Investigating the Effect of Hydrogel Thickness on Bacterial Adhesion

The rise of antibiotic resistant bacteria is a growing epidemic that necessitates the use of alternative antibiotic-free approaches to delay the onset of bacterial adhesion and subsequent infection. This is especially true for hospital settings and the healthcare industry. Healthcare-associated infections (HAIs) stemming from medical devices, like catheters, are preventable financial and physical burdens that too often lead to morbidity. In practice, the surface of these catheters are coated with thin, lubricious polymer hydrogels to passively prevent microbial adhesion. However, the prevention of bacterial adhesion using the poly(ethylene glycol) (PEG) hydrogels erodes over time, leading to significantly increased infection rates with prolonged catheterization. While many chemical alterations on this coating have been tested, their basic geometry and material properties have remained constant, namely their thickness and stiffness. It has previously been determined that key cellular processes, including adhesion and proliferation, are mechanically responsive for both mammalian and microbial species. However, the depth of this mechanical sensitivity has not been investigated. Therefore, the aim of this current study is to elucidate the effect hydrogel thickness has on bacterial attachment and subsequent growth. This was accomplished through a series of bacterial attachment and mechanical characterization experiments of hydrogels with stiffness of 40 â?? 6500 kPa and thicknesses of 10-150 µm. For the bacterial attachment experiments, gram-negative Escherichia coli and gram-positive Staphylococcus aureus bacteria were utilized due to the highly different mechanical sensitivity mechanisms the species exhibit. By also accomplishing in situ atomic force microscopy (AFM) characterization of hydrogel surface features for each relevant hydrogel, we were able to decouple the effect hydrogel thickness and hydrogel stiffness has on bacterial adhesion. Our findings indicate that bacteria â??feelâ? through a hydrogel to an underlying support layer unless a sufficiently thick hydrogel is utilized. Determining bacteriaâ??s depth-dependent mechanical sensitivity mechanism holds potential to improve the design of next generation medical device coatings that can limit bacterial adhesion without the use of antibiotics.