Biology Reference
In-Depth Information
containing membrane proteins (e.g., porins). Among Gram-positive bacteria,
mycobacteria contain an unusual lipid monolayer (mycolic acids, glycolipids,
complex lipids) with inserted porins which mimics the inner lealet of the
outer membrane of Gram-negative bacteria. The mycolic acids are bound to
an underlying arabinogalactan polysaccharide layer that is, in turn, linked to
peptidoglycan. For many bacterial strains, cell wall constituents are covered
by additional surface layers in the form of polysaccharide capsules, surface
appendages (imbriae, pili, ibrils, lagella) or crystalline S-layers. Strong
cell walls are formed in yeasts and ilamentous fungi by the aggregation of
polysaccharide polymers. In yeasts, these are made of a microibrillar array of
B
B
1-6 glucan and mannoproteins. The walls of fungal
hyphae consist of microibrillar polysaccharides, chitin or cellulose, covered
by layers of proteins and glucans. Fungal spores are often covered by an outer
layer of regularly arranged proteins, referred to as rodlets. Although much
progress has been made in elucidating the structure and biosynthesis of cell
envelope constituents, their three-dimensional organization, assembly, and
interactions remain poorly understood at the molecular level.
Since van Leeuwenhoek, microscopy and microbiology have been inti-
mately connected. Light microscopy is a fundamental tool of microbiologists,
enabling counting and identiication of the cells as well as determination of
their general morphological details. Valuable information on the cell wall
organization, assembly and dynamics can be obtained using luorescence
microscopy,
3
but the resolution is generally limited to the wavelength of the
light source. Our current view of the cell wall ultrastructure essentially relies
on the tremendous development of electron microscopy techniques. Elegant
techniques have been developed for transmission electron microscopy
(TEM) such as the use of freeze-fracture and surface replica to visualize
for example cell surface layers, and negative staining for studying puriied
structures such as lagella and imbriae.
4,5
These approaches, however, are
limited by the requirement of vacuum conditions during the analysis, i.e.,
native hydrated samples cannot be directly investigated unless sophisticated
cryoTEM methods are employed.
Besides microscopy techniques, molecular biology and proteomic app-
roaches have allowed to identify the main components of the cell envelope.
6
Here, dificulties often arise in solubilizing and separating the different
constituents. Also, large ensembles of molecules and cells are probe, meaning
information at the single-molecule level is not available. Hence, there is a clear
need to complement traditional ensemble measurements with non-invasive
single-cell and single-molecule techniques.
7,8
Various force-measuring techniques are available to probe single
molecules,
9-12
including low chamber experiments, microneedles, the
1-3 glucan, overlaid by
Search WWH ::
Custom Search