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Bacteria are microorganisms that have evolved over 3.5 billion years
and are responsible for a wide range of phenomena in the world around us, ranging from causing
diseases to helping to digest food to shaping the surface and sub-surface of the Earth. To
survive in a broad range of environmental conditions and changes to these conditions, they have
developed an amazing range of fascinating and unique biomaterials and mechanisms.
We have developed new techniques and refined existing techniques for the study of different
important properties of bacterial cells. Current PSI projects on the physics of bacteria are:
Viscoelastic properties of bacterial cells & biofilms
We have developed a simple, AFM-based experiment to measure the time dependent mechanical response
of individual bacterial cells to a constant force applied by an AFM tip. Measuring the time-dependent
deformation of a material in response to a constant force is referred to as creep deformation. Our
experiment on bacterial cells is the nanoscale equivalent of standard engineering measurements on
bulk materials to evaluate the stability of the materials under load. We can interpret the results
of the nanocreep experiment in terms of simple viscoelastic models. In addition to measuring the
mechanical properties of different kinds of bacterial cells, we have also used this experiment
to study the mechanical properties and cohesiveness of bacterial biofilms. We are now using
this experiment to evaluate the action of different antimicrobial compounds on the mechanical integrity
of the bacterial cells.
Fluorescence measurements of protein oscillations & patterns in bacterial cells
Self-assembly of proteins within bacterial cells creates many different structural elements for the cell
that perform key functions such as locomotion (flagella and pili), reinforcing the mechanical integrity of the
cells (cytoskeleton), and ensuring the proper occlusion of nucleoids and formation of the division
septum during cell division. In the rod-shaped bacterium Escherichia coli, an important part of the
cell division process is the oscillation of Min proteins along the major axis of the cell. This system
has been the focus of much experimental and theoretical work during the past decade, since it can be
studied using fluorescence microscopy by tagging the Min system with fluorescent proteins, and using
coupled reaction-diffusion models that can reproduce many aspects of the system dynamics. By oscillating
from pole to pole of the bacterial cell, a family of Min proteins (MinC, MinD and MinE) prevent the
FtsZ protein division septum from forming anywhere except for the midpoint of the cell. This ensures
that an equal amount of genetic material is transferred to the two daughter cells. We use a strain of
E. coli in which the MinD proteins are tagged with green fluorescent protein (GFP), allowing
fluorescence visualization of the MinD oscillation. We use high-resolution total internal reflection
fluorescence (TIRF) microscopy and a custom, temperature controlled flow cell to observe the effect of
exposure to antimicrobial agents on the MinD oscillation period and, more generally, to analyze spatio-temporal
patterns of the MinD proteins within the cells. These measurements provide insight into the mechanisms
of antimicrobial action.
Bacterial biomaterials
There are a large number of unique biomaterials in bacterial cells that self-assemble to perform key
functions for the cell. One particularly striking example is the outer bag of the cell, which is called
the peptidoglycan sacculus. This is one of Nature's largest and strongest biopolymers with fascinating
structure and properties. It is needed to withstand the internal cytoplasmic turgor (osmotic) pressure
while allowing the bacteria to grow and expand. The intricate nature of its structure and biosynthesis
pathway make peptidoglycan a marvel of nanotechnology but also allow it to be the principal target of
many antibiotics1 and one of the main microbial products recognized by the immune system. It is a
covalent macromolecular structure of stiff glycan chains that are crosslinked by flexible peptide
bridges. Although the peptidoglycan has been the subject of decades of investigation, its construction
and higher-order structure are still debated. Two models have been proposed for the architecture of
the peptidoglycan network (orientation of the glycan strands parallel or perpendicular to the
cytoplasmic membrane), but it has not been possible to date to determine unambiguously which model
is correct. We use atomic force microscopy (AFM) imaging and the collection of force-distance curves
on purified intact Escherichia coli K12 sacculi, as well as computer modeling of the force-distance
curves in terms of the polymer network, to learn about the architecture of the peptidoglycan network
and the effect of amidases on the network.
Surface motility of bacteria
The most commonly known form of bacterial motion or motility is swimming in liquid, which is facilitated by the
flagellum, a self-assembled protein filament that is anchored in the bacterial cell envelope. Bacteria
have a strong tendency to adsorb to almost any surface, ultimately leading to the colonization of the
surface as bacterial biofilms. Some bacteria have evolved mechanisms to move on surfaces, such as Type
IV pili which leads to twitching motility. We are quantifying the twitching motility of Pseudomonas aeruginosa
bacteria on agar films under different enviromental conditions.
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