Deciphering the Adaption of Bacterial Cell Wall Mechanical Integrity and Turgor to Different Environments: Modelling and Experimentation

Abstract

Bacteria have to adapt the mechanical properties of their cell envelope, including cell wall stiffness, turgor and cell wall tensions and deformation, to be able to grow and survive in harsh environments. However, it is technically challenging to simultaneously determine these mechanical properties on a single cell level. Here, we combined theoretical modelling and finite element modelling with an experimental approach using atomic force microscopy (AFM) to quantify the mechanical properties and turgor of Staphylococcus epidermidis and Staphylococcus aureus (SH1000 and ∆pbp4). To determine the elastic modulus (apparent cells modulus) of spherical bacteria using AFM fitted with a pyramid probe, we developed a modified Sneddon model to overcome the limitations of the Hertz and Sneddon models. Experimental validation was performed on engineering materials. When adopting this model for bacteria, we have revealed that the apparent cell modulus of Staphylococcus epidermidis and Staphylococcus aureus (SH1000 and ∆pbp4) decreases with an increase in osmolarity. Following that, we adopted a combined finite element modelling and theoretical modelling approach to simultaneously determine bacterial cell wall stiffness and turgor of an individual bacterial cell using a pyramid probe. The cell wall stiffness and turgor of Staphylococcus epidermidis and Staphylococcus aureus (SH1000 and ∆pbp4), determined by our method are consistent with other independent studies. We have revealed that bacterial cell wall stiffness increased linearly with an increase in turgor, and higher osmolarity leads to a decrease in both cell wall stiffness and turgor. We also demonstrated that the change of turgor is associated with a change in viscosity of the bacterial cell. All these are possibly related to ion interactions with bacterial cell wall. For AFM indentation using a spherical probe, we have presented a mathematical model which enables the determination of cell wall tensions and deformation on top of cell wall stiffness and turgor. The new findings are that cell wall tensions decrease with increasing osmolality. We have also revealed that external forces increase cell wall deformation, strengthening the bacterial adherence to surfaces, with a greater impact in low osmolarity environments. The cell wall stiffness and turgor determined using this model were consistent with what was found when using a pyramid probe. The key findings about how these key mechanical properties change with osmolarity conditions were also consistent with what was demonstrated using a pyramid probe. Our work highlights can be adapted to other bacteria with different shapes by adjusting cell shape parameters in the analytical model. Our findings have important implications for understanding how bacterial mechanics contribute to survival in harsh environments such as growth in the presence of antibiotics or within a host or under external force.