Microbiology and Immunology

Radiation-resistant bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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This principle has been used experimentally to deliver

radiation to specific cellular components. A cumulative effect

of radiation has been observed in animal models. This means

that if a population is repeatedly exposed to radiation, a higher

frequency of mutations is observed that is due to additive

effect. Intensive efforts to determine the mutagenic risk of low

dose exposure to ionizing radiation have been an ongoing con-

cern because of the use of nuclear energy and especially

because of the exposure to radon gas in some indoor environ-

ments. Radon is estimated by the United States Environmental

Protection Agency to be the cause of more than 20,000 cases

of lung cancer annually.

The relative efficiencies of the different types of radia-

tions in producing mutations is assessed as the mutagenic

effect. The mutagenic effect of radiation is generally assumed

to be due to direct damage to DNA, but the identity of the spe-

cific lesions remains uncertain.

Investigation of radiation’s mutagenic effects on differ-

ent tissues, cells, and subcellular compartments is becoming

possible by the availability of techniques and tools that allow

the precise delivery of small doses of radiation and that pro-

vide better monitoring of effects. Reactive oxygen species

released in irradiated cells are believed to act directly on

nuclear DNA and indirectly by modifying bases that will be

incorporated in DNA, or deactivating DNA repair enzymes.

Novel microbeam alpha irradiation techniques have allowed

researchers to investigate radiation-induced mutations in non-

irradiated DNA. There is evidence that radiation induces

changes in the cytosol that—in eukaryotes—are transmitted to

the nucleus and even to neighboring cells. Direct measurement

of DNA damage caused by ionizing radiation is performed by

examining micronucleus formation or analysis of DNA frag-

ments on agarose gels following treatment with specific

endonucleases such as those that only cleave at certain sites.

The polymerase chain reaction(PCR) is also used to detect the

loss of some marker genes by large deletions. The effect of

ionizing radiation on cells can also be measured by evaluating

the expression level of the stress inducible p21 protein.

Critical lesions leading to mutations or killing of a cell

include induction of DNA strand breaks, damaged bases, and

production of abasic sites (where a single base is deleted), and—

in multichromosomal organisms—large chromosomal dele-

tions. Except for large deletions, most of these lesions can be

repaired to a certain extent, and the lethal and mutagenic effect

of radiation is assumed to result principally from incompletely

or incorrectly repaired DNA. This view is supported by experi-

mental studies which showed that mice given a single radiation

dose, called acute dose, develop a significantly higher level of

mutations than mice given the same dose of radiation over a

period of weeks or months. The rapid activation of the DNA-

repair pathway through p53 protein and the stress-inducible p21

protein as well as the extreme sensitivity of cells with genetic

defects in DNA repair machinery support the view that the abil-

ity of the cell to repair irradiation-induced DNA damage is a

limiting factor in deciding the extent of the mutagenic effects.

See also Evolution and evolutionary mechanisms;

Evolutionary origin of bacteria and viruses; Immunogenetics;

Molecular biology and molecular genetics; Phage genetics;

Radiation resistant bacteria; Radioisotopes and their uses;

Viral genetics

RRadiation-resistant bacteriaADIATION-RESISTANT BACTERIA

Radiation-resistant bacteriaencompass eight species of bacte-

ria in a genus known as Deinococcus. The prototype species is

Deinococcus radiodurans. This and the other species are capa-

ble of not only survival but of growth in the presence of radi-

ation that is lethal to all other known forms of life.

Radiation is measured in units called rads. An instanta-

neous dose of 500 to 1000 rads of gamma radiation is lethal to

a human. However, Deinococcus radioduransis unaffected by

exposure to up to 3 million rads of gamma radiation. Indeed,

the bacterium, whose name translates to “strange berry that

withstands radiation,” holds a place in The Guinness Book of

World Records as “the world’s toughest bacterium.”

The bacterium was first isolated in the 1950s from tins

of meat that had spoiled in spite of being irradiated with a dose

that was thought to be sterilizing. The classification of the

bacterium as Deinococcus radiodurans, and the isolation,

characterization, and designation of the other species has been

almost exclusively due to Robert Murrayand his colleagues

at the University of Western Ontario. The various species

of Deinococcushave been isolated from a variety of locations

as disperse as elephant feces, fish, fowl, and Antarctic

rocks.

The reason for the development of such radiation resist-

ance is still speculative. But, the current consensus is that it

enabled the ancient form of the bacterium to survive in regions

where available water was scarce. Other organisms developed

different survival strategies, one example being the ability to

form the metabolically dormant spore.

Deinococcusis an ancient bacteria, believed to be some

two billion years old. They may have evolved at a time when

Earth was bathed in more energetic forms of radiation than

now, due to a different and less screening atmosphere. One

theory even suggests that the bacteria originated on another

world and were brought to Earth via a meteorite.

The extraterrestrial theory is likely fanciful, however,

because the bacteria are not heat resistant. Exposure to tem-

peratures as low as 113ºF (45ºC) can be lethal to the microor-

ganism.

There are two known reasons for the radiation resist-

ance of species of Deinococcus. Firstly, the structure of the

two membranes that surround the Gram-negative bacterium

contributes, albeit in a minor way. By far the major reason for

the radiation resistance is the bacterium’s ability to rapidly and

correctly repair the extensive damage caused to its genetic

material by radiation.

The high energy of radioactive waves literally cut apart

the double stranded molecule of deoxyribonucleic acid(DNA).

These cuts occur in many places, effectively shattering the

genome into many, very small fragments. Deinococcusis able

to quickly reassemble the fragments in their correct order and

then slice them back together. In contrast, bacteria such as

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