Controlling Microbial Growth

Radiation

Ultraviolet light, ionizing radiation, and microwave ovens are all used to control microbial growth.

Electromagnetic radiation encompasses all light, both in the visible spectrum and beyond. Short-wavelength radiation is associated with high energy, while long-wavelength radiation is associated with low energy. Radiation can be harmful to organisms and can thus be used to disinfect and sterilize surfaces and fluids.

Ultraviolet (UV) radiation occurs at wavelengths of light between 10 and 400 nanometers, just beyond the range of the visible spectrum. UV light at 100 to 280 nanometers is effective at killing microbes and inactivating viruses, with the peak being 265 nanometers. This light damages proteins and genetic material by disrupting their molecular bonds. In DNA, UV light causes the formation of dimers, two of the same molecule bonded together, in adjacent bases of C-C, C-T, and T-T within the same DNA strand. Disrupting the chemical bonds in DNA causes microbes to be unable to perform basic cellular functions. Bacteria and fungi contain cellular mechanisms that repair damaged DNA, so UV light is more effective against viruses, which do not have these mechanisms. UV light does not pass through most materials, including glass, paper, and plastic, which limits its use, but it does penetrate air. UV light is given off by the sun, making exposure to sunlight sufficient to control microbes on surfaces such as clothing. UV light is harmful to skin and eyes, so care must be taken when working with UV lights. UV light bulbs lose efficiency over time and need to be replaced often in order to maintain effectiveness.
When ultraviolet (UV) light strikes a cell or virus, it damages its DNA by causing a reaction between two molecules of thymine, one of the four bases of DNA, that form a stable dimer. This kills bacteria and inactivates viruses.
Ionizing radiation includes X-rays, which occur at wavelengths of light between 0.1 and 10 nanometers (about the size of an atom), and gamma rays, which occur at wavelengths of light shorter than 0.01 nanometers. The term ionizing radiation describes the effects this radiation can have—it can cause electrons to be discharged from molecules, creating ions. Ionizing radiation damages DNA and RNA and also stimulates formation of oxides, powerful oxidizing agents that further damage cells. Ionizing radiation is dangerous to all forms of life, but the amount of radiation that causes damage depends greatly on the size of the organism. Because microbes are so much smaller than other organisms, radiation sufficient to kill microbes can still be safe for other living things. Many foods, including fruits, vegetables, seafood, poultry, and meats, are irradiated to preserve them. Ionizing radiation is also used to sterilize medical equipment and pharmaceuticals. Contrary to popular belief, foods and medicines that undergo irradiation in this manner do not themselves become radioactive.

Microwave radiation occurs at wavelengths between 1 millimeter and 1 meter. Microwave ovens use frequencies that excite water molecules, causing them to quickly absorb energy and release it as heat. Sufficient heat can be released to kill many microbes. Materials lacking water, such as porcelain and paper, thus do not heat up in microwave ovens. Microwave ovens heat materials unevenly, so materials placed in them must be physically rotated, and solids often retain cool areas. Thus microwave ovens are useful only for disinfection, not sterilization.