Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Lipopolysaccharide and its constituents

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pounds (e.g., amino acids, nucleic acids and sugars). The pos-

sibility of such a process was demonstrated in 1953 by Stanley

Miller and Harold Urey, who simulated the effects of lightning

storms in a primordial atmosphere by subjecting a refluxing

mixture of water, methane, ammonia and hydrogen to an elec-

tric discharge for about a week. The resulting solution con-

tained significant amounts of water-soluble organic

compounds including amino acids.

The American scientist, Norman H. Horowitz proposed

several criteria for living systems, saying that they all must

exhibit replication, catalysis and mutability. One of the chief

features of living organisms is their ability to replicate. The

primordial self-replicating systems are widely believed to

have been nucleic acids, like DNA and RNA, because they

could direct the synthesis of molecules complementary to

themselves. One hypothesis for the evolution of self-replicat-

ing systems is that they initially consisted entirely of RNA.

This idea is based on the observation that certain species of

ribosomal RNA exhibit enzyme-like catalytic properties, and

that all nucleic acids are prone to mutation. Thus, RNA can

demonstrate the three Horowitz criteria and the primordial

world may well have been an “RNA world”. A cooperative

relationship between RNA and protein could have arisen when

these self-replicating protoribosomes evolved the ability to

influence the synthesis of proteins that increased the efficiency

and accuracy of RNA synthesis. All these ideas suggest that

RNA was the primary substance of life and the later participa-

tion of DNA and proteins were later refinements that increased

the survival potential of an already self-replicating living sys-

tem. Such a primordial pond where all these reactions were

evolving eventually generated compartmentalization amongst

its components. How such cell boundaries formed is not

known, though one plausible theory holds that membranes

first arose as empty vesicles whose exteriors served as attach-

ment sites for entities such as enzymes and chromosomesin

ways that facilitated their function.

See alsoDNA (Deoxyribonucleic acid); Evolution and evolu-

tionary mechanisms; Evolutionary origin of bacteria and

viruses; Miller-Urey experiment; Ribonucleic acid (RNA)

LIGHT MICROSCOPY•seeMICROSCOPE AND

MICROSCOPY

LIPOPOLYSACCHARIDE AND ITS

CONSTITUENTSLipopolysaccharide and its constituents

Lipopolysaccharide (LPS) is a molecule that is a constituent of

the outer membrane of Gram-negative bacteria. The molecule

can also be referred to as endotoxin. LPS can help protect the

bacterium from host defenses and can contribute to illness in

the host.

The LPS comprises much of the portion of the outer

membrane that is oriented towards the outside of the bac-

terium. There are fewer phospholipid molecules in this outer

“leaflet” of the membrane than there are on the inner side of

the membrane. Thus, because of the presence of lipopolysacc-

ahride, the construction of the outer membrane is asymmetric.

In contrast, the inner membrane of Gram-negative bacteria

and the single membrane of Gram-positive bacteria are more

symmetric, with both leaflets of the membrane comprised of

much the same molecules.

A complete LPS consists of a lipid portion and a chain

of sugar. The lipid region is anchored into the inner portion of

the membrane by a molecule called lipid A. A core polysac-

charide is also considered part of the lipid region. This core

contains a compound known as 2-keto-3-deoxyoctonic acid,

or KDO. The lipid A and KDO portions of LPS are common

to all bacterial LPS.

The other region of LPS is the sugar chain. This portion

is also known as the O-antigen. The O-antigen gets its name

from the fact that it is exposed to the external environment and

will be the target of antibody formationby the host. The

hydrophilic (“water-loving”) sugar side chain extends outward

from the surface of the cell into the watery environment that

typically surrounds many Gram-negative bacteria. There are

many chemical arrangements of the sugar chain.

The manufacture of LPS is a multi-step process involv-

ing many enzymes. The complete LPS molecule is incorpo-

rated into the outer membrane. The biosynthetic pathway of

LPS was deduced by the isolation of mutantsdefective in LPS

assembly.

LPS can be detected using microscopic techniques fol-

lowing the binding of LPS specific antibody. Additionally, a

biochemical test can be done. The test utilizes a compound

that is obtained from the horseshoe crab.

Not all bacteria have a complete sugar chain.

Depending on the bacterial species a portion of the sugar chain

can be present, or sugar chain may be entirely absent. The var-

ious LPS chemistries have an affect on the appearance of the

bacteria when they are grown as colonies on solid growth

media. Those bacteria with the complete side chain can appear

smooth and even wet, whereas those with no side chain often

appear crinkly and dry. For this reason, bacteria having the

complete LPS are known as smooth strains and those bacteria

with no sugar side chain are designated as rough strains. Those

species of bacteria that are in between, having a portion of the

sugar side chain, are called semi-rough strains.

The composition of the LPS also affects the overall

chemistry of the bacterial surface. Because the sugar chains

protruding from the surface are hydrophilic, the bacterium

tends to prefer watery environments. In contrast the lack of the

side chains exposes the hydrophobic(“water hating”) lipid por-

tion of the LPS. The surface of such bacteria tends to be more

hydrophobic. In solution, the rough bacteria tend to clump

together in an effort to avoid the water. Antibacterial com-

pounds that are hydrophobic are more likely to penetrate into

rough strains than into smooth strains. In the intestinal tract of

warm-blooded animals, where many Gram-negative bacteria

live, the possession of a complete LPS is advantageous for the

absorption of hydrophilic nutrients by the bacteria.

The LPS structure has a profound influence on the

potential of infectious Gram-negative bacteria to establish an

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