A large part of the human gut microbiota is still difficult to grow outside the body. One reason may be that conventional culture does not recreate much of the physical environment that bacteria encounter in the gastrointestinal tract. In a recent Nature-published study, researchers out of Montreal, QC, Canada explored hydrogel stiffness and bacterial growth by encapsulating individual gut-associated bacteria inside PEG4MAL microbeads, measuring the mechanics of those beads with a CellScale MicroTester, and then watching how the bacteria grew inside them.
The work moves beyond using a hydrogel simply as a container. PEG4MAL concentration, network structure, swelling, osmolality, and mechanical properties were all varied or measured. The researchers then looked at whether those physical differences corresponded with changes in bacterial viability and colony formation.
That makes the study an interesting example of hydrogel stiffness and bacterial growth intersecting with an emerging area sometimes described as mechanomicrobiology.
Why Hydrogel Stiffness and Bacterial Growth Matter for Gut Microbiome Culture
Gut microbiome research has leaned heavily on sequencing, in part because many microorganisms are difficult to maintain in conventional culture. The authors note estimates suggesting that up to 71% of bacterial taxa capable of colonizing the human gut remain uncultured.
There are several reasons for that. Oxygen concentration, nutrients, pH, microbial neighbours, and local chemistry all change through the gastrointestinal tract. Physical conditions change too.
Bacteria living near the intestinal mucosal layer are not suspended in a mechanically neutral environment. They interact with a soft, hydrated material while also experiencing fluid movement and other mechanical cues. For difficult-to-culture gut bacteria, recreating at least some of those surroundings may affect what happens after the cells are taken out of the gut.
This is where hydrogel stiffness and bacterial growth become connected.
The researchers used four-arm polyethylene glycol maleimide, or PEG4MAL, as a synthetic hydrogel. Its formulation could be adjusted without changing the overall experimental platform, making it possible to create microbeads with different physical properties. For researchers working with similar materials, hydrogel mechanical testing becomes one way of putting numbers to an otherwise fairly subtle part of the bacterial microenvironment.
Can gut bacteria be encapsulated in hydrogel microbeads?
Instead of growing bacteria together in a conventional culture vessel, the team used microfluidics to compartmentalize them inside individual PEG4MAL beads.
The longer-term idea is that if individual bacteria can be physically separated while still having access to nutrients and waste diffusion, slower-growing or low-abundance species may have more opportunity to develop without being immediately overrun by fast-growing neighbours.
Microfluidic Encapsulation of Gut Bacteria in PEG4MAL
The encapsulation setup used two junctions rather than forming and crosslinking the beads in a single step. The first junction handled droplet formation, with the bacteria suspended in PEG4MAL and introduced alongside a mineral oil phase.
A PEG4MAL solution containing bacteria entered the first junction alongside a mineral-oil phase. Flow focusing generated individual droplets.
Crosslinking happened farther downstream. DTT was introduced at the second junction, where it could diffuse into the droplets and react with the PEG4MAL. By the time the droplets left the device, they had become crosslinked microbeads with bacteria trapped inside.
The researchers also had to make the setup work without oxygen. They moved the microfluidic system into an anaerobic chamber for experiments with Akkermansia muciniphila, which cannot tolerate the aerobic conditions used for the earlier E. coli work.
PEG4MAL concentration was another variable they changed during development of the system. They compared 2%, 5%, 10%, and 20% w/v formulations. As the polymer concentration increased, the fluid going through the chip became more viscous and its interfacial tension shifted as well.
After crosslinking, network density, swelling, and stiffness changed as well.
That distinction becomes important later. Hydrogel stiffness and bacterial growth were related in the experiment, but stiffness was not the only material variable changing.
Measuring Hydrogel Stiffness and Bacterial Growth With the MicroTester
To characterize the mechanics of individual microbeads, the researchers performed microbead mechanical testing using the MicroTester.
Washed PEG4MAL beads were resuspended in PBS and placed on the sample stage individually. A 2 mm by 2 mm compression plate was used to load the beads. The softer 2% and 5% PEG4MAL beads were tested with a 76.2 µm diameter microbeam, while a 203.2 µm microbeam was used for the stiffer 10% and 20% formulations.
This type of micro-mechanical testing is useful when the specimen itself is small enough that bulk hydrogel measurements may not describe the object being used in the biological experiment.
How is Young's modulus measured in small hydrogel beads?
The MicroTester recorded force and deformation as each microbead was compressed. Young’s modulus was calculated from the linear portion of the stress-strain curve up to 20% strain.
In practice, that gave the researchers a mechanical measurement for the same kinds of beads being used for bacterial encapsulation.
Young’s modulus increased from roughly 1.5 kPa at 2% PEG4MAL to about 12 kPa at 20% PEG4MAL.
The increase was substantial, but it came alongside other changes in the material. Higher PEG4MAL concentrations had a denser microstructure and lower swelling. Osmolality of the precursor solution also increased sharply with concentration.
This is one reason the connection between hydrogel stiffness and bacterial growth needs to be interpreted as part of a broader physical microenvironment rather than as a stiffness-only experiment.
For readers interested more generally in how modulus values depend on the way soft materials are tested, we recently looked at that issue in a separate study on hydrogel stiffness measurement.
The softer 2% and 5% PEG4MAL networks were also more porous. At 2%, E. coli appeared broadly distributed through the bead. At higher concentrations, the distribution changed, and the researchers observed more escape and preferential association near the surface of the 20% beads.
The 2% formulation was not automatically the best choice, though. Those very soft beads were difficult to recover during washing and filtration, with substantial bead loss during processing.
The researchers therefore settled on 5% PEG4MAL for the next stage as a compromise between handling and bacterial response.
That practical tradeoff is worth keeping in mind when thinking about hydrogel stiffness and bacterial growth. A formulation still has to survive the experiment.
What Happened After the Bacteria Were Encapsulated?
After getting the microfluidic encapsulation process working, the researchers turned to a more practical question: would the bacteria actually grow differently inside the PEG4MAL beads?
They started with E. coli, which gave them a relatively straightforward model for testing the system. The same approach was then moved into an anaerobic chamber and repeated with Akkermansia muciniphila, a strict anaerobe associated with the intestinal mucus layer.
Growing Akkermansia muciniphila Inside Hydrogel Microbeads
For an anaerobic gut bacteria culture system, simply keeping the cells alive after encapsulation is an important first check. After three days in 5% PEG4MAL microbeads, A. muciniphila remained at roughly 90% viability.
The researchers also looked at how the cells were distributed inside the beads. They did not see an obvious dead region developing in the centre of the bacterial aggregates, which suggests that transport of nutrients and waste through the hydrogel was not severely limiting growth over this three-day period.
The colony images are probably the more interesting part of the result. Both E. coli and A. muciniphila developed larger cell aggregates when encapsulated than when the same bacteria were grown in suspension. With A. muciniphila, that difference became especially easy to see by Day 3.
So while the study was looking at hydrogel stiffness and bacterial growth, the encapsulation step itself was already changing the way the bacteria occupied their local environment.
For gut microbiome culture, that is the more interesting result than viability alone. The beads were not simply keeping the cells alive. They provided an environment in which isolated bacteria could develop into higher-density colonies.
The study does not establish exactly which physical property caused that response. Still, the combination of hydrogel stiffness and bacterial growth, porosity, swelling, and confinement gave the researchers several variables that could now be adjusted deliberately.
Tuning Hydrogel Stiffness and Bacterial Growth With RGD
The next experiment tried to change the physical properties of the 5% PEG4MAL network without returning to the difficult-to-handle 2% formulation.
The researchers added 9 µM RGD peptide to the PEG4MAL precursor. In mammalian cell systems, RGD is normally discussed in the context of cell adhesion. Here, its intended role was different. The researchers expected it to interfere with crosslinking through steric effects and act more like a plasticizer.
It appears to have done that.
Adding RGD to 5% PEG4MAL reduced Young’s modulus from roughly the 4 to 5 kPa range to closer to 2 to 3 kPa. Swelling increased at the same time. Interfacial tension also decreased, while viscosity remained similar.
The biological response moved as well.
On Day 1, A. muciniphila colonies in the RGD-containing beads were significantly larger than colonies in unmodified 5% PEG4MAL. By Day 3, the RGD group still contained some very large colonies, but variability had widened enough that the difference between groups was no longer statistically significant.
That is why it would be too tidy to describe the study as proof that softer hydrogels make gut bacteria grow faster.
The better interpretation is that changing the encapsulating network changed several physical properties at once, and bacterial colony development changed with it. Hydrogel stiffness and bacterial growth were part of the same experimental picture, but not the only two variables in it.
How Does Hydrogel Stiffness Affect Bacterial Growth?
This study gives a useful answer, but not a single-number answer.
Lower PEG4MAL concentrations produced softer and more porous beads. Those conditions generally corresponded with bacterial behaviour that the researchers considered more favourable for encapsulation. Modifying the 5% network with RGD also lowered its modulus and increased early A. muciniphila colony size.
At the same time, PEG4MAL concentration changed osmolality, swelling, network density, and probably transport through the material.
So, can hydrogel mechanical properties influence bacterial growth? It appears that they can be part of the influence. This experiment cannot isolate stiffness from everything around it.
That makes hydrogel stiffness and bacterial growth a useful starting point rather than an endpoint.
It also fits with the broader idea of mechanomicrobiology, where bacteria are considered not only in terms of nutrients and biochemical signalling, but also in terms of the forces and physical environments they encounter.
For gut bacteria, that could eventually mean looking more closely at the stiffness of intestinal mucus, confinement, osmotic pressure, fluid movement, and forces associated with peristalsis.
The authors point in that direction themselves. Their current experiments focused on isolated bacteria and short culture periods, while community-level encapsulation, co-culture, and longer experiments remain open questions.
Small Hydrogel Beads Are Becoming Their Own Mechanical Testing Problem
One thing that stands out in this study is how much information came from testing the microbeads themselves.
That has parallels with other microscale biomaterial systems. In previous research highlights, we have looked at pullulan microbeads used in intervertebral disc research and hydrogel microspheres carrying mesenchymal stromal cells for diabetic wound healing.
The biology in those studies is completely different, but there is a recurring experimental issue: once a material is reduced to a bead, microsphere, aggregate, or other small construct, bulk mechanical measurements may no longer describe the object the cells or microorganisms actually experience.
Here, that meant using compression testing directly on individual PEG4MAL microbeads.
Using the MicroTester for Hydrogel Microbead Compression Testing
The CellScale MicroTester is used for mechanical measurements on small and low-force specimens such as microtissues, hydrogels, spheroids, and individual biomaterial structures.
In this study, the researchers used it to isolate individual PEG4MAL microbeads, compress them with a small platen, record force and deformation, and calculate Young’s modulus for each material formulation.
That measurement became useful because hydrogel stiffness and bacterial growth were being investigated at the same physical scale. Rather than characterizing a large bulk sample and assuming the microbeads behaved similarly, the researchers measured the actual crosslinked structures used for encapsulation.
The study also points toward a less familiar use of microscale mechanical testing. Bacteria are usually discussed in terms of culture media, metabolism, genetics, or community composition. Here, the mechanical properties of the surrounding material became another experimental variable.
There is still quite a bit to untangle. A softer bead is also a differently crosslinked bead, and that can affect porosity, swelling, diffusion, and osmotic conditions. But that is part of what makes hydrogel stiffness and bacterial growth interesting in this context. The microenvironment is physical as well as chemical, and researchers are beginning to measure more of it.
Citation
Wheatley, S.K., Dupeyroux, L., Rodger, M. et al. Microfluidic encapsulation of the human gut microbiota—a tool for research and beyond. Microsyst Nanoeng 12, 245 (2026). https://doi.org/10.1038/s41378-026-01264-7
