Radiation as a Microbial Control Agent
Another form of energy that can serve as an antimicrobial agent is radiation. Radiation is defined as energy emitted from atomic activities and dispersed at high velocity through matter or space. It is characterized by a range of wavelengths known as the electromagnetic spectrum (figure 1). Although radiation exists in many energy states, we will consider only those types suitable for microbial control: gamma rays, X-rays, and ultra violet radiation.
Fig1. The electromagnetic spectrum. Waves range from the longest radio waves to the shortest gamma rays. The frequency of the wave (measured in Hertz, Hz) is simply the number of wave cycles that occur in 1 second. Only ionizing radiation (X-rays and gamma rays), ultraviolet and infrared have applications in microbial control. Fouad A. Saad/Shutterstock
Modes of Action of Ionizing Versus Nonionizing Radiation
The actual physical effects of radiation on microbes can be under stood by visualizing the process of irradiation, or bombardment with radiation, at the cellular level (figure 2). When a cell is bombarded by certain waves or particles, its molecules absorb some of the available energy, leading to one of two consequences: (1) If the radiation ejects orbital electrons from an atom, it causes ions to form; this type of radiation is termed ionizing radiation. One of the most sensitive targets for ionizing radiation is DNA, which will undergo mutations on a broad scale. Secondary lethal effects appear to be chemical changes in organelles and the production of toxic substances. Gamma rays, X-rays, and high-speed electrons are all ionizing in their effects. (2) Nonionizing radiation, best exemplified by UV, excites atoms by raising them to a higher energy state, but it does not ionize them. This atomic excitation, in turn, leads to the formation of abnormal bonds within molecules such as DNA and is thus a source of mutations (see figure 4).
Fig2. Cellular effects of irradiation. (a) Ionizing T A radiation penetrates a cell, dislodging electrons from molecules throughout the cytoplasm and causing DNA to shatter. Without the ability to replicate DNA, organisms that could cause spoilage or illness cannot grow. (b) Nonionizing radiation enters a cell, strikes molecules, and excites them. The effect on DNA is the creation of abnormal bonds, primarily between adjacent thymine bases (thymine dimers). These abnormal bases distort the DNA helix and interfere with replication and transcription, preventing growth of the targeted cell. An additional difference between the two processes is that ionizing radiation can penetrate solid barriers, while nonionizing radiation acts only at the surface. Barry Chess/McGraw Hill
Ionizing Radiation: Gamma Rays and X-Rays
Over the past several years, ionizing radiation has become safer and more economical to use, and its applications have expanded. It is a highly effective alternative for sterilizing materials that are sensitive to heat or chemicals. Because it sterilizes in the absence of heat, irradiation is a type of cold (or low-temperature) sterilization. Devices that emit ionizing rays include gamma-ray machines containing radioactive cobalt and X-ray machines similar to those used in medical diagnosis. Items are placed in these machines and irradiated for a short time with a carefully chosen dosage. The dosage of radiation is measured in grays. Depending on the ap plication, exposure ranges from 5 to 50 kilograys (kGy; a kilogray is equal to 1,000 grays). Although all ionizing radiations can penetrate liquids and most solid materials, gamma rays are more penetrating and X-rays less so.
Irradiation as a Method of Microbial Control
Foods have been subject to irradiation in limited circumstances for more than 50 years. From flour to pork and ground beef, to fruits and vegetables, radiation is used to kill bacterial pathogens, insects, and worms, and even to inhibit the sprouting of potatoes. As soon as radiation is in volved, however, consumers become concerned that ionizing radiation could make food less nutritious, unpalatable, or even unsafe. But irradiated food has been extensively studied, and each of these concerns has been addressed. Figure 3 demonstrates how irradiation of fruit can greatly reduce spoil age and improve shelf life.
Fig3. Sterilization with ionizing radiation. Many foods can be effectively sterilized by utilizing the penetrating power of ionizing radiation. The “radura” symbol (meaning treated with radiation; see the green image on the package on the left) applied to fresh foods signifies that the food has been irradiated and is required to be displayed on all food so treated. T (inset): U.S. Food and Drug Administration; (raspberries): McGraw Hill
Irradiation may lead to a small decrease in the amount of thiamine (vitamin B1) in food, but this change is small enough to be inconsequential. The irradiation process does produce short lived free-radical oxidants, which disappear almost immediately (this same type of chemical intermediate is produced through cooking as well). Certain foods do not irradiate well and are not good candidates for this type of treatment. The whites of eggs become milky, and alfalfa seeds do not germinate properly. It should be emphasized that food is not made radioactive by the irradiation process, and many studies, in both animals and hu mans, have concluded that there are no ill effects from eating irradiated food. In fact, NASA relies on irradiated meat for its astronauts.
One of the potential benefits of irradiated foods has to do with infection control. It has been estimated that irradiation of 50% of the meat and poultry in the United States would result in 900,000 fewer cases of infection, 8,500 fewer hospitalizations, and 350 fewer deaths each year. Radiation is currently approved in the United States for the reduction of bacterial pathogens such as E. coli and Salmonella in beef and chicken, reduction of Trichinella worms in pork, and the reduction of pathogens and pests on fruits and vegetables. An additional benefit of irradiation is that microbes responsible for food spoilage are killed along with pathogens, leading to an increased shelf life. In any event, no irradiated food can be sold to consumers without clear labeling that this method has been used (figure 3).
Sterilizing medical products with ionizing radiation has become routine. Drugs, vaccines, medical instruments (especially plastics), and other delicate materials can be irradiated without harming them. Its main advantages include speed, high penetrating power (it can sterilize materials through outer packages and wrappings), and the absence of heat. Its main disadvantages are potential dangers from factory exposure to radiation and possible damage to some materials. See table 1 for a summary of the ap plications of ionizing radiation.
Table1. Applications Using Radiation for Sterilization and Disinfection
Nonionizing Radiation: Ultraviolet Rays
Ultraviolet (UV) radiation ranges in wavelength from approximately 100 nm to 400 nm. It is most lethal from 240 nm to 280 nm (with a peak at 260 nm). In everyday practice, the source of UV rays is the germicidal lamp, which generates radiation at 254 nm. Owing to its lower energy state, UV radiation is not as penetrating as ionizing radiation. Because UV radiation passes readily through air, slightly through liquids, and only poorly through solids, the object to be disinfected must be directly exposed to it for full effect.
As UV radiation passes through a cell, it is initially absorbed by DNA. Specific molecular damage occurs on the pyrimidine bases (thymine and cytosine), which form abnormal linkages with each other called pyrimidine dimers. These bonds occur between adjacent bases on the same DNA strand and interfere with normal DNA replication and transcription. The results are inhibition of growth and cellular death. In addition to altering DNA directly, UV radiation disrupts cells by generating toxic photochemical products called free radicals. These highly reactive molecules interfere with essential cell processes by binding to DNA, RNA, and proteins. Ultraviolet rays are a powerful tool for destroying fungal cells and spores, bacterial vegetative cells, protozoa, and viruses. Bacterial spores are about 10 times more resistant to radiation than are vegetative cells, but they can be killed by increasing the time of exposure.
Ultraviolet radiation is usually aimed more at disinfection than at sterilization. Germicidal lamps can cut down on the concentration of airborne microbes by as much as 99%. Ultraviolet disinfection of air has recently become far more widely used. Spurred by the COVID-19 pandemic, ultraviolet light systems are now commonly built into air conditioning and ventilation equipment to lessen the airborne spread of pathogens. A variety of portable, high-intensity UV lamps have been introduced for use in hospitals, sports stadiums, subway cars, and shopping malls (figure 4). For small areas, like hospital rooms, the lights can be turned on for several minutes (for safety reasons, no one can remain in the room while the light is on), after which employees may enter the room to flip the mattress and call button along with opening drawers and cabinets. The room is then subjected to a second light treatment. For large areas, like shopping malls and sports stadiums, self-propelled UV robots wander the building for hours after the crowd has gone home, bathing every surface in microbe-killing ultraviolet light.
Fig4. Nonionizing radiation. Portable ultraviolet units can be used to disinfect hospital rooms, train cars, and airplanes. Self propelled units can be used to disinfect larger spaces like warehouses and shopping malls. Halfway through the disinfection process, drawers and cabinets will be opened to allow light to reach interior surfaces because ultraviolet light cannot penetrate solid barriers. Jbruiz/Shutterstock
Ultraviolet irradiation of liquids requires special equipment to spread the liquid into a thin, flowing film that is exposed directly to a lamp. This method can be used to treat water (figure 5) and to purify other liquids (milk and fruit juices) as an alternative to heat. The surfaces of solid, nonporous materials such as walls and floors, as well as meat, nuts, tissues for grafting, and drugs, can be successfully disinfected with UV. Several types of small UV disinfection appliances have been marketed for per sonal items.
Fig5. An ultraviolet (UV) treatment system for disinfection of water. Water is directed through channels at a water treatment plant, past racks of UV lamps (green glow). This system has a capacity of several million gallons per day and can be used as an alternative to chlorination. Home systems that fit under the sink are also available. Dmitry Rogachev/Shutterstock
One major disadvantage of UV is its poor powers of penetration through solid materials such as glass, metal, cloth, plastic, and even paper. Another drawback to UV is the damaging effect of overexposure on human tissues, including sunburn, retinal damage, cancer, and skin wrinkling. These harmful results occur only if a person is directly exposed to the UV rays, but it necessitates placement of barriers to cut down on this possibility. Table 1 summarizes UV applications.
Filtration—A Physical Removal Process
Filtration is an effective method to remove microbes from air and liquids. In practice, a fluid is strained through a filter with openings large enough for the fluid to pass through but too small for microorganisms to pass through (figure 6a).
Fig6. Membrane filtration. (a) Vacuum assembly for achieving filtration of liquids through suction. Inset shows filter as seen in cross section, with tiny passageways (pores) too small for the microbial cells to enter but large enough for liquid to pass through. (b) Scanning electron micrograph of Enterococcus trapped on the surface of a membrane filter (10,400×). Average cells measure about 0.5 μm to 1 μm in diameter, and average pore size is about 0.3 μm. (b) (b): Janice Haney Carr/CDC
Most modern microbiological filters are thin membranes of cellulose acetate, polycarbonate, and a variety of plastic materials (Teflon, nylon) whose pore size can be carefully controlled and standardized. Ordinary substances such as charcoal, diatomaceous earth, or unglazed porcelain are also used in some applications. Viewed microscopically, most filters are perforated by very precise, uniform pores (figure 11.13b). The pore diameters vary from coarse (8 μm) to ultrafine (0.02 μm), permitting selection of the minimum particle size to be trapped. Those with even smaller pore diameters permit true sterilization by removing viruses, and some will even remove large proteins. A sterile liquid filtrate is typically produced by suctioning the liquid through a sterile filter into a presterilized container. These filters are also used to separate mixtures of microorganisms and to enumerate bacteria in water analysis.
Applications of Filtration Sterilization
Sterilization by means of filtration is used to prepare liquids that cannot withstand heat, including serum and other blood products, vaccines, drugs, IV fluids, enzymes, and media. Filtration has been employed as an alternative method for sterilizing milk and beer without altering their flavor. It is also an important step in water purification. Its use extends to filtering out particulate impurities (crystals, fibers) that can cause severe reactions in the body. It has the disadvantage of not removing soluble molecules (toxins) that can cause disease.
Filtration is a very effective means of removing airborne contaminants that are a common source of infection and spoil age. High-efficiency particulate air (HEPA) filters are widely used to provide a flow of sterile air to hospital rooms and sterile rooms.
