Fluorescence-activated droplet sorting (FADS): efficient microfluidic cell sorting based on enzymatic activity

TLDR

The authors present FADS, a microfluidic fluorescence‑activated droplet sorter that merges the high‑throughput advantages of microtitre‑plate screening with the precision of traditional FACS. FADS isolates single cells in emulsion droplets and sorts them by dielectrophoresis‑based fluorescence detection at up to 2000 droplets per second, validated with β‑galactosidase reporter E. coli at ~300 droplets per second.

Abstract

We describe a highly efficient microfluidic fluorescence-activated droplet sorter (FADS) combining many of the advantages of microtitre-plate screening and traditional fluorescence-activated cell sorting (FACS). Single cells are compartmentalized in emulsion droplets, which can be sorted using dielectrophoresis in a fluorescence-activated manner (as in FACS) at rates up to 2000 droplets s−1. To validate the system, mixtures of E. colicells, expressing either the reporter enzyme β-galactosidase or an inactive variant, were compartmentalized with a fluorogenic substrate and sorted at rates of ∼300 droplets s−1. The false positive error rate of the sorter at this throughput was <1 in 104 droplets. Analysis of the sorted cells revealed that the primary limit to enrichment was the co-encapsulation of E. colicells, not sorting errors: a theoretical model based on the Poisson distribution accurately predicted the observed enrichment values using the starting cell density (cells per droplet) and the ratio of active to inactive cells. When the cells were encapsulated at low density (∼1 cell for every 50 droplets), sorting was very efficient and all of the recovered cells were the active strain. In addition, single active droplets were sorted and cells were successfully recovered.