Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Extraterrestrial microbiology

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eukaryotic life argues for the origin of viruses as an evolu-

tionary offshoot of prokaryotes.

Scientists are in general agreement that the first virus

was a fragment of DNAor ribonucleic acid(RNA) from a even-

tual prokaryotic or eukaryotic host. The genetic fragment

somehow was incorporated into a eukaryotic and became

replicated along with the host’s genetic material. Over evolu-

tionary time, different viruses developed, having differing

specificities for the various bacteria and eukaryotic cells that

were arising.

The evolutionary origin of viruses will likely remain

conjectural. No fossilized virus has been detected. Indeed, the

minute size of viruses makes any distinction of their structure

against the background of the rock virtually impossible.

Likewise, bacterial fossilization results in the destruction of

internal detail. If a virus were to be present in a fossilizing

bacterium, any evidence would be obliterated over time.

Some details as to the evolutionary divergence of

viruses from a common ancestor are being realized by the

comparison of the sequence of evolutionarily maintained

sequences of genetic material. This area of investigation is

known as virus molecular systematics.

The comparison of a number of genesequences of viral

significance, for example the enzyme reverse transcriptase

that is possessed by retrovirusesand pararetroviruses, is con-

sistent with the evolutionary emergence of not one specific

type of virus, but rather of several different types of viruses.

The present day plethora of viruses subsequently evolved

from these initial few viral types (or “supergroups” as they

have been dubbed). So, in contrast to an evolutionary “tree.”

viral evolutionary origin resembles more of a bush. Each of

the several branches of the bush developed independently of

one another. Furthermore, the consensus among virologists

(scientists who study viruses) is that this independent evolu-

tiondid not occur at the same time or progress at the same rate.

In scientific terms, the viral evolution is described as being

“polyphyletic.”

The evolution of viruses with life forms, including bac-

teria, likely occurred together. On other words, as bacteria

increased in diversity and in the complexity of their surfaces,

new viruses evolved to be able to utilize the bacteria as a repli-

cation factory. Similarly, as more complex eukaryotic life

forms appeared, such as plants, insects, birds, and mammals,

viruses evolved that were capable of utilizing these as hosts.

See also Bacterial kingdoms; Evolution and evolutionary

mechanisms; Mitochondrial DNA

EXOTOXIN•seeENTEROTOXIN AND EXOTOXIN

EExtraterrestrial microbiologyXTRATERRESTRIAL MICROBIOLOGY

Extraterrestrial microbiology is the study of microbiological

processes that could occur outside of the boundaries of Earth,

or on other bodies in the solar system. While such microor-

ganismshave not yet been found, recent findings of living

bacteriain very inhospitable environments on Earth, com-

bined with the existence of water on planets such as Mars,

have buttressed the possibility that life in similar conditions on

other planets is not inconceivable.

The scientific search for extraterrestrial life began in

1860, when the microbiologist Louis Pasteurattempted and

failed to culturebacteria from the Orgueil meteorite.

The search for extraterrestrial life has always been one

of the curiosities that has pulled man into the exploration of

space. As the chemistries of the planets in our solar system

became clearer, the possibilities for human-like life faded.

However, at about the same time, the diversity of microbial

life on Earth became more apparent. In particular, a type of

evolutionarily ancient microorganism known as archaebacte-

ria was isolated from extremely harsh environments, such as

hot springs, thermal hot vents on the ocean floor, and from

deep in the subsurface of the planet. In contrast to life forms

that require oxygen and organic carbon, archaebacteria live on

hydrogen and carbon dioxide. Planetary bodies such as Mars

and Europa contain atmospheres of hydrogen and carbon

dioxide. Thus, theoretically, archaebacteria could find such

planets hospitable. Moreover, the finding of bacterial life

below the Earth’s surface makes the probability of similar life

elsewhere greater. Other worlds are more likely to have, or

have had, hot and oxygen-limited conditions similar to ther-

mal vents or the subsurface, rather than the sunlight, oxygen-

rich atmosphere of Earth’s surface. Furthermore, the now

prevailing view that archaebacteria are very ancient indicates

that life on Earth may have arisen from environments now

considered inhospitable. The environment on other solar bod-

ies may be similar to what Earth experienced when microbial

life first arose.

Unmanned probes have explored a variety of bodies in

our solar system. One such stellar body, Europa, has so far not

proved to be a source of life. Probes sent to scan the planet’s

surface found only lifeless slush. However, two of the moons,

which orbit the gas planet Saturn, are of interest. Enceladus

has visual signs and chemical signals consistent with the pres-

ence of liquid water. The other moon, Titan, is icy and spectral

monitoring of the surface has detected signals indicative of

organic compounds.

By far the bulk of interest in extraterrestrial microbiol-

ogy has centered on the planet Mars. Interest in Mars as a

potential supporter of microbiological life prompted the

Vikingmission that occurred in 1976. The, in two separate

missions, proves landed on different regions of the planet ad

conducted experiments designed to detect signature molecules

of microbiological activity. One experiment, the gas exchange

experiment, sought to detect alterations in the composition of

gases in a test chamber. The alterations would presumable be

due to microbial decomposition of nutrients, with the con-

sumption of some gases and the release of others. The results

were equivocal at first, but with examination were thought to

be the result of abiological activity, specifically solar ultravio-

let radiation. In a second experiment, radioactive nutrient was

released into wetted Martian soil. Bacterial metabolismwould

be evident by the appearance of different radioactive com-

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