High-Throughput Monitoring of Bacterial Cell Density in Nanoliter Droplets: Label-Free Detection of Unmodified Gram-Positive and Gram-Negative Bacteria

Droplet microfluidics disrupted analytical biology with the introduction of digital polymerase chain reaction and single-cell sequencing. The same technology may also bring important innovation in the analysis of bacteria, including antibiotic susceptibility testing at the single-cell level. Still, despite promising demonstrations, the lack of a high-throughput label-free method of detecting bacteria in nanoliter droplets prohibits analysis of the most interesting strains and widespread use of droplet technologies in analytical microbiology. We use a sensitive and fast measurement of scattered light from nanoliter droplets to demonstrate reliable detection of the proliferation of encapsulated bacteria. We verify the sensitivity of the method by simultaneous readout of fluorescent signals from bacteria expressing fluorescent proteins and demonstrate label-free readout on unlabeled Gram-negative and Gram-positive species. Our approach requires neither genetic modification of the cells nor the addition of chemical markers of metabolism. It is compatible with a wide range of bacterial species of clinical, research, and industrial interest, opening the microfluidic droplet technologies for adaptation in these fields. ; © 2020 American Chemical Society. Received: August 11, 2020; Accepted: November 10, 2020; Published: December 10, 2020. N.P. was supported by the National Science Centre funding based on decision number DEC-2014/12/W/NZ6/00454 (Symphony). P.G. was supported by EVO Drops no. 813786. M.H. was supported by the Foundation for Polish Science through the program TEAM TECH/2016-2/10. K.M. has received funding from the Polish National Agency for Academic Exchange. J.B. and M.W. were supported within the 2 × 2 PhotonVis project no. POIR.04.04.00-00-3D47/16-00 which is carried out within the TEAM TECH programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund. This publication is part of a project that has received funding from the European ...