Machine vision-enabled OD600 measurement and mass spectrometric determination of metabolites of microbial cell-containing microfluidic droplets in real-time.

Droplet microfluidics has become a versatile tool for high-throughput phenotypic analysis of cells, employed in enzyme engineering, drug screening, and antibiotic assays. An important metric in such bioanalyses is the optical density (OD600) of the culture, but current methods for measuring OD600 within cultured droplets are limited. Herein, we present a machine-vision method for the measurement of OD600 between 0.1 and 4.3 in flowing microdroplets from 0.5-50 nL in volume. This system can make real-time decisions based on cell density and volume measurements and was demonstrated to remove vacant droplets from an emulsion by dielectrophoretic sorting at up to 10 droplets per s based on cell density with an error rate below 10%. We show that droplets can also be subjected to subsequent on-line analysis such as mass spectrometry (MS) and their growth levels correlated to metabolite abundances. We show that growth level is correlated with lysine abundance in droplets incubated with engineered E. coli cells.