Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Bacterial ultrastructure

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BBacterial surface layersACTERIAL SURFACE LAYERS

Bacterial surface layers are regularly arranged arrays, often

comprised of the same component molecule, which are

located on the surface of bacteria. The prototype surface layer

is referred to as a S layer.

S layers are found on many bacteria that are recovered

their natural environment, as well as on most of the known

archaebacteria. Examples of bacteria that possess S layers

include Aeromonas salmonicida, Caulobacter crescentus,

Deinococcus radiodurans, Halobacterium volcanii, and

Sulfolobus acidocaldarius. In many bacteria, the production of

the surface layer proteins and assembly of the surface array

ceases once the bacteria are cultured in the artificial and nutri-

ent-rich conditions of most laboratory media.

The S layer of a particular bacterium is composed

entirely of one type of protein, which self-assembles into the

two-dimensional array following the extrusion of the proteins

to the surface of the bacterium. The array visually resembles

the strings of a tennis racket, except that the spaces between

adjacent proteins are very small. In some Gram-positive bac-

teria the surface layer proteins are also associated with the

rigid peptidoglycanlayer than lies just underneath. The com-

bination of the two layers confers a great deal of strength and

support to the bacterium.

Bacterial surface layers are the outermost surface com-

ponent of bacteria. As such, they modulate the interaction of

the bacterium with its external environment, and are the first

line of defense against antibacterial compounds. S layers, for

example, act as sieves, by virtue of the size of the holes in

between adjacent protein molecules. The layer can physically

restrict the passage of molecules, such as destructive enzymes,

that are larger than the pores. The S layer around the bacterium

Bdellovibrio bacteriovoranseven precludes attack from pred-

ators of the bacterium.

Some disease-causing bacteria possess S layers. These

bacteria include Corynebacterium diphtheriaeand Bacillus

anthracis. Microscopic examination of bacteria found in the

mouth has also revealed S layers. Possession of surface layers

by these bacteria aids the bacteria in avoiding the process of

phagocytosis. This is thought to be because the protein sur-

face layer makes the bacteria more hydrophobic(“water hat-

ing”) than bacteria of the same species that does not have the

surface layer. The increasingly hydrophobic cells are not read-

ily phagocytosed.

BBacterial ultrastructureACTERIAL ULTRASTRUCTURE

Bacterial ultrastructure is concerned with the cellular and

molecular construction of bacteria. The bulk of research in

bacterial ultrastructure investigates the ultrastructure of the

cell wall that surrounds bacteria.

The study of bacterial ultrastructure began with the

development of the staining regimen by Danish pathologist

Christian Gram(1853–1938) that classifies the majority of

bacteria as either Gram-negative or Gram-positive. The latter

bacteria retain the crystal violet stain, while Gram-negative

bacteria do not retain this stain and are stained by the second

stain that is applied, safranin. While the basis for this differ-

ence was not known at first, scientists suspected that the struc-

ture of the wall surrounding the contents of the bacteria might

be involved.

Subsequent to the time of Gram, scientists have discov-

ered that the cell wall plays only a secondary role in the Gram

stain reactions. However, the cell wall of Gram-positive bac-

teria is indeed much different than that of Gram-negative bac-

teria. The study of bacterial ultrastructure relates these

constituent differences to the intact cell wall. In other words,

ultrastructure explores the structure of each constituent and

the chemical and other associations that exist between these

constituents.

The exploration of bacterial ultrastructure requires sam-

ples that are as undisturbed as possible from their natural, or

so-called native, state. This has been challenging, since much

of the information that has been obtained has come from the

use of electron microscopy. The techniques of conventional

transmission electron microscopy and scanning electron

microscopy require the removal of water from the sample.

Because the bulk of living things, including bacteria, are com-

prised of water, the removal of this fluid can have drastic con-

sequences on the structure of the bacteria. Much effort has

gone into the development of regimens that chemically “fix”

bacteria, so that structure is maintained following the subse-

quent removal of water.

Techniques have also been developed that prepare bac-

teria for transmission electron microscopy without the neces-

sity of removing water from the specimen. One technique uses

an embedding resin (a substance in which the bacteria are

immersed and, when the resin is hardened, allows thin slices

of the bacteria to be cut) that mixes with water. This resin is

harder to work with than the conventional resins that are not

water-soluble. Thus, while valuable information can be

obtained using water-soluble resins, a great deal of experience

is necessary to produce high quality results.

Light micrograph of Klebsiella bacteria showing “halo” created by the

capsule.

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