Shape Shifting

Our Project

Background

Most of the bacteria has evolved to have a cell wall - a rigid structure which protects the bacteria from the variety of enviromental hazards such as mechanical stress, osmotic rupture and lysis. The cell wall often serves as a docking point to many proteins including various receptors and adherence sites. Along with these properties cell wall provides the cell with a rigid boundary and helps bacteria to acquire and preserve their shape. In Bacillus subtilis along with other proteins, a group of proteins termed Penicillin Binding Proteins (pbp), usually anchored in the cell wall, is involved in the formation of the rod shape. When the cells lose their cell due to the actwall they automatically lose these proteins to the environment as they are being made. The cells lose the support and turn into a sphere as it is the most energetically favourable state (ratio of surface area to volume is minimal, and membrane curvature is more-or-less constant).

It has been previously observed that these cells can become elongated and 'squeeze' into the spaces with a smaller diameter than theirs.

This fact has sparked our interest, because the l-form B.subtilis cells can sometimes grow to fairly large sizes before they divide, and we thought it may be possible to fill spaces of various shapes and sizes with them. This ability may be useful to our fellow scientists in many ways such as the following:

L-forms can also be a decent secretory machine, as recombinant proteins which are targeted to the cell will be secreted into the environment. Their flexibility will allow very efficient delivery of said proteins to the otherwise hard to reach places ranging from intercellular space to the micro-cracks (smaller than 1 μm) in solid material.

Modelling

To show the flexibility of the L-forms we have planned to trap them in a microfluidics chamber the shape of which is different from that of a normal l-form cell e.g. star, square, triangle.

Before we even had begun to design the experiments, to illustrate the process which we predict to occur inside of the terminal chamber as the cell grows we have constructed a predicted model of the cell behaviour as it grows inside a square, based on the knowledge that we have about the processes inside the cell which are involved in membrane synthesis and growth. We would like to thank Dr. David Swailes from the School of Mechanical Engineering at Newcastle University for his massive help with the mathematical side of the modelling. We couldn't have done what we have without his help.

For the purposes of the study the complex model of the growing cell inside of the confined space can be broken down to simpler models of the system at two phases. The first phase would be a constantly growing cell, followed by a model of the cell, gradually adopting the shape of the boundaries.

The full description of the model can be found on this page.

This model can be improved by conducting experiments which would allow us to find parameters such as the rate of membrane synthesis and maximum membrane torsion, or to make the model free of a few assumptions i.e. find out the maximum size the cell could grow to before dividing, measure the effects of nutrient depletion, and develop a protocol which would allow us to test our hypothesis on the cells which are in the growth phase of their cell cycle. It can also be expanded in terms of factoring in the bending energy of the membrane in a particular shape and then comparing the models of different shapes and evaluating which would me more readily assumed.

It would also be interesting to see whether the state of the cell (i.e. l-form or rod) before it enters the chamber would have any difference on the way it fills the space.

Plans and Outlook To the Future