Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Radioisotopes and their uses in microbiology and immunology

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Escherichia colican only tolerate one or several cuts to the

DNA before the radiation damage is either lethal or causes the

formation of drastic mutations.

The molecular nature of this repair ability is not yet

clear. However, the completion of the sequencing of the

genome of Deinococcus radioduransin late 1999 should pro-

vide the raw material to pursue this question. The genome is

unique among bacteria, being comprised of four to ten pieces

of DNA and a large piece of extrachromosomal DNA that is

part of a structure called a plasmid. The genome of other bac-

teria typically consists of a single circle of DNA (although

plasmid DNA can also be present). Within the chromosome-

like regions of Deinococcusthere are many repeated stretches

of DNA. In an analogy to a computer, the bacterium has

designed many backup copies of its information. If some back

up copies are impaired, the information can be recovered from

the other DNA.

This DNA repair ability has made the genus the subject

of intense scrutiny by molecular biologists interested in the

process of DNA manufacture and repair. Furthermore, the

radiation resistance of Deinococcushas made the bacteria an

attractive microorganism for the remediation of radioactive

waste. While this use is not currently feasible at the scale that

would be required to clean up nuclear contamination, small-

scale tests have proved encouraging. The bacteria still need to

be engineered to cope with the myriad of organic contami-

nants and heavy metals that are also typically part of nuclear

waste sites.

See alsoBioremediation; Extremophiles

RADIOISOTOPES AND THEIR USES IN

MICROBIOLOGY AND IMMUNOLOGYRadioisotopes and their uses in microbiology and immunology

Radioisotopes, containing unstable combinations of protons

and neutrons, are created by neutron activation that involves

the capture of a neutron by the nucleusof an atom. Such a cap-

ture results in an excess of neutrons (neutron rich). Proton rich

radioisotopes are manufactured in cyclotrons. During radioac-

tive decay, the nucleus of a radioisotope seeks energetic sta-

bility by emitting particles (alpha, beta or positron) and

photons (including gamma rays).

The history of radioisotopes in microbiology and

immunology dates back to their first use in medicine.

Although nuclear medicine traces its clinical origins to the

1930s, the invention of the gamma scintillation camera by

American engineer Hal Anger in the 1950s brought major

advances in nuclear medical imaging and rapidly elevated the

use of radioisotopes in medicine. For example, cancer and

other rapidly dividing cells are usually sensitive to damage by

radiation. Accordingly, some cancerous growths can be

restricted or eliminated by radioisotope irradiation. The most

common forms of external radiation therapy use gamma and

x rays. During the last half of the 20th century the radioiso-

tope cobalt-60 was a frequently used source of radiation used

in such treatments. Iodine-131 and phosphorus-32 are also

commonly used in radiotherapy. More radical uses of

radioisotopes include the use of Boron-10 to specifically

attack tumor cells. Boron-10 concentrates in tumor cells and

is then subjected to neutron beams that result in highly ener-

getic alpha particles that are lethal to the tumor tissue. More

modern methods of irradiation include the production of x

rays from linear accelerators.

Because they can be detected in low doses, radioiso-

topes can also be used in sophisticated and delicate biochemi-

cal assays or analysis. There are many common laboratory

tests utilizing radioisotopes to analyze blood, urine and hor-

mones. Radioisotopes are also finding increasing use in the

labeling, identification and study of immunological cells.

The study of microorganismsalso relies heavily on the

use of radioisotopes. The identification of protein species,

labeling of surface components of bacteria, and tracing the

transcriptionand translationsteps involved in nucleic acid

and protein manufacture all utilize radioisotopes.

A radioisotope can emit three different types of radia-

tion. The first of these is known as alpha radiation. This radi-

ation is due to alpha particles, which have a positive charge.

An example is the decay of an atom of a substance called

Americium to an atom of Neptunium. The decay is possible

because of the release of an alpha particle.

The second type of radiation is called beta radiation.

This radiation results from the release of a beta particle. A

beta particle has a negative charge. An example is the decay

of a carbon atom to a nitrogen atom, with the release of a beta

particle.

The final type of radiation is known as gamma radia-

tion. This type of radiation is highly energetic.

The various types of radiations can be selected to pro-

vide information on a sample of interest. For example, to

examine how quickly a protein is degraded, an isotope that

decays very quickly is preferred. However, to study the adher-

ence of bacteria to a surface, a radiolabel that persisted longer

would be more advantageous.

Furthermore, various radioactive compounds are used

in microbiological analyses to label different constituents of

the bacterial cell. Radioactive hydrogen (i.e., tritium) can be

used to produce radioactive deoxyribonucleic acid. The

radioactive DNAcan be detected by storing the DNA sample

in contact with X-ray film. The radioactive particles that are

emitted from the sample will expose the film. When the film

is developed, the result is an image of the DNA. This process,

which is known as autoradiography, has long been used to

trace the elongation of DNA, and so determine the speed at

which the DNA is replicating.

DNA can also be labeled, but in a different location

within the molecule, by the use of radioactive phosphorus.

Bacterial and viral proteins can be labeled by the addi-

tion of radioactive methionine to the growth mixture. The

methionine, which is an amino acid, will be incorporated into

proteins that are made. Several paths can then be followed. For

instance, in what is known as a pulse-chase experiment, the

radioactive label is then followed by the addition of nonra-

dioactive (or “cold”) methionine. The rate at which the

radioactivity disappears can be used to calculate the rate of

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