LAUSR

Label-free and sensitive detection of synthesis products from single microbial cells remains the bottleneck for determining the specific turnover numbers of individual whole-cell biocatalysts. We demonstrate the detection of lysine synthesized by only a few living cells in microfluidic droplets via mass spectrometry. Biocatalyst turnover numbers were analyzed using rationally designed reaction environments compatible with mass spectrometry, decoupled from cell growth, with high specific turnover rates (~1 fmol cell-1 h-1), high conversion yields (25%), and long-term catalyst stability (>14h). The heterogeneity of cellular reactivity of only 15±5 single biocatalysts per droplet could be demonstrated for the first time by parallelizing droplet incubation. These results enable resolving biocatalysis beyond the averages of populations. This is a key step towards quantifying specific reactivities of single cells as minimal functional catalytic units.