Ionizing Radiation: (Radiation Sterilization)
Mode of Action:
Both, X rays and Gamma rays have wavelength shorter than the wavelength of ultraviolet light. X rays, which have wavelength of 0.1 to 40 nm, and gamma rays, which have even shorter wavelength, are forms of ionizing radiation, so named because it can dislodge electrons from atoms, creating ions. (Longer wavelengths comprise nonionizing radiation.) These forms of radiation also kill microorganisms and viruses and ionizing radiation damages DNA and produces peroxides, which act as powerful oxidizing agents in cells. This radiation can also kill or cause mutations in human cells if it reaches them.
Production of Gamma rays:
Gamma rays are emitted by certain radioactive elements such as cobalt, and electron beams are produced by accelerating electrons to high energies in special machines.
Production of X rays:
X rays, which are produced by machines in a manner similar to the production of electron beams, are similar in nature to gamma rays.
Advantage of Gamma rays:
Gamma rays penetrate deeply but may requite hours to sterilize large masses.
High energy Electron Beams:
Effectiveness of Electron Beam:
High energy electron beams have much lower penetration power but usually require only a few seconds of exposure.
Mode of Action of Ionizing Radiations:
The principal effect of ionizing radiation is the ionization of water, which forms highly reactive hydroxyl radicals. These radicals react with organic cellular components, especially DNA.
The so-called target theory of damage by radiation supposes that ionizing particles, or packets of energy, pass through or close to vital portions of the cell; these constitute "hits." One, or a few, hits may only cause nonlethal mutations, some of them conceivably useful. More hits are likely to cause sufficient mutations to kill the microbe.
Application of Method:
The food industry has recently renewed it interest in the use of radiation for food preservation. It can be used to prevent spoilage in seafood by doses of 100 to 250 kilorads, in meats and poultry by doses of 50 to 100 kilorads, and in fruits by doses of 200 to 300 kilorads. (one kilorad equals 1000 rads) many consumers in the United States reject irradiation foods for fear o receiving radiation, but such foods are quite safe --- free of both pathogens and radiation. In Europe, mil and other foods are often irradiated to achieve sterility. Especially high energy electron beams, is used for the sterilization pharmaceuticals and disposable dental and medical supplies, such as plastic syringes, surgical gloves, suturing materials, and catheters. As a protection against Bioterrorism, the postal service often uses electron beam radiation to sterilize certain classes of mail. These radiations can be used to differentiate between Gram positive and negative bacteria. Gram-positive bacteria are more sensitive to ionizing radiations than gram-negative bacteria. Ionizing radiations are currently used to sterilize such heat sensitive pharmaceuticals as vitamins, hormones, and antibiotics, as well as certain plastics and suture materials.
Worldwide Importance and Controversy:
Ionizing radiations have also been approved for controlling microorganisms, and for preserving foods, as noted in MIcrofocus 21.4. The approval has generated much controversy, fueled by activists concerned about the safety of factory workers and consumers. First used in 1921 to inactivate Trichinella spiralis, the agent of trichinosis, irradiation is now used as a preservative in more than 40 countries for over 100 food items, including potatoes, onions, cereals, flour, fresh fruit, and poultry. The US Food and Drug Administration (FDA) approved cobalt-60 irradiation to preserve poultry in the early 1990s, and in 1997, it extended the approval to preserve red meat such as beef, lamb and pork.
Strong Visible Light: (Radiation Sterilization)
Sunlight has been known for years to have a bactericidal effect; nut the effect is due primarily to ultraviolet rays in the sunlight.
Introduction :
Strong visible light, which contains light of wavelength from 400 to 700 nm (violet to red light).
Mode of Action:
Visible light can have direct bactericidal effects by oxidizing light-sensitive molecules such as riboflavin and porphyrins (components of oxidative enzymes) in bacteria. For that reason, bacterial cultures should be exposed to strong light during laboratory manipulations.
Application of Strong Light:
The fluorescent dyes cosin and methylene blue can denature proteins in the presence of strong light because they absorb energy and cause oxidation of proteins and nucleic acids. The combination of a dye and strong light can used to rid materials of both with bacteria and viruses.
Microwave: (Radiation Sterilization)
Introduction :
Microwave radiation, in contrast with gamma, X ray, and ultraviolet radiation, falls at the longer wavelength end of the electro-magnetic spectrum. It has wavelengths of approximately 1 mm to 1 m, a range that includes television and police radar wavelengths.
Sterilization by Microwave Oven:
A specialized microwave oven has recently become available that can be used to sterilize media in just 10 minutes. It has 12 pressure vessels, each of which holds 100 ml of medium. Microwave energy increases the pressure of the medium inside the vessels until sterilizing temperatures are reached.
Applications of Microwaves:
Microwave oven frequencies are tuned to match energy levels in water molecules. In the liquid state, water molecules quickly absorbs the microwave energy and than release it to surrounding materials as heat. The molecules are set into high-speed motion, and the heat of friction is transferred of foods, which become hot rapidly. Thus, materials that do not contain water, such as plates made of paper, china, or plastic, remain cool while the moist food on them becomes heated.
Limitations of Microwave Ovens:
Other than the heat generated, there is no specific activity against microorganisms.
For this reason the home microwave cannot be used to sterilize items such as bandages and glassware. Conduction of energy is metals leads to problems such as sparking, which makes most metallic items also unsuitable for microwave sterilization. Moreover, bacterial endospores, which contain almost no water, are not destroyed by microwaves.
Laser:
A final form of radiation we shall consider is light energy. When concentrated by sophisticated devices, light energy forms a laser beam. The word laser is an acronym of light amplification by stimulated emission of radiation.
Advantage of Lasers
Recent experiments indicate that laser beams can be used to sterilize instruments and the air in operating rooms, as well as a wound surface. Microorganisms are destroyed in a fraction of a second, but the laser beam must reach all parts of the material to effect sterilization.
clip_image002
Caution:
Caution should be observed in cooking foods in the home microwave oven. Geometry and differences in density of the food being cooked can cause certain regions to become hotter than other, sometimes leaving very cold spots. Consequently, to cook foods thoroughly in a microwave oven, it is necessary to rotate the items either mechanically or by hand. For example, pork roasts must be turned frequently and cooked thoroughly to kill any cysts of the pork roundworm Trichinella. Failure to kill such cysts could lead to the disease trichinosis, in which cysts of the worm become embedded in human muscles and other tissues. All experimentally infected pork roasts, when microwaved without rotation, showed live worm remaining in some portion at the end of standard cooking time.
Radiation: A Sterilization Method
Nonionizing Radiation:
Nonionizing radiation has a wavelength longer than that or ionizing radiation, usually greater than about 1 nm. The best example of non ionizing radiation is ultraviolet (UV) light.
Mode of Action:
When microorganisms are subjected to UV light, cellular DNA absorbs the energy by purines and pyrimidine bases, and adjacent thymine molecules link together, as figure illustrates. Linked thymine molecules are unable to encode adenine on messenger RNA molecules during the process of protein synthesis. Moreover, replication of the chromosome in binary fission is impaired. The damaged organism can no longer produce critical proteins or reproduce, and it quickly dies. Ultraviolet light is especially effective in inactivating viruses. However, it kills far fewer bacteria than one might expect because of DNA repair mechanisms. Once DNA is repaired, new molecules of RNA and protein can be synthesized to replace the damaged molecules.
clip_image002
Conditions for Microbe control:
Ultraviolet (UV) light consists of light of wavelengths between 40 to 390 nm, but wavelength in the 200 nm range are most effective in killing microorganisms But according to some books wavelength between 260 – 265 nm is most effective.
Uses of Ultraviolet:
Ultraviolet light effectively reduces the microbial population where direct exposure takes place. It is used to limit airborne or surface contamination in a hospital room, morgue, pharmacy, toilet facility, or food service operation. In some communities, ultraviolet light is replacing chlorine in sewage treatment. When chlorine-treated sewage effluent is discharged into streams or other bodies of water, carcinogenic compounds form and may enter the food chain. The cost of removing chlorine before discharging treated effluent could add as much as $100 per year to the sewage bills of the average American family, and very few sewage plants do this. Running the sewage effluent under ultraviolet light before discharging it can destroy microorganism without altering the odor, pH, or chemical composition of the water and without forming carcinogenic compounds.
Advantages:
It does penetrate air, effectively reducing the number of airborne microorganism and killing them on surfaces on operating rooms and rooms that will contain caged animals. To help sanitize the air without irradiation humans, these lights can be turned on when there rooms are not in use. . Hanging laundry outdoors on bright, sunny days takes advantage of the ultraviolet light present in sunlight. Although the quantity of UV rays in sunlight is small, these rays may help kill bacteria on clothing, especially diapers.
Disadvantage of UV Light:
A major disadvantage of UV light as a disinfectant is that the radiation is not very penetrating, so the organism to be killed must be directly exposed to the rays. It is noteworthy microorganisms in the air and upper layers of the soil, but it may not the effective against all bacterial spores. Organisms protected by solids and such coverings as paper, glass, and textiles are not affected. Another potential problem is that UV light can damage human eyes, and prolonged exposure can cause burns and skin cancer in humans. And it may cause damage in human skin cells and permanent damage the eyes.
SUN--Free Source of UV:
Sunlight contains some UV radiation, but the shorter wavelengths – those most effective against bacteria – are screened out by the ozone layer of the atmosphere. The antimicrobial effect of sunlight is due almost entirely to the formation of singlet oxygen on the cytoplasm. Many pigments produced by bacteria provide protection from sunlight.