Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Extremophiles

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probe is designed to detect minute amounts of carbon, and all

metabolic forms of the atom. For example, experiments will

look for the presence of methane, such as would be produced

by methanogenic bacteria.

In addition to extraterrestrial microbial life, interest has

arisen over the possibility that extraterrestrial microorganisms

could find their way to Earth. Transport of microorganisms via

meteorites and on material ejected from the solar body by a

meteorite impact has been proposed. In the 1990s, the electron

microscopic examinationof meteorite ALH84001, which

originated from Mars, found bacteria-like objects. Their

shape, size and chemistry were at the time consistent with a

biological origin. However, further study negated this possi-

bility and no other such observations have been made.

See alsoAnaerobes and anaerobic infections; Biogeochemical

cycles; Extremophiles

EExtremophilesXTREMOPHILES

Extremophiles is a term that refers to bacteriathat are able to

exist and thrive in environments that are extremely harsh, in

terms of those environments classically envisioned as hos-

pitable to the growth of bacteria.

The discovery of extremophiles, beginning in the 1970s,

has had three major influences on microbiology and the

biotechnologyindustry. Firstly, the discovery of bacteria

growing in environments such as the hot springs of

Yellowstone National Park and around the hydrothermal vents

located on the ocean floor (where the bacteria are in fact the

fundamental basis of the specialized ecosystem that is fueled

by the vents) has greatly increased the awareness of the possi-

bilities for bacterial life on Earth and elsewhere. Indeed, the

growth of some extremophiles occurs in environments that by

all indications could exist on planets such as Mars and other

stellar bodies. Thus, extremophilic bacteria might conceivably

not be confined to Earth.

The second major influence of extremophiles has been

the broadening of the classification of the evolutionary devel-

opment of life on Earth. With the advent of molecular means

of comparing the genetic sequences of highly conserved

regions from various life forms, it became clear that

extremophiles were not simply offshoots of bacteria, but

rather had diverged from both bacteria and eukaryotic cells

early in evolutionary history. Extremophilic bacteria are

grouped together in a domain called archaea. Archae share

similarities with bacteria and with eukaryotes.

Thirdly, extremophiles are continuing to prove to be a

rich trove of enzymesthat are useful in biotechnological

processes. The hardiness of the enzymes, such as their ability

to maintain function at high temperatures, has been crucial to

the development of biotechnology. A particularly well-known

example is the so-called tag polymerase enzyme isolated from

the extremophile Thermus aquaticus. This enzyme is funda-

mental to the procedures of the polymerase chain reaction

(PCR) procedure that has revolutionized biotechnology.

There are several environments that are inhospitable to

all but those extremophilic bacteria that have adapted to live

in them. The best studied is elevated temperature. Heat-loving

bacteria are referred to as thermophiles. More than 50 species

of thermophiles have been discovered to date. Such bacteria

tolerate temperatures far above the tolerable limits known for

any animal, plant, or other bacteria. Some thermophiles, such

as Sulfolobus acidocaldarius, are capable of growth and repro-

duction in water temperatures that exceed 212° F [100° C] (the

boiling point of water at sea level). The most heat-tolerant

thermophile known so far is Pyrolobus fumarii, that grows in

the walls of the hydrothermal vents where temperatures

exceed 200° F [93.33° C]. In fact, the bacterium requires a

temperature above 194° F [90° C] to sustain growth. The basis

of the thermophile’s ability to prevent dissolution of cell wall

constituents and genetic material at such high temperatures is

unknown.

Other examples of extreme environments include ele-

vated salt, pressure, and extreme acid or base concentrations.

Salt-loving, or halophilic, bacteria grow in environ-

ments where the sodium concentration is extremely high, such

as in the Dead Sea or Great Salt Lake. In such an environment,

a bacterium such as Escherichia coliwould compensate for

the discrepancy in sodium concentration between the bac-

terium’s interior and exterior by shunting all the internal fluid

to the exterior. The result would be the collapse and death of

the bacterium. However, salt-loving bacteria such as

Halobacterium salinarumcontent with the sodium discrep-

ancy by increasing the internal concentration of potassium

chloride. The enzymes of the bacterium operate only in a

potassium chloride-rich environment. Yet the proteins pro-

duced by the action of these enzymes need to be tolerant of

high sodium chloride levels. How the enzymes are able to

accommodate both demands is not clear.

Acid-loving extremophiles prefer environments where

the pHis below pH=5, while alkaline-loving bacteria require

pHs above pH=9. Thriving populations of acid-loving bacteria

have been isolated in the runoff from acidic mine drainage,

where the pH is below one, which is more acidic than the con-

tents of the stomach. Interestingly, these bacteria are similar to

other bacteria in the near neutral pH of their interior. Very

acidic pHs would irreversibly damage the genetic material.

Acid-loving bacteria thus survive by actively excluding acid.

The enzymes necessary to achieve this function at very acidic

pH levels.

Similarly, alkaline-loving bacteria maintain a near neu-

tral interior pH. The enzymes that function at such alkaline

conditions are of interest to manufacturers of laundry deter-

gents, which operate better at alkaline pHs.

Some extremophiles grow and thrive at very low tem-

peratures. For example, Polaromonas vacuolatahas an ideal

growth temperature of just slightly above the freezing point of

water. These bacteria are finding commercial applications in

enzymatic processes that operate at refrigeration temperatures

or in the cold cycle of a washing machine.

The discovery of bacteria in environments that were

previously disregarded as being completely inhospitable for

bacterial life argues that more extremophiles are yet to be

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