Colorimetric CRISPR Biosensor: A Case Study with Salmonella Typhi

There is a critical need to implement a sensitive and specific point-of-care (POC) biosensor that addresses the instrument limitations and manufacturing challenges faced in resource-constrained contexts. In this paper we focus on enteric fever which is a highly contagious and prevalent infection in low- and middle-income countries. Although easily treatable, its ambiguous symptoms paired with a lack of fast, accurate and affordable diagnostics lead to incorrect treatments which exacerbate the disease burden, including increasing antibiotic resistance. In this study, we develop a readout module for CRISPR-Cas12a that produces a colorimetric output that is visible to the naked eye and can act as a cascade signal amplifier in any CRISPR assay based on trans-cleavage. We achieve this by immobilizing an oligo covalently linked to a β-galactosidase (LacZ) enzyme, which is cleaved in the presence of DNA target-activated CRISPR-Cas12a. Upon cleavage, the colorimetric enzyme is released, and the supernatant transferred to an environment containing X-Gal producing an intense blue color. This method is capable of detecting amplified bacterial genomic DNA and has a lower limit of detection (LoD) to standard fluorescent assays while removing the requirement for costly equipment. Furthermore, it remained active 4 weeks after lyophilization, allowing for the possibility of shipment without cold chain, significantly reducing deployment costs.