Methane oxidizing and producing bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY378•
on the embryonic development of fish and crustaceans. He
taught at the university for the next six years before moving to
the University of Odessa on the Black Sea where he studied
marine animals.
During the summer of 1880, he spent a vacation on a
farm where a beetle infection was destroying crops. In an
attempt to curtail the devastation, Metchnikoff injected a fun-
gus from a dead fly into a beetle to see if he could kill the pest.
Metchnikoff carried this interest in infection with him when he
left Odessa for Italy, following the assassination of Czar
Alexander II in 1884. A zoologist up to that point, Metchnikoff
began to focus more on pathology, or the study of diseases.
This transformationwas due primarily to his study of
the larva of the Bipinniara starfish. While studying this larva,
which is transparent and can be easily observed under the
microscope, Metchnikoff saw special cells surrounding and
engulfing foreign bodies, similar to the actions of white blood
cells in humans that were present in areas of inflammation.
During a similar study of the water flea Daphniae, he
observed white blood cells attacking needle-shaped spores
that had invaded the insect’s body. He called these cells
phagocytes, from the Greek word phagein,meaning, to eat.
While scientists thought that human phagocytes merely
transported foreign material throughout the body, and there-
fore spread disease, Metchnikoff realized they performed a
protective function. He recognized that the human white blood
cells and the starfish phagocytes were embryologically homol-
ogous, both being derived from the mesoderm layer of cells.
He concluded that the human cells cleared the body of disease-
causing organisms. In 1884, he injected infected blood under
the skin of a frog and demonstrated that white blood cells in
higher animals served a similar function as those in starfish
larvae. The scientific community, however, still did not accept
his idea that phagocytic cells fought off infections.
Metchnikoff returned to Odessa in 1886 and became the
director of the Bacteriological Institute. He continued his
research on phagocytes in animals and pursued vaccines for
chicken cholera and sheep anthrax. Hounded by scientists and
the press because of his lack of medical training, Metchnikoff
fled Russia a year later. A chance meeting with French scien-
tist Louis Pasteurled to a position as the director of a new lab-
oratory at the Pasteur Institute in Paris. There, he continued his
study of phagocytosisfor the next twenty-eight years.
But conflict with his fellow scientists continued to fol-
low him. Many scientists asserted that antibodies triggered the
body’s immune response to infection. Metchnikoff accepted
the existence of antibodies but insisted that phagocytic cells
represented another important arm of the immune system. His
work at the Pasteur Institute led to many fundamental discov-
eries about the immune response, and one of his students,
Jules Bordet, contributed important insights into the nature of
complement, a system of antimicrobial enzymestriggered by
antibodies. Metchnikoff received the Nobel Prize for physiol-
ogy and medicine in 1908 jointly with Paul Ehrlichfor their
work in initiating the study of immunology and greatly influ-
encing its development.
Metchnikoff’s interest in immunityled to writings on
aging and death. His book The Nature of Man,published in1903, extolled the health virtues of “right living,” which for
him included consuming large amounts of fermented milk or
yogurt made with a Bulgarian bacillus. In fact, his own name
became associated with a popular commercial preparation of
yogurt, although he received no royalties. With the exception
of yogurt, Metchnikoff warned of eating uncooked foods,
claiming that the bacteriapresent on them could cause cancer.
Metchnikoff claimed he even plunged bananas into boiling
water after unpeeling them and passed his silverware through
flames before using it.
On July 15, 1916, after a series of heart attacks,
Metchnikoff died in Paris at the age of 71. He was a member
of the French Academy of Medicine, the Swedish Medical
Society, and the Royal Society of London, from which he
received the Copley Medal. He also received an honorary doc-
torate from Cambridge University.See alsoPhagocyte and phagocytosisMETHANE OXIDIZING AND PRODUCING
BACTERIAMethane oxidizing and producing bacteriaMethane is a chemical compound that consists of a carbon
atom to which are bound four hydrogen atoms. The gas is a
major constituent of oxygen-free mud and water, marshes, the
rumen of cattle and other animals, and the intestinal tract of
mammals. In oxygen-free (anaerobic) environments, methane
can be produced by a type of bacteriaknown as methanogenic
bacteria. Methane can also be used as an energy source by
other bacteria that grow in the presence of oxygen (aerobic
bacteria), which break down the compound into carbon diox-
ide and water. These bacteria are known as methane oxidizing
bacteria.
Bacteria from a number of genera are able to oxidize
methane. These include Methylosinus, Methylocystis,
Methanomonas, Methylomonas, Methanobacter, and
Methylococcus. A characteristic feature of methane-oxidizing
bacteria is the presence of an extensive system of membranes
inside the bacterial cell. The membranes house the enzymes
and other biochemical machinery needed to deal with the se of
methane as an energy source.
The oxidation of methane by bacteria requires oxygen.
The end result is the production of carbon dioxide and water.
Methane oxidation is restricted to prokaryotes. Eukaryotic
microorganisms such as algae and fungido not oxidize
methane.
The production of methane is a feature of anaerobic
bacteria. Examples of methane producing genera are
Methanobacterium, Methanosarcina, Methanococcus, and
Methanospirillum. Methanogenic bacteria are widespread in
nature, and are found in mud, sewage, and sludge and in the
rumen of sheep and cattle. Some methanogenic bacteria have
adapted to live in extreme environments. For example,
Methanococcus jannaschiihas an optimum growth tempera-
ture of 85° C (185° F), which is achieved in hot springs and
thermal vents in the ocean. Such anaerobic bacteria are amongwomi_M 5/7/03 7:52 AM Page 378