Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Metchnikoff, Élie

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directly as signals in the control of their own breakdown and

synthesis. Feedback control can be negative or positive.

Negative feedback results in the inhibition by an end product,

of the activity or synthesis of an enzyme or several enzymes

in a reaction chain. The inhibition of the synthesis of enzymes

is called enzyme repression. Inhibition of the activity of an

enzyme by an end product is an allosteric effect and this type

of feedback control is well known in many metabolic path-

ways (e.g., lactose operon). In positive feedback, an endprod-

uct activates an enzyme responsible for its own production.

Many reactions in metabolism are cyclic. A metabolic

cycle is a catalytic series of reactions, in which the product of

one bimolecular (involving two molecules) reaction is regen-

erated as follows: A + B →C + A. Thus, A acts catalytically

and is required only in small amounts and A can be regarded

as carrier of B. The catalytic function of A and other members

of the metabolic cycle ensure economic conversion of B to C.

B is the substrate of the metabolic cycle and C is the product.

If intermediates are withdrawn from the metabolic cycle, e.g.,

for biosynthesis, the stationary concentrations of the metabolic

cycle intermediates must be maintained by synthesis.

Replenishment of depleted metabolic cycle intermediates is

called anaplerosis. Anaplerosis may be served by a single

reaction, which converts a common metabolite into an inter-

mediate of the metabolic cycle. An example of this is pyruvate

to oxaloacetate reaction in the tricarboxylic acid cycle.

Alternatively, it may involve a metabolic sequence of reac-

tions, i.e., an anaplerotic sequence. An example of this is the

glycerate pathway which provides phosphoenol pyruvate for

anaplerosis of the tricarboxylic acid cycle.

Prokaryotes exhibit a great diversity of metabolic

options, even in a single organism. For example, Escherichia

coli can produce energy by respiration or fermentation.

Respiration can be under aerobic conditions (e.g., using O 2 as

the final electron acceptor) or anaerobically (e.g., using some-

thing other than oxygen as the final electron acceptor).

Compounds like lactose or glucose can be used as the only

source of carbon. Other bacteriahave other metabolic capa-

bilities including the use of sunlight for energy.

Some of these mechanisms are also utilized by eukary-

otic cells. In addition, prokaryotes have a number of energy-

generating mechanisms that do not exist in eukaryotic cells.

Prokaryotic fermentation can be uniquely done via the phos-

phoketolase and Enter-Doudoroff pathways. Anaerobic respi-

ration is unique to prokaryotes, as is the use of inorganic

compounds as energy sources or as carbon sources during bac-

terial photosynthesis. Archaebacteria possess metabolic path-

ways that use H 2 as the energy source with the production of

methane, and a nonphotosynthetic metabolism that can con-

vert light energy into chemical energy.

In bacteria, metabolic processes are coupled to the syn-

thesis of adenosine triphosphate (ATP), the principle fuel

source of the cell, through a series of membrane-bound pro-

teins that constituent the electron transport system. The

movement of protons from the inside to the outside of the

membrane during the operation of the electron transport sys-

tem can be used to drive many processes in a bacterium, such

as the movement of the flagella used to power the bacterium

along, and the synthesis of ATP in the process called oxidative

phosphorylation.

The fermentative metabolism that is unique to some

bacteria is evolutionarily ancient. This is consistent with the

early appearance of bacteria on Earth, relative to eukaryotic

organisms. But bacteria can also ferment sugars in the same

way that brewing yeast(i.e., Saccharomyces cerevesiaefer-

ment sugars to produce ethanol and carbon dioxide. This fer-

mentation, via the so-called Embden Myerhoff pathway, can

lead to different ends products in bacteria, such as lactic acid

(e.g., Lactobacillus), a mixture of acids (Enterobacteriacaeae,

butanediol (e.g., Klebsiella, and propionic acid (e.g.,

Propionibacterium).

See alsoBacterial growth and division; Biochemistry

MMetchnikoff, Élie ETCHNIKOFF, ÉLIE(1845-1916)

Russian immunologist

Élie Metchnikoff was a pioneer in the field of immunologyand

won the 1908 Nobel Prize in physiology or medicine for his

discoveries of how the body protects itself from disease-caus-

ing organisms. Later in life, he became interested in the effects

of nutrition on aging and health, which led him to advocate

some controversial diet practices.

Metchnikoff, the youngest of five children, was born in

the Ukrainian village of Ivanovka on May 16, 1845, to Emilia

Nevahovna, daughter of a wealthy writer, and Ilya Ivanovich,

an officer of the Imperial Guard in St. Petersburg. He enrolled

at the Kharkov Lycee in 1856, where he developed an espe-

cially strong interest in biology. At age 16, he published a

paper in a Moscow journal criticizing a geology textbook.

After graduating from secondary school in 1862, he entered

the University of Kharkov, where he completed a four-year

program in two years. He also became an advocate of the the-

ory of evolutionby natural selectionafter reading Charles

Darwin’s On the Origin of Species by Means of Natural

Selection.

In 1864, Metchnikoff traveled to Germany to study,

where his work with nematodes (a species of worm) led to the

surprising conclusion that the organism alternates between

sexual and asexual generations. His studies at Kharkov, cou-

pled with his interest in Darwin’s theory, convinced him that

highly evolved animals should show structural similarities to

more primitive animals. He pursued his studies of inverte-

brates in Naples, Italy, where he collaborated with Russian

zoologist Alexander Kovalevsky. They demonstrated the

homology (similarity of structure) between the germ layers—

embryonic sheets of cells that give rise to specific tissue—in

different multicellular animals. For this work, the scientists

were awarded the Karl Ernst von Baer Prize.

Metchnikoff was only twenty-two when he received the

prize and had a promising career ahead of himself. However,

he soon developed severe eye strain, a condition that ham-

pered his work and prevented him from using the microscope

for the next fifteen years. Nevertheless, in 1867, he completed

his doctorate at the University of St. Petersburg with a thesis

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