Biomedical Engineering Reference
In-Depth Information
It's also useful to make measurements of bacteria by non-imaging modes of AFM,
because the high positioning resolution of AFM allows such measurements to directly
address individual bacterial cells, which is difficult by other techniques [611]. For
example, nanomechanical measurements (e.g. nanoindentation ) of bacteria have been
shown to be sensitive to treatment with antimicrobial agents [169, 629, 632], bacterial
species and strain [155, 635, 636], physiological state of the organisms and the environ-
ment in which the measurements are made [155]. With the AFM it's relatively simple to
perform nanoindentation experiments on individual micro-organisms, and even to differ-
entiate one part of a cell from another by stiffness measurements [171]. For this sort of
experiment, it's important to remember that the response of the probe will be different
when the cell surface is perpendicular to the probe motion, than when it's at an angle,
however [637]. Thus, all measurements should normally be carried out only on the upper
portion of the cell which is relatively flat [382].
Other non-imaging experiments which may be carried out on bacteria using AFM
include force spectroscopy in order to measure the distribution of specific adhesion factors
on cell surfaces [156], cell hydrophobicity/hydrophilicity [475, 638], or the distribution of
other molecules across the cell surface [611, 637, 639].
Bacterial colonization of surfaces is an important process, and reducing the process
requires knowledge of individual bacteria-surface interactions. Bacteria-surface adhesion
studies can be carried out using a number of experimental methodologies, the most
commonly applied ones being direct force spectroscopy with bacteria immobilized on
the AFM probe and lateral force microscopy measurements of the force required for
removal of cells [637, 640-644]. AFM allows the combination of studies of cell-surface
adhesion, with measurements of the surface itself, which can help to understand how
factors such as roughness, hydrophobicity, etc. can affect colonization by bacteria [645].
7.3.3 Lipid membrane imaging
Plasma membranes are ubiquitous in animal cells, forming a barrier between the intracel-
lular components and the extracellular environment. The membrane's purpose is to
selectively allow molecules in and out of the cell, while blocking unwanted material. In
addition the membranes form a scaffold for a large number of cell surface molecules,
mainly proteins, which regulate activities such as cell adhesion, recognition, signalling,
etc. By AFM it is possible to study the cell membrane in its native environment, i.e. as part
of a cell (see next section), but for increased stability and higher resolution, it's useful to
use a model system. The major component of the plasma membrane is a bilayer of
phospholipids, with their hydrophilic heads pointing out into solution, and the hydropho-
bic tails on the inside of the bilayer, so that they are shielded from the aqueous environ-
ment. Due to their importance in biology such lipid bilayers have been widely studied by a
number of techniques. While they are simple to study by other techniques in solution (they
form spherical vesicles), for AFM they can be deposited easily on a flat surface. This
creates a flat, stable model for the plasma membrane, which is an ideal sample for AFM
studies.
Formation of lipid bilayers for AFM studies has been carried out using a number of
different methodologies. The two most common of these, however, are Langmuir-Blodgett
film deposition, and fusion of vesicles from solution directly onto the substrate surface.
