Microbiology and Immunology

Magnetotactic bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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article’s significance, it was later hailed as the beginning of a

revolution that led to the formation of molecular biology as a

scientific discipline.

MacLeod married Elizabeth Randol in 1938; they even-

tually had one daughter. In 1941, MacLeod became a citizen

of the United States, and was appointed professor and chair-

man of the department of microbiology at the New York

University School of Medicine, a position he held until 1956.

At New York University he was instrumental in creating a

combined program in which research-oriented students could

acquire both an M.D. and a Ph.D. In 1956, he became profes-

sor of research medicine at the Medical School of the

University of Pennsylvania. MacLeod returned to New York

University in 1960 as professor of medicine and remained in

that position until 1966.

From the time the United States entered World War II

until the end of his life, MacLeod was a scientific advisor to

the federal government. In 1941, he became director of the

Commission on Pneumonia of the United States Army

Epidemiological Board. Following the unification of the mili-

tary services in 1949, he became president of the Armed

Forces Epidemiological Board and served in that post until

1955. In the late 1950s, MacLeod helped establish the Health
      Research Council for the City of New York and served as its
      chairman from 1960 to 1970. In 1963, President John F.
      Kennedy appointed him deputy director of the Office of
      Science and Technology in the Executive Office of the
      President; from this position he was responsible for many pro-
      gram and policy initiatives, most notably the United
      States/Japan Cooperative Program in the Medical Sciences.
      In 1966, MacLeod became vice-president for Medical
      Affairs of the Commonwealth Fund, a philanthropic organiza-
      tion. He was honored by election to the National Academy of
      Sciences, the American Philosophical Society, and the
      American Academy of Arts and Sciences. MacLeod was en
      route from the United States to Dacca, Bangladesh, to visit a
      cholera laboratory when he died in his sleep in a hotel at the
      London airport in 1972. In the Yearbook of the American
      Philosophical Society,Maclyn McCarty wrote of MacLeod’s
      influence on younger scientists, “His insistence on rigorous
      principles in scientific research was not enforced by stern dis-
      cipline but was conveyed with such good nature and patience
      that it was simply part of the spirit of investigation in his lab-
      oratory.”

See alsoBacteria and bacterial infection; Microbial genetics;

Pneumonia, bacterial and viral

MAD COW DISEASE•seeBSE ANDCJD DISEASE

MMagnetotactic bacteriaAGNETOTACTIC BACTERIA

Magnetotactic bacteriaare bacteria that use the magnetic field

of Earth to orient themselves. This phenomenon is known as

magnetotaxis. Magnetotaxis is another means by which bacte-

ria can actively respond to their environment. Response to

light (phototaxis) and chemical concentration (chemotaxis)

exist in other species of bacteria.

The first magnetotactic bacterium, Aquasprilla magne-

totactumwas discovered in 1975 by Richard Blakemore. This

organism, which is now called Magnetospirillum magneto-

tacticum, inhabits swampy water, where because of the

decomposition of organic matter, the oxygen content in the

water drops off sharply with increasing depth. The bacteria

were shown to use the magnetic field to align themselves. By

this behavior, they were able to position themselves at the

region in the water where oxygen was almost depleted, the

environment in which they grow best. For example, if the bac-

teria stray too far above or below the preferred zone of habi-

tation, they reverse their direction and swim back down or up

the lines of the magnetic field until they reach the preferred

oxygen concentration. The bacteria have flagella, which

enables them to actively move around in the water. Thus, the

sensory system used to detect oxygen concentration is coordi-

nated with the movement of the flagella.

Magnetic orientation is possible because the magnetic

North Pole points downward in the Northern Hemisphere. So,

magnetotactic bacteria that are aligned to the fields are also

pointing down. In the Northern Hemisphere, the bacteria

would move into oxygen-depleted water by moving north

along the field. In the Southern Hemisphere, the magnetic

North Pole points up and at an angle. So, in the Southern

Hemisphere, magnetotactic bacteria are south-seeking and

also point downward. At the equator, where the magnetic

North Pole is not oriented up or down, magnetotactic bacteria

from both hemispheres can be found.

Since the initial discovery in 1975, magnetotactic bac-

teria have been found in freshwater and salt water, and in oxy-

gen rich as well oxygen poor zones at depths ranging from the

near-surface to 2000 meters beneath the surface.

Magnetotactic bacteria can be spiral-shaped, rods and spheres.

In general, the majority of magnetotactic bacteria discovered

so far gather at the so-called oxic-anoxic transition zone; the

zone above which the oxygen content is high and below which

the oxygen content is essentially zero.

Magnetotaxis is possible because the bacteria contain

magnetically responsive particles inside. These particles are

composed of an iron-rich compound called magnetite, or var-

ious iron and sulfur containing compounds (ferrimagnetite

greigite, pyrrhotite, and pyrite). Typically, these compounds

are present as small spheres arranged in a single chain or sev-

eral chains (the maximum found so far is five) in the cyto-

plasmof each bacterium. The spheres are enclosed in a

membrane. This structure is known as a magnetosome. Since

many bacterial membranes selectively allow the movement of

molecules across them, magnetosome membranes may func-

tion to create a unique environment within the bacterial cyto-

plasm in which the magnetosome crystal can form. The

membranes may also be a means of extending the chain of

magnetosome, with a new magnetosome forming at the end of

the chain.

Magnetotactic bacteria may not inhabit just Earth.

Examination of a 4.5 billion-year-old Martian meteorite in

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