diagnose the category A select agent Francisella tularensis. Detection and differentiation of F. tularensis subspecies tularensis (strain TI0902 - Type A) and subspecies holarctica (strain LVS - Type B) were demonstrated using a single-mode multi-cavity fiber Fabry-Perot interferometric sensor. Sensors were prepared by depositing seven polymer bilayers onto the fiber tip followed by attaching one of two DNA probes: (a) 101-bp probe from the yhhW gene unique to Type A strains, or (b) 117-bp probe of the lpnA gene, common to both Type A and Type B. The yhhW probe gave a signal only with Type A strains and not with Type B strains. Probe lpnA gave a positive signal with both Type A and type B. Specific detection of nanogram quantities of target DNA (without the need for PCR amplification) was seen, highlighting the sensitivity of the method when small quantities of target DNA are present. An optical fiber biosensor was tested using a standard transmission mode long period fiber grating (LPFG) of length 15 mm and period 260 µm and the IgG fraction from antiserum to F. tularensis. Antibody thin films were first fabricated on planar substrates and the refractive index change that occurred upon antigen binding was evaluated using spectroscopic ellipsometry. Index changes on the order of 0.02 were observed, which correspond to typical LPFG wavelength shifts of 20-30 nm. The capability to deposit active IgG directly onto the optical fiber surface in a nanostructured film with controlled optical properties was seen, and the presence of F. tularensis detected from the decrease of peak wavelength caused by binding of specific antigen. This assay will fill a void that exists in rapid diagnosis of F. tularensis, but can also be applied to other select agents.