A colorimetric strategy for quantifying amino acids using E. coli auxotrophs displaying gold-binding proteins.

We present a cell-based colorimetric assay for amino acid quantification using Escherichia coli auxotrophs engineered to grow exclusively in the presence of their corresponding target amino acids and to display gold-binding proteins (GBPs) on their surface. Upon the addition of gold nanoparticles (AuNPs), the surface-expressed GBPs mediate AuNP aggregation, resulting in a distinct color shift from pink to blue. This approach enables the detection of ten essential amino acids through characteristic colorimetric changes and allows for quantitative analysis via absorbance measurements with high specificity, strong linearity (R2 > 0.96), and low detection limits ranging from 0.43 to 1.04 μM. The clinical applicability of the assay was demonstrated by accurately detecting elevated amino acid levels associated with phenylketonuria (PKU), homocystinuria, and maple syrup urine disease (MSUD), with excellent analytical precision (coefficients of variation <4 %) and high recovery rates (98-102 %). This technology holds strong potential as a novel platform for amino acid quantification, providing an easily observable colorimetric readout without the need for bulky or specialized instrumentation.