University of Warwick scientists has found that firmer, lower water content hydrogels limit bacterial growth, with implications for designing antibacterial coatings, infection models, and advanced medical materials.
Hydrogels are soft, jelly-like materials that can absorb large amounts of water. They are widely used in medical technologies such as contact lenses and wound dressings, and are also a staple of laboratory research, where they are used to grow bacteria. But scientists have long struggled to explain why some hydrogels readily support bacterial growth while others appear to suppress it.
New research from the University of Warwick, published in Communications Materials, shows that the answer lies not only in the physical properties of the material, particularly its stiffness and hydration, but also on bacterial cell surface characteristics.
The researchers tested four common bacterial species, including Gram-negative (Escherichia coli), Pseudomonas fluorescens (P.fluorescens) and Gram-positive (Staphylococcus aureus (S. aureus), Bacillus subtilis (B. subtilis)) using hydrogels with different stiffness and water contents, covering 120 conditions. To better mimic real-world conditions, the gel surfaces were gently pricked with bacteria, allowing them to enter the material through small defects, as they might in damaged medical devices or dressings.
Across all species tested, bacteria grew more rapidly in softer, more hydrated gels. In contrast, firmer, lower water content materials consistently slowed bacterial expansion, both on the surface and within the gel.
"Due to their enhanced hydration and elasticity, softer, wetter gels give bacteria room to expand and make it easier for nutrients to move through the material," says Andrea Dsouza, Research Associate, Warwick Medical School. "Stiffer gels with lower water content create more physical resistance and a less favourable environment for growth."
The team also found that the nutrient solutions used to feed the bacteria affected growth mainly by changing the gel's physical properties - such as stiffness and water retention - rather than by acting as better or worse food sources. This insight may help explain why previous studies of bacterial growth in gels have often produced conflicting results.
The researchers also identified a more selective mechanism at work. In some cases, negatively charged gels repelled bacteria harbouring negatively charged groups on the cell surface, particularly at higher gel concentrations, further limiting growth through electrostatic interactions.
"By studying stiffness, hydration, nutrient conditions and surface charge together, we were able to build a much clearer picture of how hydrogels regulate bacterial growth," says senior author Jérôme Charmet. "It's the combination of these factors that matters."
The findings have implications in both directions. For microbiologists, they offer guidance on how to design gels that better support bacterial cultures in the lab. For materials scientists and biomedical engineers, they point to physical design strategies - rather than chemical additives or antibiotics - that could help prevent harmful bacteria from gaining a foothold.
Andrea adds: "Moisture helps wounds heal, but too much softness can also help bacteria. The challenge is designing dressings that stay wet enough for tissue repair while remaining mechanically hostile to microbes."
As antibiotic resistance continues to rise, designing materials that are physically hostile to bacteria may provide a valuable new line of defence against infection.
