E.COLI CULTURE
IN DROPLETS
USING FLUIGENTFLUOROSURFACTANT
INTRODUCTION
In vitro cell culture is a fundamental component of biological production systems and biotechnological research. The ability to grow cells outside of their natural environment offers many possibilities ranging from high quantity production of enzymes to cell toxicity studies.
Traditionally, in vitro cell culture methods use petri dishes or culture flasks for laboratory cellculture. In culture flasks, cells are grown in a homogenous liquid medium, while petri dishes
allow the growth of colonies on a solid or semi-solid substrate surface. Nevertheless, these two
methods have limitations including the difficulty to compartmentalize clones and single cells.Compartmentalization allow single cells or clones to be individually assayed or exposed to different
culture conditions or stimuli for versatile (or multiparametric) analysis of cells heterogeneity.
To tackle this issue droplet based microﬂuidics appears as an emerging cell culture method enabling compartmentalized culture of single cells and clones in droplets.
Using droplet based microfluidics allows to encapsulate single or multiple cells into tiny dropletof pL volume which are generated at a rate of thousand per seconds. This method is based on two
different non-miscible solutions, mostly aqueous solution and oil solution (as carrier phase) which
are used to generate stable droplet and encapsulate cells in them
Perfluorinated oils have shown several advantages compare to other carrier fluids such as mineraloils. Their low viscosity allows easy handling in microfluidic systems without need of high-pressure
pumps.

This application note was made in collaboration with Oksana Shvydkiv and her lab.
For more information visit the team website: Leibniz-hki.de/en/droplet-based-microfluidics.html

APPLICATION NOTELong term cell culture experiments have been successfully performed with water-in-perfluorinatedoils[1] droplets. Moreover, fluorinated oils show lower organic molecule transfer drop to drop[2].
Finally, while mineral oils swell PDMS chips, fluorinated oils show a better compatibility[3]. Also,
oxygen is well soluble in the fluorinated oils which ensures sufficient gas diffusion between oil anddroplet content. Though perfluorinated oils offer benefits for applications such as cell cultivation
or dPCR, to find an adequate and efficient surfactant for droplet stabilization in this oil remains achallenge. To date, most commercially available surfactants present limitations.
The objective of this study is to highlight the biocompatibility of the new surfactant dSURF byculturing microorganisms in picoliter droplets.
For this purpose, a model microorganism that is commonly used in droplet based microfluidic
for the development and testing of new methods has been employed: Escherichia coli ECJW922

MATERIALS AND METHODSReagents:
Continuous phase Reagents:
Novec HFE-7500(Sigma Aldrich) containing 0.5%
or 3% dSURF (Fluigent, P/N DR-RE-SU-A1)
Dispersed phase Reagents:
500 µl suspension of E. coli ECJW922 in TB (Terrific broth)medium with OD600 (optical density) of 0.005 (5 x 106:
CFU*/ml) or 0.01 (10 x 107 CFU/ml), , which means cellconcentration in the first suspension was 5 x 106 CFU/ml
and in second suspension 10 x 107 CFU/ml. CFU .
The idea is to obtain cell concentrations in the suspension
such that after their encapsulation into droplets approximatively 50% of droplets will be occupied mainly with
one or two cells and in the second solution approximatively 90% of droplets will be occupied with single, two,
three or more cells according to the Poisson distribution.
*CFU: colony forming unit

Droplet Generation:

Droplet generation was performed withcustomer designed PDMS chip but wewould recommend to use the EZ dropchip (Fluigent, P/N DropKit01)
The Fluigent Drplet Starter Pack

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APPLICATION NOTE
To generate droplets, fluid handling devices such as Pressure pump, Syringe pump or peristaltic
pump could be used with a flow focusing PDMS chip[4]. In this case Syringes pump have been used..
Syringes that are loaded into the syringe pump are connected to the chip with 1/32 OD 250 µm ID
PTFE tubing in order to inject continuous and dispersed phases at determined flow rates into thechip to achieve desired droplet size and droplet generation frequency.
Flow EZTM flow controller can be also used to increase droplet generation performances such as
Flow rate stability and droplet monodispersity. In this case, E Coli suspension and dOIL with dSURF
are loaded into vials. A pressure is applied to the reservoir to ensure a continuous and pulselessinjection of both phases into the chip. The determined flow rate is monitored and controled byusing the A-i-O software or local control on the Flow EZTM to achieve the desired droplet size andfrequency.
SoftwareMineral+ Span

Flow-Rate Platform

FLOW UNITSFlow Unit

DropboxE. Coli
OTA + PI (resin)
Suspension
Flow-Rate Platform

FLOW UNITS

Flow Unit

LINK

Flow EZ™

dOILOil+ dSURF
Mineral+ Span 80
Experimental Setup for Cell Encapsulation

Droplet incubation and Monitoring:
Droplets were incubated at 28 or 37 degree Celsius on an Eppendorf shaker (but could be incubatedin any incubation devices or chip[5]).
To monitor droplet stability and bacterial growth, droplets were transferred into the observationchambers of another microfluidic chip.This was placed on an inverted microscope (Axio Observer
Z1, Zeiss). The standard halogen lamp of the microscope was used as light source for bright anddark field illumination images. For image acquisition a PCO camera and 10x magnification objective
were used.
Droplet stability and presence/absence of microbial growth were checked after 2 or 4h and after
20h. Time points of 2 or 4 h were necessary, because after prolonged incubation time, the quality
and stability of droplet emulsion could be affected by the growth of bacteria.
Therefore, droplets have been checked after 2 or 4h of incubation when some growth of bacteria
could be observed and later after 20h.
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APPLICATION NOTERESULTS
Test with E.Coli at OD=0.005 and 0.5% dSURF, incubation at 37°C:
Stable and monodisperse droplet emulsion and E. coli in some droplets were observed.

ABCD

Legend:(A) Image of droplets under bright-field after 4h incubation (B) Image of droplets under brightfield after 24h incubation (C) Images of droplets under dark-field after 4h incubation (D) Imagesof droplets under dark-field after 24h incubation.
Stable, monodispersed droplet emulsions were observed on both images. High numbers of E. coli
cells were observed in almost half of the droplets, particularly in 46± 0.8% of droplets, which is ina good agreement with the theoretical number of 54% calculated from the Poisson distributionof cells over 67.2µm droplets (160pL) at a given concentration of 5 x 106 CFU/ml. The variability
between theoretical and experimental numbers could be explained by the choice to use a low
E.coli concentration and fractional droplet occupation with cells as well as by small variation in theexperimental procedure, such as general pipetting imprecision
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APPLICATION NOTE

Test with E.Coli at OD=0.01 and 0.5% dSURF, incubation at 37°C:
Stable and monodisperse droplet emulsion and E. coli in some droplets were observed

ABCD

Legend:(A) Images of droplets under bright-field after 4h incubation (B) Image of droplets under brightfield after 20h incubation C) Images of droplets under dark-field after 4h incubation (D) Imagesof droplets under dark-field after 20h incubation
Stable, monodispersed droplet emulsion could be observed on both images. High numbers ofE.coli cells were observed in most droplets, particularly in 89± 1% of droplets, which is in a good
agreement with the theoretical number of 86% calculated from the Poisson distribution of cells
over 72.4µm droplets (200pL) at a given concentration of 10 x 107 CFU/ml.

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APPLICATION NOTE
Test with E.Coli at OD=0.005 and 3% dSURF, incubation at 28°C:
Stable and monodisperse droplet emulsion and E. coli in some droplets were observed

AB

CD

Legend:(A) Images of droplets under bright-field after 4h incubation (B) Image of droplets under brightfield after 20h incubation C) Images of droplets under dark-field after 4h incubation (D) Imagesof droplets under dark-field after 20h incubation
Stable, monodispersed droplet emulsions could be observed on both images. High numbers ofE. coli cells were observed in many droplets, particularly in 63± 1.1% of droplets, which is in a perfect agreement with the theoretical number of 63% calculated from the Poisson distribution ofcells over 72.4µm droplets (200pL) at a given concentration of 5 x 106 CFU/ml

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APPLICATION NOTECONCLUSION

Growth of E. coli after 20 h of incubation in all above described cases has highlighted
that dSURF is biocompatible at concentrations ranging from 0.5% to 3%. For routine
applications and cost reduction, 0.5% concentration of dSURF can be used. It is also
suitable for experiments starting with single and multiple microbial cells per droplet.
The emulsion stability after 20 h shows the good droplet stabilization which will be ofuse for longer duration experiments.

REFERENCES[1] C. H. J. Schmitz, A. C. Rowat, S. Koester and D. A. Weitz, Dropspots: a picoliter array in a microfluidic device, Lab Chip, 2009, 9, 44–49.
[2] Bai Yunpeng, He Ximin, Liu Dingsheng, Patil Santoshkumar N, Bratton Dan, Huebner Ansgar,
Hollfelder Florian, Abell Chris, Huck WilhelmTS. A double droplet trap system for studying masstransport across a droplet–droplet interface. Lab Chip May 2010;10(10):1281–5.
[3] Lee JN, Park C, Whitesides GM. Solvent compatibility of poly(dimethylsiloxane)-based microfluidic devices. Anal Chem 2003, 75:6544-54.
[4] Svensson C-M*, Shydkiv O*, Dietrich S, Mahler L, Weber T, Choudhary M, Tovar M+, Figge MT+**,
Roth M**. Coding of experimental conditions in microfluidic droplet assays using colored beads
and machine learning supported image analysis 2018[5] Mahler L, Tovar M, Weber T, Brandes S, Rudolph MM, Ehgartner J, Mayr T, Figge MT, Roth M,
Zang E (2015) Enhanced and homogeneous oxygen availability during incubation of microfluidic
droplets. RSC Advances 5, 101871-101878.

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