Radiation

Radiation

Energy transmitted through space in a variety of forms is generally called radiation. For our purposes, the most significant type of radiation is probably electromagnetic radiation, of which light and x-rays are examples. Electromagnetic radiation has the dual properties of a continuous wave phenomenon and a discontinuous particle phenomenon; the particle are packets, or quanta of energy, sometimes called photons, which vibrate at different frequencies. Radiation of a given frequency can also be described by its wavelength, it is measured in angstroms.

Gamma rays and x-rays, which have energies of more than about 10eV, are called ionizing radiations because they have enough energy to knock electrons away from molecules and ionize them. When such radiations pass through cells, they create free hydrogen radicals, hydroxyl radicals, and some peroxides, which in turn can cause different kinds of intracellular damage. Moreover, since this damage is produced in a variety of materials, ionizing radiations are rather non-specific in their effects. Less energetic radiation, particularly ultraviolet light, does not ionize; it is absorbed quite specifically by different compounds because it excites electrons and raises them to higher energy levels, thus creating different chemical species that can engage in a variety of chemical reactions not possible for unexcited molecules.

In addition to electromagnetic radiation, organisms may be subjected to acoustic radiation (sound wave) and to subatomic particles, such as those released in radioactive decay. The atomic era has alerted us to the damaging potential of radiation. Consequently, a tremendous expenditure of research effort is being directed toward determining the minimum dosage which affects cells, how radiations damage cells, and how the damage can be prevented. Microorganisms have been used for the major part of this research for the same reasons they are used in so many other areas of basic biological research; they are easy to grow and lend themselves to rapid, efficient experimentation.

Besides the fundamental research in radiation microbiology, there have been many developments in the application of ionizing radiation to sterilize biological materials. This method is called cold sterilization because ionizing radiations produce relatively little heat in the material irradiated. Thus it is possible to sterilize heat-sensitive substances, and such techniques are being developed in the food and pharmaceutical industries.

Ultraviolet light

The ultraviolet portion of the spectrum includes all radiations from 150-3900 . Wavelengths around 2650 have the highest bactericidal efficiency. Although the radiant energy of sunlight is partly composed of ultraviolet light, most of the shorter wavelengths of this type are filtered out by the earth’s atmosphere (ozone, clouds, and smoke). Consequently, the ultraviolet radiation at the surface of the earth is restricted to the span from about 2670 to 3900 . From this, we may conclude that sunlight, under certain conditions has the microbicidal capacity, but to a limited degree.

Many lamps are available which emit a high concentration of ultraviolet light in the most effective region, 2600 to 2700 . Germicidal lamps, which emit ultraviolet radiations, are widely used to reduce microbial populations. For example, they are used extensively in hospital operating rooms, in aseptic filling rooms, in the pharmaceutical industry, where sterile products are being dispensed into vials or ampules, and in the food and dairy industries for the treatment of contaminated surfaces.

An important practical consideration in using this means of destroying microorganisms is that ultraviolet light has very little ability to penetrate matter. Even a thin layer of glass filters off a large percentage of the light. Thus, only the microorganisms on the surface of an object where they are exposed directly to the ultraviolet light are susceptible to destruction.

Mode of action: Ultraviolet light is absorbed by many cellular materials but most significantly by the nucleic acids, where it does the most damage. The absorption and subsequent reactions are predominantly in the pyrimidines of the nucleic acid. One important alternation is the formation of a pyrimidine dimer in which two adjacent pyrimidines become bonded. Unless diners are removed by specific intracellular enzymes, DNA replication can be inhibited and mutations can result.

X-rays (Roentgen rays)

X-rays are lethal to microorganisms and higher forms of life. Unlike ultraviolet radiations, they have considerable energy and penetration ability. However, they are impractical for purposes of controlling microbial populations because (1) they are very expensive to produce in quantity and (2) they are difficult to utilize efficiently since radiations are given off in all directions from their point of origin. However, X-rays have been widely employed experimentally to produce microbial mutants.

Gamma rays

Gamma radiations are high-energy radiations emitted from certain radioactive isotopes such as ⁶⁰Co. As a result of the major research programs with atomic energy, large quantities of radioisotopes have become available as by-products of atomic fission. These isotopes are potential sources of gamma radiations. Gamma rays are similar to x-rays but are of shorter wavelength and higher energy. They are capable of great penetration into matter, and they are lethal to all life, including microorganisms.

Because of their great penetrating power and their microbial effect, gamma rays are attractive for use in commercial sterilization of materials of considerable thickness or volume, e.g., packaged foods and medical devices. However, certain technical problems must be resolved for practical applications, e.g., development of radiation sources for large-scale use and the design of equipment to eliminate any possible hazards to the operators.

Results of quantitative studies on the effect of ionizing radiations on the cells have resulted in the establishment of the “target” theory of action. This implies that the radiant-energy particle makes a “direct hit” on some essential substance such as DNA within the bacterial cell, causing ionization which results in the death of the cell.

Cathode rays (electron-beam radiation)

When a high-voltage potential is established between a cathode and an anode in an evacuated tube, the cathode beams of electrons, called cathode rays or electron beams. Special types of equivalent have been designed which produce accelerated to extremely high intensities (millions of volts), and these electrons are accelerated to extremely high velocities. These intense beams of accelerated electrons are microbicidal as well as having other effects on biological and non-biological materials.

The electron accelerator, a type of equivalent which produces the high-voltage electron beam, is used today for the sterilization of surgical supplies, drugs, and other materials. One of the unique features of the process is that the material can be sterilized after it has been packaged (the radiations penetrate the wrappings) and at room temperature. Electron-beam radiation has limited power of penetration; but within its limits of penetration, sterilization is accomplished by very brief exposure.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.