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Method Article
The development of microbial communities depends on a combination of factors, including environmental architecture, member abundance, traits, and interactions. This protocol describes a synthetic, microfabricated environment for the simultaneous tracking of thousands of communities contained in femtoliter wells, where key factors such as niche size and confinement can be approximated.
The development of microbial communities depends on a combination of complex deterministic and stochastic factors that can dramatically alter the spatial distribution and activities of community members. We have developed a microwell array platform that can be used to rapidly assemble and track thousands of bacterial communities in parallel. This protocol highlights the utility of the platform and describes its use for optically monitoring the development of simple, two-member communities within an ensemble of arrays within the platform. This demonstration uses two mutants of Pseudomonas aeruginosa, part of a series of mutants developed to study Type VI secretion pathogenicity. Chromosomal inserts of either mCherry or GFP genes facilitate the constitutive expression of fluorescent proteins with distinct emission wavelengths that can be used to monitor community member abundance and location within each microwell. This protocol describes a detailed method for assembling mixtures of bacteria into the wells of the array and using time-lapse fluorescence imaging and quantitative image analysis to measure the relative growth of each member population over time. The seeding and assembly of the microwell platform, the imaging procedures necessary for the quantitative analysis of microbial communities within the array, and the methods that can be used to reveal interactions between microbial species area all discussed.
Microbial communities are shaped by both deterministic factors, such as the structure of the environment, and stochastic processes, which are associated with cell death, division, protein concentration, number of organelles, and mutation1. Within the natural environment, it can be nearly impossible to parse the individual impact of these influences on community composition and activity. Obscured by natural structures and buried within a chemical and biological milieu, identifying community members and further resolving their spatiotemporal distribution within the natural environment is extremely challenging. Nonetheless, recent efforts have underscored the importance of spatial organization on community function and point towards the need to account for both member abundance and organization in ongoing studies2,3,4.
It is clear that the local chemical environment (i.e. the availability of nutrients and secondary metabolites), the physical structure (e.g., soil architecture, plant roots, ocean particles, or the intestinal microvilli), the presence or absence of oxygen, and the introduction of pathogenic species all affect the composition, architecture, and function of microbial communities5,6,7,8,9,10,11. Nonetheless, traditional techniques for cultures that neglect to capture these factors continue to prevail. Community composition (e.g., the presence of co-dependent species), physical attachment, signaling molecule concentration, and direct cell-cell contact are all important factors for shaping a microbial community and can be lost in conventional culture conditions. These properties are difficult to replicate in a bulk liquid culture or on an agar plate. The availability of microfluidic, micropatterning, and nanofabrication techniques that allow for the replication of key physical and chemical features of natural environments has, however, enabled many researchers to build bacterial communities to study their interactions12,13,14 and to develop synthetic environments that mimic natural conditions4,15,16,17,18,19,20.
This protocol describes a method to fabricate a microwell array device and provides detailed experimental procedures that can be used to functionalize the wells in the array and to grow bacteria, both as single-species colonies and in multi-member communities. This work also demonstrates how bacteria modified to produce fluorescent reporter proteins can be used to monitor bacterial growth within wells over time. A similar array was presented previously and showed that it is possible to track the growth of single-species colonies of Pseudomonas aeruginosa (P. aeruginosa) in microwells. By modulating well size and seeding density, the starting conditions of thousands of growth experiments can be varied in parallel to determine how the initial inoculation conditions affect the ability of the bacteria to grow21. The current work uses a slightly modified version of the microwell array that builds on the previous work by enabling the simultaneous comparison of multiple arrays and by using a more robust experimental protocol. The array used in this work contains multiple subarrays, or array ensembles, containing wells of different sizes, ranging from 15 - 100 µm in diameter, that are arranged at three different pitches (i.e. 2x, 3x, and 4x the well diameter). The arrays are etched into silicon, and the growth of the bacteria seeded in the silicon arrays is enabled by sealing the arrays with a coverslip that has been coated with a medium-infused agarose gel. P. aeruginosa mutants designed to study the Type VI secretion system are used in this demonstration.
The results presented here build toward the ultimate goal of analyzing multimember communities within microwell arrays, enabling researchers to monitor the abundance and organization of bacteria in situ while controlling and probing the chemical environment. This should ultimately provide insights into the "rules" that govern community development and succession.
1. Silicon Microwell-array Fabrication
2. Bacterial Culture and Seeding (Figure 1a)
Figure 1: Fabrication and Cell Seeding Procedure. (a) Microwell arrays are etched into silicon wafers coated with a thin layer of parylene (i). To wet the wells and/or functionalize the surface, a protein solution is added in a droplet on top of the arrays (ii). The protein solution is removed, the wafers are dried, and a new solution containing the desired bacteria is added (iii). The bacterial solution is removed after an incubation period, and the wafers are allowed to dry, leaving behind bacteria in the wells and on the surface (iv). The surface-associated bacteria are removed with parylene lift-off, leaving behind bacteria seeded cleanly in the microwells and still viable due to the 2% glycerol medium, which helps to keep the wells hydrated(v). The silicon chips are then placed array-side down on an agarose gel-coated glass coverslip, which feeds bacterial growth in the microwells (vi). (b) Layout of sub-arrays on a single silicon device. Each sub-array contains a set of identical wells. The diameter of the microwells across all sub-arrays range in diameter from 5-100 µm and are organized at 2x, 3x, or 4x the well diameter pitch, which is denoted by the white to dark-gray colors on the bottom panel schematic. When the well depths are shallow (<10 µm), the 5 and 10 µm well diameters are rarely useful, generally because of a lack of cells colonizing these very small wells. In this work, only the data from wells with 15-100 µm diameters were analyzed. Please click here to view a larger version of this figure.
NOTE: As shown in Figure 1b, a complete chip contains sub-arrays of wells, with diameters ranging from 5 to 100 µm, with three different pitches (i.e. 2x, 3x, and 4x the diameter) repeating 4 times.
3. Microscope Set-up
4. Preparation of Agarose-coated Glass Coverslips
5. Sealing the Wells with an Agarose-coated Coverslip and Imaging
6. Analysis
The experimental platform presented here is designed for high-throughput and high-content studies of bacterial communities. The design enables thousands of communities, growing in wells of various sizes, to be analyzed simultaneously. With this microwell array design, the dependence of the final community composition on initial seeding densities, well size, and chemical environment can be determined. This work demonstrates the growth of a two-member community in the microwell array and pu...
This article presented a microwell array device and experimental protocols designed to enable high-throughput and high-content live-cell imaging-based analysis of bacterial community development. While the focus of the demonstration here was to study the effects of contact-mediated Type VI secretion on community development, the arrays were designed to be flexible and accommodate the study of a broad range of microbial communities and microbe-microbe interactions. The work here focuses solely on the use of bacteria that ...
The authors have nothing to disclose.
Microwell arrays were fabricated and characterized at the Center for Nanophase Materials Sciences User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Financial support for this work was provided through the Oak Ridge National Laboratory Director's Research and Development Fund. The authors would also like to thank the J. Mougous Laboratory (University of Washington, Seattle, WA) for the supply of P. aeruginosa strains used in these studies.
Name | Company | Catalog Number | Comments |
Parylene N | Specialty Coating Systems | CAS NO.:1633-22-3 | |
Parylene coater | Specialty Coating Systems | Labcoter 2 Parylene Deposition Unit PDS2010 | |
Silicon Wafer | WRS Materials | 100mm diameter, 500-550μm thickness, Prime, 10-20 resistivity, N/Phos<100>, | |
adhesion promoter | Shin-Etsu Microsci | MicroPrime P20 adhesion promoter | |
postive tone photoresist | Rohm and Haas Electronics Materials LLC (Owned by Dow) | Microposit S1818 Positive Photoresist (code 10018357) | |
Quintel Contact Aligner | Neutronix Quintel Corp | NXQ 7500 Mask Aligner | |
Reactive Ion Etching Tool | Oxford Instruments | Plasmalab System 100 Reactive Ion Etcher | |
R2A Broth | TEKnova | R0005 | |
Bovine Serum Albumin | Sigma | A9647 | |
Multimode Plate Reader | Perkin Elmer | Enspire, 2300-0000 | |
Fluorescent Microscope | Nikon | Eclipse Ti-U | |
Automated Stage | Prior | ProScan III | |
CCD camera | Nikon | DS-QiMc | |
Stage-top environmental control chamber | In Vivo Scientific | STEV ECU-HOC | |
Phosphate Buffered Saline | ThermoFisher Scientific | 14190144 | |
UltraPure Agarose | ThermoFisher Scientific | 16500500 | |
25 x 75 mm No. 1.5 coverslip | Nexterion | High performance #1.5H coverslips | |
Fluorescence Reference Slides | Ted Pella | 2273 | |
Physical Stylus Profilometer | KLA Tencor | P-6 | |
lab wipes | Kimberly Clark | Kimipe KIMTECH SCIENCE Brand, 34155 | |
commercial software | Nikon | NIS Elements | |
Zeiss 710 Confocal Microscope | Zeiss | ||
filter cubes | Nikon | Nikon FITC (96311), Nikon Texas Red(96313) |
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