Biomedical Engineering Reference
In-Depth Information
Fig. 7.19. An example of real-time measurements of DNA-protein interactions. These images were
obtained with high-speed IC-AFM, and the numbers in each frame indicate the time after imaging
began. The images show a complex of EcoP15l, a DNA restriction enzyme, with DNA. The images
from 1 to 10 seconds show the DNA loop passing through the enzyme. Reproduced with permission
from [2], copyright (2007) National Academy of Sciences, USA.
7.3.2 Bacterial cell measurements
AFM is a highly suitable tool to examine bacteria, and has been widely applied to their
study. Bacteria are commonly studied by optical microscopy, which can give an overall
idea about gross cell morphology (via a two-dimensional projection), and is also useful for
cell-counting studies. In comparison, AFM is slower, and thus is less useful for quantita-
tive cell-counting, but allows measurement of a variety of other cellular properties,
particularly by nanoindentation and force spectroscopy experiments [611]. In addition,
the greatly increased resolution of AFM allows for the imaging of finer details of
cell morphology and sub-cellular features such as pili and fimriae [612]. The three-
dimensional information from AFM can also be useful in differentiating morphologies
which would look the same in optical microscopy [6]. Various other micro-organisms
have been studied by AFM such as spores [178, 613-615], fungi [616, 617], including
yeasts [171, 618], viruses [287, 619], and others [620] but here we concentrate on bacteria
for the sake of brevity.
Some species of bacteria that have been well-studied include
E. coli
[169, 621] and
various species of
Staphylococcus
[169, 317, 622],
Bacillus
[178, 615],
Streptococcus
[623, 624] and
Salmonella
[625, 626], see Figure 7.20. Bacteria generally need to be
immobilized on a surface for imaging, and a number of different procedures have been
used. For studies in air, drying onto a surface, or even flaming can work well, although one
