The authors thank the FIRST microfabrication facility at ETH Zurich. This work was supported by an Australian Research Council Discovery Early Career Researcher Award DE180100911 (to D.R.B.), a Gordon and Betty Moore Marine Microbial Initiative Investigator Award GBMF3783 (to R.S.), and a Swiss National Science Foundation grant 1-002745-000 (to R.S.).

1. Fabrication of the Microfluidic Device for the Single Chemical-pulse Experiment
<ol>
	<li>Design the channel using computer-aided design (CAD) software and print it onto a transparency film to create the photo mask (Figure 1A).</li>
	<li>Fabricate the master by soft lithography (under clean-room conditions).
	<ol>
		<li>Clean a silicon wafer (4 inches) in quick succession with acetone, methanol and isopropanol, then dry using nitrogen. Bake the wafer in the oven at 130 ...

<ol>
	<li>Armitage, J. P., Lackie, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The biology of the chemotactic response. , (1991).</li><li>Azam, F., Malfatti, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microbial+structuring+of+marine+ecosystems.">Microbial structuring of marine ecosystems.</a> Nature Reviews Microbiology. 5 (10), 782-791 (2007).</li><li>Buchan, A., LeCleir, G. R., Gulvik, C. A., Gonz&#225;lez, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Master+recyclers:+features+and+functions+of+bacteria+associated+with+phytoplankton+blooms.">Master recyclers: features and functions of bacteria associated with phytoplankton blooms.</a> Nature Reviews Microbiology. 12 (10), 686-698 (2014).</li><li>Whitesides, G. M., Ostuni, E., Takayama, S., Jiang, X., Ingber, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Soft+lithography+in+biology+and+biochemistry.">Soft lithography in biology and biochemistry.</a> Annual Review of Biomedical Engineering. 3, 335-373 (2001).</li><li>Segall, J. E., Block, S. M., Berg, H. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Temporal+comparisons+in+bacterial+chemotaxis.">Temporal comparisons in bacterial chemotaxis.</a> Proceedings of the National Academy of Sciences of the United States of America. 83 (23), 8987-8991 (1986).</li><li>Waite, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Non-genetic+diversity+modulates+population+performance.">Non-genetic diversity modulates population performance.</a> Molecular Systems Biology. 12 (12), 895 (2016).</li><li>Stocker, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Marine+Microbes+See+a+sea+of+gradients.">Marine Microbes See a sea of gradients.</a> Science. 338 (6107), 628-633 (2012).</li><li>Ellis-Davies, G. C. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Caged+compounds:+photorelease+technology+for+control+of+cellular+chemistry+and+physiology.">Caged compounds: photorelease technology for control of cellular chemistry and physiology.</a> Nature Methods. 4 (8), 619-628 (2007).</li><li>Kaplan, J. H., Somlyo, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flash+photolysis+of+caged+compounds:+New+tools+for+cellular+physiology.">Flash photolysis of caged compounds: New tools for cellular physiology.</a> Trends in Neurosciences. 12 (2), 54-59 (1989).</li><li>Ando, H., Furuta, T., Tsien, R. Y., Okamoto, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photo-mediated+gene+activation+using+caged+RNA/DNA+in+zebrafish+embryos.">Photo-mediated gene activation using caged RNA/DNA in zebrafish embryos.</a> Nature Genetics. 28 (4), 317-325 (2001).</li><li>England, P. M., Lester, H. A., Davidson, N., Dougherty, D. 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R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemotactic+signal+integration+in+bacteria.">Chemotactic signal integration in bacteria.</a> Proceedings of the National Academy of Sciences of the United States of America. 92 (21), 9757-9761 (1995).</li><li>Sagawa, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-Cell+E.+coli+Response+to+an+Instantaneously+Applied+Chemotactic+Signal.">Single-Cell E. coli Response to an Instantaneously Applied Chemotactic Signal.</a> Biophysical Journal. 107 (3), 730-739 (2014).</li><li>Xie, L., Lu, C., Wu, X. 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R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bacteria+push+the+limits+of+chemotactic+precision+to+navigate+dynamic+chemical+gradients.">Bacteria push the limits of chemotactic precision to navigate dynamic chemical gradients.</a> Proceedings of the National Academy of Sciences of the United States of America. 116 (22), 10792-10797 (2019).</li><li>Aran, K., Sasso, L. A., Kamdar, N., Zahn, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Irreversible,+direct+bonding+of+nanoporous+polymer+membranes+to+PDMS+or+glass+microdevices.">Irreversible, direct bonding of nanoporous polymer membranes to PDMS or glass microdevices.</a> Lab on a Chip. 10 (5), 548 (2010).</li><li>ZoBell, C. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+cultural+requirements+of+heterotrophic+aerobes.">The cultural requirements of heterotrophic aerobes.</a> Journal of Marine Research. 4, 42-75 (1941).</li><li>Canepari, M., Nelson, L., Papageorgiou, G., Corrie, J. E. T., Ogden, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photochemical+and+pharmacological+evaluation+of+7-nitroindolinyl-and+4-methoxy-7-nitroindolinyl-amino+acids+as+novel,+fast+caged+neurotransmitters.">Photochemical and pharmacological evaluation of 7-nitroindolinyl-and 4-methoxy-7-nitroindolinyl-amino acids as novel, fast caged neurotransmitters.</a> Journal of Neuroscience Methods. 112 (1), 29-42 (2001).</li><li>Trigo, F. F., Corrie, J. E. T., Ogden, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser+photolysis+of+caged+compounds+at+405nm:+Photochemical+advantages,+localisation,+phototoxicity+and+methods+for+calibration.">Laser photolysis of caged compounds at 405nm: Photochemical advantages, localisation, phototoxicity and methods for calibration.</a> Journal of Neuroscience Methods. 180 (1), 9-21 (2009).</li><li>Ribeiro, A. C. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mutual+diffusion+coefficients+of+L-glutamic+acid+and+monosodium+L-glutamate+in+aqueous+solutions+at+T=298.15K.">Mutual diffusion coefficients of L-glutamic acid and monosodium L-glutamate in aqueous solutions at T=298.15K.</a> The Journal of Chemical Thermodynamics. 74, 133-137 (2014).</li><li>Sharqawy, M. H., Lienhard, J. H., Zubair, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermophysical+properties+of+seawater:+a+review+of+existing+correlations+and+data.">Thermophysical properties of seawater: a review of existing correlations and data.</a> Desalination and Water Treatment. 16 (1-3), 354-380 (2010).</li><li>. Nikon depth of field calculator Available from: <a target="_blank" href="https://www.microscopyu.com/tutorials/depthoffield">https://www.microscopyu.com/tutorials/depthoffield</a> (2019)</li><li>Ki&#248;rboe, T., Thygesen, U. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluid+motion+and+solute+distribution+around+sinking+aggregates:+II.+Implications+for+remote+detection+by+colonizing+zooplankters.">Fluid motion and solute distribution around sinking aggregates: II. Implications for remote detection by colonizing zooplankters.</a> Marine Ecology Progress Series. 211, 15-25 (2001).</li><li>Blackburn, N., Fenchel, T., Mitchell, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microscale+nutrient+patches+in+planktonic+habitats+shown+by+chemotactic+bacteria.">Microscale nutrient patches in planktonic habitats shown by chemotactic bacteria.</a> Science. 282 (5397), 2254-2256 (1998).</li><li>Stocker, R., Seymour, J. R., Samadani, A., Hunt, D. E., Polz, M. 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L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bacterial+chemotaxis+in+an+optical+trap.">Bacterial chemotaxis in an optical trap.</a> PLoS ONE. 6 (4), 18231 (2011).</li><li>Zhu, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Frequency-Dependent+Escherichia+coli+Chemotaxis+behavior.">Frequency-Dependent Escherichia coli Chemotaxis behavior.</a> Physical Review Letters. 108 (12), (2012).</li><li>Baraban, L., Harazim, S. M., Sanchez, S., Schmidt, O. 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The authors have nothing to disclose.

This method allows researchers to study bacterial chemotaxis under controlled, dynamic gradients in micro- and millifluidic devices, enabling reproducible data acquisition. The near-instantaneous creation of chemical pulses at the microscale by photolysis aims to reproduce the types of nutrient pulses that bacteria encounter in the wild from a range of sources, for example, the diffusive spreading of plumes behind sinking marine particles25, or the nutrient spreading from lysed phytoplankton cells...

Microbes rely on chemotaxis, the process of detecting chemical gradients and modifying motility in response1, to navigate chemical landscapes, approach nutrient sources and hosts, and escape noxious substances. These microscale processes determine the macroscale kinetics of interactions between microbes and their environment2,3. Recent advances in microfluidics and microfabrication technologies, including soft lithography4, have revolutionized our ability to create controlled microenvironments in which to study the interactions of microbes. For example, past experiments have studied bacterial chemotaxis by generating highly controlled, stable gradients of intermediate to high nutrient concentrations5,6. However, in natural environments, microscale chemical gradients can be short-lived&#8213;dissipated by molecular diffusion&#8213;and background conditions are often highly dilute7. To directly measure the chemotactic response of microbial populations first exposed to unsteady chemical environments, we devised and here describe methods to combine microfluidic technology with photolysis, thereby mimicking gradients that wild bacteria encounter in nature.
Uncaging technology employs light sensitive probes that functionally encapsulate biomolecules in an inactive form. Irradiation releases the caged molecule, allowing the targeted perturbation of a biological process8. Due to the rapid and precise control of cellular chemistry that the uncaging affords9, photolysis of caged compounds has traditionally been employed by biologists, physiologists and neuroscientists to study the activation of genes10, ion channels11, and neurons12. More recently, scientists have leveraged the significant advantages of photolysis to study chemotaxis13, to determine the flagella switching dynamics of individual bacterial cells exposed to a stepwise chemoattractant stimulus14,15, and to investigate motility patterns of single sperm cells in three-dimensional (3D) gradients16.
In our approach, we implement photolysis of caged amino acids within microfluidic devices to study the behavioral response of a bacterial population to controlled chemical pulses, which become near-instantaneously available through photorelease. The use of a low-magnification (4x) objective (NA = 0.13, depth of focus approximately 40 &#956;m) allows both the observation of the population-level aggregative response of thousands of bacteria over a large field of view (3.2 mm x 3.2 mm), and the measurement of motion at the single-cell level. We present two applications of this method: 1) the release of a single chemical pulse to study bacterial accumulation&#8722;dissipation dynamics starting from uniform conditions, and 2i) the release of multiple pulses to characterize the bacterial accumulation dynamics under time-varying, spatially heterogeneous chemoattractant conditions. This method has been tested on the marine bacteria Vibrio ordalii performing chemotaxis toward the amino acid glutamate17, but the method is broadly applicable to different combinations of species and chemoattractants, as well as to biological processes beyond chemotaxis (e.g., nutrient uptake, antibiotic exposure, quorum sensing). This approach promises to help elucidate the ecology and behavior of microorganisms in realistic environments and to uncover the hidden trade-offs that individual bacteria face when navigating ephemeral dynamic gradients.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>(3-Aminopropyl) triethoxysilane (APTES)</td>
			<td>Sigma-Aldrich</td>
			<td>A3648</td>
			<td>&#62;98% purity, highly toxic</td>
		</tr>
		<tr>
			<td>CELLSTAR tube</td>
			<td>Greiner Bio-One</td>
			<td>210261</td>
			<td>50 ml</td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Eppendorf</td>
			<td>5424R</td>
			<td>to eliminate spent media from the bacterial culture</td>
		</tr>
		<tr>
			<td>Digital Incubators Incu-Line</td>
			<td>VWR-CH</td>
			<td>390-0384</td>
			<td>to bake 3D master</td>
		</tr>
		<tr>
			<td>Duster</td>
			<td>VWR-CH</td>
			<td>16650-22</td>
			<td>to clean the wafer and microchannels</td>
		</tr>
		<tr>
			<td>Hot plate</td>
			<td>VWR-CH</td>
			<td>444-0601</td>
			<td>to bond the microchannels</td>
		</tr>
		<tr>
			<td>Isopropanol</td>
			<td>Sigma-Aldrich</td>
			<td>W292907</td>
			<td></td>
		</tr>
		<tr>
			<td>LightSafe micro centrifuge tubes</td>
			<td>Sigma-Aldrich</td>
			<td>Z688312</td>
			<td>1.5 ml</td>
		</tr>
		<tr>
			<td>MATLAB</td>
			<td>Mathworks</td>
			<td></td>
			<td>for image analysis and bacterial tracking</td>
		</tr>
		<tr>
			<td>Microcentrifuge tube</td>
			<td>Eppendorf</td>
			<td>30120086</td>
			<td>1.5 ml</td>
		</tr>
		<tr>
			<td>Microscope glass slide</td>
			<td>VWR-CH</td>
			<td>631-1552</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope Nikon Eclipse TiE</td>
			<td>Nikon Instruments</td>
			<td>MEA53100</td>
			<td>with motorized stage</td>
		</tr>
		<tr>
			<td>MNI-Glutamate</td>
			<td>Tocris Bioscience</td>
			<td>1490</td>
			<td>&#62;98 % purity, photosensitive</td>
		</tr>
		<tr>
			<td>Mold printing equipment</td>
			<td>Stratasys</td>
			<td></td>
			<td>Objet30 3D printer</td>
		</tr>
		<tr>
			<td>Mold printing service</td>
			<td>3D Printing Studios</td>
			<td>Custom</td>
			<td><a href="https://www.3dprintingstudios.com/" target="_blank">https://www.3dprintingstudios.com/</a></td>
		</tr>
		<tr>
			<td>Nanodrop One UV-Vis Spectrophotometer</td>
			<td>Thermo Fisher Scientific</td>
			<td>ND-ONE-W</td>
			<td>to calibrate the uncaging</td>
		</tr>
		<tr>
			<td>NIS Elements</td>
			<td>Nikon Instruments</td>
			<td></td>
			<td>Microscope Imaging Software</td>
		</tr>
		<tr>
			<td>Oven Venti-Line</td>
			<td>VWR-CH</td>
			<td>466-3516</td>
			<td>to bake PDMS (with forced convection)</td>
		</tr>
		<tr>
			<td>Photoresist SU-8-3050</td>
			<td>MicroChem Corp.</td>
			<td>SU8-3050</td>
			<td></td>
		</tr>
		<tr>
			<td>Plasma chamber Zepto</td>
			<td>Diener Electronic</td>
			<td>ZEPTO-1</td>
			<td>to functionalize the surfaces before bonding</td>
		</tr>
		<tr>
			<td>Polycarbonate membrane</td>
			<td>Sterlitech</td>
			<td>PCT0447100</td>
			<td>0.4 &#181;m pore size, 19 % open area, 24 &#181;m thickness</td>
		</tr>
		<tr>
			<td>Polyethylene microtubing</td>
			<td>Scientific Commodities</td>
			<td>BB31695-PE/2</td>
			<td>I.D. x O.D.: 0.015&#34; x 0.043&#34; / 0.38mm x 1.09mm</td>
		</tr>
		<tr>
			<td>Polystyrene Petri dish</td>
			<td>VWR-CH</td>
			<td>25373-100</td>
			<td>bottom surface (90 mm x 15 mm) to bond the millifluidic device</td>
		</tr>
		<tr>
			<td>Scale</td>
			<td>VWR-CH</td>
			<td>611-2605</td>
			<td>to weight PDMS mixture</td>
		</tr>
		<tr>
			<td>sCMOS camera Andor Zyla</td>
			<td>Oxford Instruments</td>
			<td></td>
			<td>for phase contrast and fluorescence microscopy (max 100 fps)</td>
		</tr>
		<tr>
			<td>Sea salt</td>
			<td>Instant Ocean</td>
			<td>Product No. SS1-160p</td>
			<td></td>
		</tr>
		<tr>
			<td>SolidWorks 2015</td>
			<td>Dassault Systemes SolidWorks</td>
			<td></td>
			<td>Used to design the mold</td>
		</tr>
		<tr>
			<td>Spectra X light engine</td>
			<td>Lumencolor</td>
			<td></td>
			<td>for LED 395 nm</td>
		</tr>
		<tr>
			<td>Sylgard 184</td>
			<td>Dow Corning</td>
			<td>110-41-155</td>
			<td>PDMS Si Elastomer Kit; curing agent</td>
		</tr>
		<tr>
			<td>Syringe (Luer-Lok)</td>
			<td>B Braun Omnifix</td>
			<td>4616308F</td>
			<td></td>
		</tr>
		<tr>
			<td>Syringe Needle</td>
			<td>Agani</td>
			<td>A228</td>
			<td>from 10 to 30 ml</td>
		</tr>
		<tr>
			<td>Syringe Pump 11 Pico Plus Elite</td>
			<td>Harvard Apparatus</td>
			<td>70-4506</td>
			<td>Terumo Agani 23 gauge 5/8 inch (16mm)</td>
		</tr>
		<tr>
			<td>VeroGrey</td>
			<td>Stratasys</td>
			<td></td>
			<td>Dual Syringe Pump</td>
		</tr>
		<tr>
			<td>Vortex-Genie</td>
			<td>Scientific Industries</td>
			<td>SI-0236</td>
			<td>Mold Material</td>
		</tr>
	</tbody>
</table>

generating controlled, dynamic chemical landscapes to study microbial behavior

We demonstrate a method for the generation of controlled, dynamic chemical pulses&#8213;where localized chemoattractant becomes suddenly available at the microscale&#8213;to create micro-environments for microbial chemotaxis experiments. To create chemical pulses, we developed a system to introduce amino acid sources near-instantaneously by photolysis of caged amino acids within a polydimethylsiloxane (PDMS) microfluidic chamber containing a bacterial suspension. We applied this method to the chemotactic bacterium, Vibrio ordalii, which can actively climb these dynamic chemical gradients while being tracked by video microscopy. Amino acids, rendered biologically inert (&#39;caged&#39;) by chemical modification with a photoremovable protecting group, are uniformly present in the suspension but not available for consumption until their sudden release, which occurs at user-defined points in time and space by means of a near-UV-A focused LED beam. The number of molecules released in the pulse can be determined by a calibration relationship between exposure time and uncaging fraction, where the absorption spectrum after photolysis is characterized by using UV-Vis spectroscopy. A nanoporous polycarbonate (PCTE) membrane can be integrated into the microfluidic device to allow the continuous removal by flow of the uncaged compounds and the spent media. A strong, irreversible bond between the PCTE membrane and the PDMS microfluidic structure is achieved by coating the membrane with a solution of 3-aminopropyltriethoxysilane (APTES) followed by plasma activation of the surfaces to be bonded. A computer-controlled system can generate user-defined sequences of pulses at different locations and with different intensities, so as to create resource landscapes with prescribed spatial and temporal variability. In each chemical landscape, the dynamics of bacterial movement at the individual scale and their accumulation at the population level can be obtained, thereby allowing the quantification of chemotactic performance and its effects on bacterial aggregations in ecologically relevant environments.

We used the microfluidic and millifluidic devices (Figure 1) to study bacterial accumulation profiles under dynamic nutrient conditions. Bacterial trajectories were extracted from recorded videos acquired by phase contrast microscopy of the accumulation-dissipation dynamics of a bacterial population following a chemical pulse released by photolysis (Figure 2 and Figure 3). By averaging millions of trajectories, the spatiotemporal dy...

Watch this Scientific Journal Video about Generating Controlled, Dynamic Chemical Landscapes to Study Microbial Behavior at JoVE.com

Generating Controlled, Dynamic Chemical Landscapes to Study Microbial Behavior

A protocol for the generation of dynamic chemical landscapes by photolysis within microfluidic and millifluidic setups is presented. This methodology is suitable to study diverse biological processes, including the motile behavior, nutrient uptake, or adaptation to chemicals of microorganisms, both at the single cell and population level.

We demonstrate a method for the generation of controlled, dynamic chemical pulses&#8213;where localized chemoattractant becomes suddenly available at the ...

This method allows to elucidate the microscale ecology and behavior of microorganisms navigating dynamic chemical gradients and to uncover their hidden trade-offs, nutrient kinetics, and population dynamics in ecologically relevant microenvironments. By combining microfluidic technology with photolysis, we generate controlled chemical pulses, where localized chemoattractant becomes suddenly available at the microscale, to measure the chemotactic response of microbial populations first exposed to unsteady chemical gradients. Design the channel using CAD software, and print it onto a transparency film to create the photo mask.Fabricate the master by soft lithography in a clean room, according to the manuscript. Prepare a PDMS mixture by combining the elastomer with its curing agent at a 10-to-one ratio in a 40-milliliter beaker. With a plastic knife, mix vigorously until the liquid is homogeneous.Immediately degas the PDMS mixture in a vacuum chamber for 45 minutes at room temperature. To expedite the process, periodically release the vacuum in order to burst the bubbles that form at the interface. Remove dust from the surface of the master with a pressurized cleaner.Then pour the degassed PDMS mixture onto the master, and bake it in an oven at 80 degrees Celsius for two hours. Cut the PDMS with a blade around the microstructures at a distance of approximately five millimeters, and then carefully peel the PDMS from the master. Punch holes to serve as the inlet and outlet of the microchannel.Seal the microchannel with adhesive tape to prevent accumulation of dust. Design the 3D shape using 3D design software, and print the master for the PDMS mold with a high-resolution 3D printer. After preparing and degassing the PDMS mixture as done previously, clean the surface of the master by dabbing with adhesive tape.Put the master on a scale. Then, while avoiding the central region of the master, pour 23.4 grams of uncured PDMS mixture in order to obtain the desired height of PDMS by matching the height of the master at 0.5 millimeters. Remove any remaining bubbles with the help of compressed air.Place the PDMS cast on the master in an oven at 45 degrees Celsius to bake for at least 12 hours. Then, gently peel off the hardened PDMS layer, and punch inlet and outlet holes for the injection of the bacterial suspension. Activate both the surface of a Petri dish and the PDMS mold with oxygen plasma for two minutes.Bond the PDMS mold on the Petri dish by gently pressing the mold to the Petri dish. Avoid pressing where the features are located. Place the Petri dish bonded to the PDMS 3D mold in an oven at 45 degrees Celsius for at least 12 hours to strengthen the chemical bond.Next, activate the nanoporous polycarbonate membrane in an oxygen plasma chamber for one minute at room temperature. Under a chemical hood, dilute a commercial solution of 3-aminopropyltriethoxysilane in deionized water to 1%by volume by using a polypropylene tube. Transfer the diluted 3-aminopropyltriethoxysilane solution in a Petri dish, and immerse the activated membrane in the 3-aminopropyltriethoxysilane solution for 20 minutes.Then, remove the membrane from the 3-aminopropyltriethoxysilane solution with tweezers, and place it on a clean room wipe to dry. For the fabrication of the PDMS microfluidic channels that will lie on the membrane and allow washing of the bacterial arena, repeat the fabrication of the microfluidic device for the single chemical pulse experiment as done previously. To bond the PDMS washing channels to the functionalized membrane, first activate both the PDMS washing channel and the polycarbonate membrane with an oxygen plasma chamber for two minutes.Immediately after the plasma treatment, bring the functionalized membrane and the PDMS washing channel into contact by gently pressing the PDMS washing channel onto the membrane. It is important to gently press together the PDMS and the membrane, or the membrane might be irreversibly bonded to the ceiling of the 200-micrometer height PDMS microchannel. Then, activate both the bonded laminate PDMS washing channel polycarbonate membrane and the 3D PDMS mold previously bonded to the Petri dish by placing them into an oxygen plasma chamber for two minutes.Sandwich the membrane between two PDMS layers, and press together. Place the Petri dish bonded to the sandwich structure in an oven at 45 degrees Celsius for at least 12 hours to strengthen the chemical bond. To begin, maintain the millifluidic chamber under vacuum for 20 minutes.Then, place the Petri dish containing the millifluidic chamber onto a microscope stage. With a pipette, fill the chamber below the membrane with the dilute bacterial suspension of GFP-fluorescent Vibrio ordalii in one-millimolar 4-methoxy-7-nitroindolinyl-caged-L-glutamate solution in artificial seawater. After filling the channel with the bacterial suspension, suck the solution in excess with a paper towel, and seal inlet and outlet with PDMS plugs by gently pressing them into the holes.Fill a syringe with an artificial seawater solution of one-millimolar 4-methoxy-7-nitroindolinyl-caged-L-glutamate, and attach tubing to the inlet and outlet of the washing channel above the membrane. Connect the tubing to a waste reservoir, and ensure that the tubing is entirely submerged in the fluid waste reservoir to avoid pressure oscillations. Set the flow rate on the syringe pump to 50 microliters per minute.Start the syringe pump to establish the flow in the washing channel above the membrane. Start the software controlling the LED beam and the microscope stage, and generate user-defined sequences of pulses at different locations and with different intensities. Record video using a 4x objective at regular time intervals over a period of several hours and over multiple contiguous locations to cover a large surface.In this study, the microfluidic and millifluidic devices were used to study bacterial chemotactic response and accumulation profiles under dynamic nutrient conditions. Maximum intensity projections show the bacterial position, indicated by white traces, over a 0.5-second interval immediately following the pulse release and 40 seconds after the pulse release. The bacterial trajectories were also shown in black 60 seconds after the pulse release.The shaded region represents the chemical pulse released by photolysis at time zero second in the middle of the field of view, which subsequently diffused. The bacterial concentration, as well as the temporal dynamics of the radial drift velocity following a pulse release in the center of the field of view are shown here. Negative values of the drift velocity correspond to directed chemotactic motion towards the center of the pulse.Swimming statistics are presented here for a bacterial population in the absence of chemical gradients. Trajectories represent the two-dimensional projections of the three-dimensional bacterial motion in the microchannel. Probability distribution of measured bacterial swimming speed was fit by a gamma distribution.From the bacterial trajectories, the probability distribution of the time between successive reorientations was extracted. The reorientation pattern, the distribution of swimming speed, and the reorientation statistics of the organisms were used to inform an individual based model. This method has been tested on the marine bacteria Vibrio ordalii performing chemotaxis toward the amino acid glutamate, but the proposed methodology is broadly applicable to different combination of species and chemoattractants, as well as to biological processes beyond chemotaxis, such as population and community dynamics in spatially heterogeneous and dynamic conditions.Classical techniques in microbial ecology and microbiology, such as growths in batch cultures or chemostats, have largely ignored the spatio-temporal characteristics of microbial habitats. The near-instantaneous creation of chemical pulses at the microscales by photolysis aims to reproduce the type of nutrient pulses that marine bacteria encounter in the wild from a range of sources, for example, the diffusive spreading of plumes behind sinking organic particles or the nutrient spreading from lysed phytoplankton cells. The aminosilane solution is acutely toxic and should be handled with extreme care.We work exclusively under the fume hood and wear protective equipment like lab coat, safety goggles, and gloves. We also take care of the waste we generated.

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Research

JoVE Journal

Bioengineering

5.9K visualizações.  Institute of Environmental Engineering, Department of Civil, Environmental and Geomatic Engineering. Este método permite elucidar a ecologia de microescala e o comportamento dos microrganismos navegando em gradientes químicos dinâmicos e descobrir suas trocas ocultas, cinética de nutrientes e dinâmica populacional em microambientes ecologicamente relevantes. Ao combinar tecnologia microfluida com fotolise, geramos pulsos químicos controlados, onde o quimioattractant localizado se torna subitamente disponível na microescala, para medir a resposta quimotactica de populações microbiana...