Development of a holographic sensor for the detection of calcium dipicolinate—A sensitive biomarker for bacterial spores

An enzyme-linked holographic sensor for the detection of spore-specific calcium dipicolinate (Ca-DPA) is described. Small, acid-soluble spore proteins (SASPs) extracted from dormant spores are exploited as holographic matrices. Reflection holograms are incorporated into SASP films using a diffusion method for the distribution of ultra-fine silver bromide grains into pre-formed polymers, followed by holographic recording using a frequency doubled Nd:YAG (532 nm) laser. Changes in holographic diffraction intensity are used to characterise the structural integrity of SASP-based matrices in response to proteolytic attack by recombinant germination protease (GPRS), following activation with Ca-DPA; the holographic response manifests as an irreversible reduction in diffraction intensity due to progressive fringe removal caused by SASP degradation. This work demonstrates proof-of principle of a Ca-DPA-activated enzyme-linked holographic sensor, highlighting the potential of a system whereby Ca-DPA released from germinating spores may activate GPR in the vicinity of an SASP-based holographic matrix; such a sensor system presents a novel platform for the selective detection of bacterial spores with intrinsic signal amplification.