Mechanical interactions between bacterial cells and hydrogels : an experimental and modelling approach

Abstract

There is an increasing demand for medical implants and prosthetic devices due to increased life expectancy. It is important to understand how microorganisms affect the mechanical properties of hydrogels since they can get severely affected by bacteria associated infections which may lead to complete implant removal. Bacterial adhesion on surfaces has been a widely investigated subject in many areas including biomaterial associated infections since it is considered the starting point for colonisation and biofilm formation. It has been shown that mammalian cell structure and function can be adjusted by altering the mechanical properties, such as stiffness, of their 3D microenvironment. The effect of stiffness on cell adhesion and cell function for eukaryotic cells is investigated in many studies however little work is done for bacteria for which similar interaction may happen. This study focused on demonstrating how the encapsulated bacterial cells (Escherichia coli and Staphylococcus epidermidis) and the nutrients supplied to the hydrogels from the culture media alter their overall mechanical properties, using various characterisation techniques. In the first part of this work, stress relaxation tests were used to reveal the mechanical properties of hydrogel-bacterial cell constructs. It was shown that the type of nutrients has an effect on the mechanical properties of the hydrogels. Similar test conditions were modelled for inert particles using finite element analysis (FEA) to show the effect of encapsulated bacterial cells. Rheological tests, which represented a different type of loading, were also adopted for determination of mechanical properties of hydrogel-bacterial cell constructs where the results exhibited a different behaviour. This research overall has revealed a complicated interaction mechanism between hydrogels, nutrients supplied and bacterial cells, depending on the loading type. Therefore, it is important to take into consideration the effects of media and the characterisation technique employed for the mechanical ii characterisation of hydrogels and their interactions with the embedded bacterial cells.