Radioisotopes, containing unstable combinations of protons and neutrons, are created by neutron activation that involves the capture of a neutron by the nucleus of an atom. Such a capture results in an excess of neutrons (neutron rich). Proton rich radioisotopes are manufactured in cyclotrons. During radioactive decay, the nucleus of a radioisotope seeks energetic stability 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 radioisotope 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 energetic 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, radioisotopes can also be used in sophisticated and delicate biochemical assays or analysis. There are many common laboratory tests utilizing radioisotopes to analyze blood, urine and hormones. Radioisotopes are also finding increasing use in the labeling, identification and study of immunological cells.
The study of microorganisms also relies heavily on the use of radioisotopes. The identification of protein species, labeling of surface components of bacteria, and tracing the transcription and translation steps involved in nucleic acid and protein manufacture all utilize radioisotopes.
A radioisotope can emit three different types of radiation. The first of these is known as alpha radiation. This radiation 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 radiation. This type of radiation is highly energetic.
The various types of radiations can be selected to provide 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 adherence 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 DNA (deoxyribonucleic acid). The radioactive DNA can 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 addition 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 nonradioactive (or "cold") methionine. The rate at which the radioactivity disappears can be used to calculate the rate of turnover of the particular protein. In another experimental approach, the protein constituents of bacteria or viruses can be separated on an electrophoretic gel. The gel is then brought into contact with x-ray film. Wherever a radioactive protein band is present in the gel, the overlaying film will be exposed. Thus, the proteins that are radioactive can be determined.
The use of radiolabeled compounds that can be utilized as nutrients by bacteria allows various metabolic pathways to be determined. For example, glucose can be radiolabeled and its fate followed by various techniques, including chromatography, autoradiography, and gel electrophoresis. Furthermore, a molecule such as glucose can be radiolabeled at various chemical groups within the molecule. This allows an investigator to assess whether different regions of a molecule are used preferentially.
Radiolabeling has allowed for great advances in microbiological research. A well-known example is the 1952 experiment by Hershey and Chase, which established that DNA was the reservoir of genetic information. Bacterial viruses were exposed to either radioactive sulfur or phosphorus. The sulfur radiolabeled the surface of the virus, while the phosphorus labeled the DNA. Viruses were allowed to infect bacteria and then were mechanically sheared off of the bacteria. The sheared viruses were then collected separately from the bacteria. Radioactive sulfur was found in the virus suspension and radioactive phosphorus was found in the bacteria. Furthermore, the bacteria eventually produced new virus, some of which had radioactive DNA. Thus, radiolabeling demonstrated the relationship between DNA and genetic information.