Experimental and theoretical investigation of mechanical responses of bacteria under hypoosmotic pressure
Creators: Darabi, Marjan
Publisher: name: University of Alberta Library
Year: 2024

<p>Bacteria represent a type of ubiquitous pathogen that respond to environmental changes such<br>
as osmotic pressure due to their cellular structure. Both Gram-negative and Gram-positive<br>
bacteria have distinct structure, including a plasma membrane and cell wall. The primary<br>
difference between them can be characterized by the absence of an outer membrane and the<br>
presence of thicker peptidoglycan in Gram-positive bacteria. While both bacterial types may<br>
exhibit different osmotic responses, the osmotic pressure-dependent mechanical responses<br>
of Gram-positive bacteria have not been thoroughly investigated, compared with those of<br>
Gram-negative bacteria. Studying the osmotic pressure-dependent morphological change<br>
and its mathematical model exert some promising utilities in biomedical applications such<br>
as designing and optimizing therapeutic strategies for drug delivery. For example, changes in<br>
cell morphology due to osmotic pressure can affect the uptake and release of drugs. Besides,<br>
mathematical models can predict how different osmotic conditions influence drug transport<br>
across cell membranes. As most of the antibiotics avoid peptidoglycan synthesis in bacteria<br>
which leads to osmotic lysis, studying structural change of bacteria under osmotic condition<br>
assist antibiotics development.<br>
In this work, we employed a combination of experimental and theoretical approaches to<br>
study how Peptidoglycan and inner membrane deform under hypoosmotic pressure. Dynamic<br>
light scattering (DLS) analysis was used to monitor time-dependent changes in the size of the bacteria, Escherichia coli (E. coli, Gram-negative) and Lactobacillus acidophilus<br>
(L. acidophilus, Gram-positive), over the course of incubation at different osmotic pressure<br>
conditions. Upon exposure to 300 mOsm of hypoosmotic gradient, the hydrodynamic radius<br>
of L. acidophilus cells was observed to increase from 0.81±0.05 μm to 1.79±0.06 μm.<br>
On the other hand, the radius of E. coli was found to increase from 0.45±0.008 μm to<br>
0.67±0.01 μm at 0 mOsm. Besides, Transmission Electron Microscopy (TEM) is employed<br>
to capture morphological changes of bacteria through applying hypoosmotic pressure. The<br>
experimental results were used to develop a mathematical model. Through this model, the<br>
mechanical behavior of bacteria cell envelope is predicted by formulating equilibrium equations<br>
which describe deformations of the membrane attached to the meshed structure of<br>
peptidoglycan. This results in nonlinear partial differential equations which are solved using<br>
the custom-built Finite Element (FE) scheme. Finite Element Method, a prominent continuum<br>
approach, intricately dissects complex problems into a finite set of interconnected<br>
elements. These elements are considered continuous and deformable entities, facilitating a<br>
comprehensive analysis of the material’s behavior through the examination of these discrete<br>
components. We used Finite Element Method to elucidate the correlation between applied pressure<br>
and bacteria deformation. The numerical solver used for solving the governing equations was<br>
the Newton-Raphson method, seamlessly integrated into the FENICS platform—a Pythonbased<br>
open-source software dedicated to solving partial differential equations. Based on our<br>
model, it is predicted that Gram-positive bacteria experience significant out of plane deformation<br>
on z direction by increasing lateral pressure from 5 MPa to 125.30 MPa. This<br>
deformation is depicted by the red regions on the deformation contours of the model, and<br>
vividly shows that the bacteria is stretched under various pressure conditions. Furthermore,<br>
the heightened tension experienced by the bacteria in different pressure conditions is presented<br>
on Material displacement contour. It provides valuable insights into the susceptibility of specific regions within Gram-positive bacteria to rupture under increased pressure conditions.<br>
Particularly, areas with concentrated stress and heightened strain exhibit vulnerability<br>
to rupture when exposed to elevated pressures. Notably, the maximum displacement is observed<br>
in regions of the cell where mobility along the vertical z-axis is prominent. Despite<br>
the need for further research, it was shown that theoretical predictions were well aligned<br>
with experimental findings, emphasizing the observed shape change of bacteria matching<br>
the predicted deformation.</p>