Important pathogenic implications. Mycobacterium Therefore gamma and X-radiations are referred to as

tuberculosis is a classic example of an organism capa- ionizing radiations. Free radicals are highly reactive

ble of withstanding severe desiccation and still re- chemical species that can lead to polymerization and

maining infective. In contrast, Treponema pallidum, the other chemical reactions disruptive totFte biochemi

bacterium that causes syphilis, is extremely sensitive cal organization of microorganisms. Viruses and

to drying and dies almost instantly in the air or on a other microorganisms are inactivated by exposure to

dry surface. ionizing radiation (FIG. 11-7).

Some microorganisms produce specialized spores that can withstand the desiccating conditions of the Exposure to radiation may cause microbial muta-

, R, , „ , ,1 . I II tions, induces the formation of toxic free radicals,

atmosphere. Such spores generally have thick walls _{inereases micro}bial death rates, and is used as a

that retain moisture within the cell. Many fungal sterilization method.

spores can be transmitted over long distances

through the atmosphere; some spores even travel Sensitivities to ionizing radiation vary. Nonrepro- from one continent to another. The transmission of ducing (dormant) stages of microorganisms tend to fungal spores through the air is a serious problem in be more resistant to radiation than growing organ- agriculture because it permits the spread of fungal isms. For example, endospores are more resistant diseases of plants from one field to another. than the vegetative cells of many bacterial species.

Salem Witch Hunts

In the 1690s, witch hunts were a regular practice in Salem and surrounding regions of New England. People accused of being witches were burned at the stake. In most cases, these people were accused of casting spells or performing supernatural acts. Re­cent scientific evidence suggests that the accusors were suffering from ergotism, a form of food poison­ing that is characterized by hallucinations. The dis­ease occurs when fungi grow on grains and produce chemicals called ergot alkaloids. Apparently, exces­sive rain wet the grains and without other means of food preservation, the fungi were able to grow on them. The spoilage of grains was widespread, which explains the occurrence of mass hallucinations. The accusors were hallucinating, which is why they thought they saw witches.

FIG. 11 -7 Ionizing radiation effectively kills microorgan­isms, including viruses.

CONTROL OF MICROORGANISMS BY PHYSICAL ENVIRONMENTAL FACTORS 319

HIGHLIGHT

Safety of Irradiated Food

Research on irradiation as a food preservation technol­ogy began after World War II when the U.S. Army be­gan a series of experiments irradiating fresh foods for troops in the field. Since 1963, the United States Food and Drug Administration (FDA) has passed rules per­mitting irradiation to curb insects in food and microor­ganisms in spices, control parasites in pork, and retard spoilage in fruits and vegetables. On May 2, 1990, the FDA approved the use of irradiation as a safe and effec­tive means to control a major source of foodborne ill­ness— Salmonella and other foodborne bacteria in raw chicken, turkey, and other poultry. Food safety ex­perts believe that up to 60% of all poultry sold in the United States is contaminated with Salmonella and that perhaps all chicken may be contaminated with Campy­lobacter organisms. Eating poultry contaminated with these organisms may cause disease, with symptoms ranging from a simple stomach ache to incapacitating stomach and intestinal disorders, occasionally resulting in death.

Although the FDA has concluded that irradiation of food is safe, the pubUc remains frightened by any use of radiation. They are fearful that irradiated foods may be contaminated and carry dangerous radioactivity. They associate radiation with atomic bombs and nuclear re­actor accidents like Chernobyl and Three Mile Island. Action groups have formed to block the distribution of foods sterilized by irradiation. Three states (Maine, New York, and New Jersey) have banned or issued moratoriums on the sale of irradiated foods. Irradiation

opponents charge that the FDA, the World Health Or­ganization, and the nuclear power industry are conspir­ing to promote the technique as a way to dispose of nu­clear waste.

To counter fears about radiation, the FDA points out that irradiation does not make food radioactive. The specified exposure times and energy levels of radiation sources approved for food cannot induce radioactivity in the food. Food irradiation does not leave a residue that is harmful to people. It removes potentially harm­ful pathogens and food spoilage microorganisms. Dur­ing the irradiation process, the genetic material of bac­teria is damaged such that they can no longer survive or multiply. No radioactive material is ever added to the product. The same technique is used to sterilize many disposable medical devices.

The FDA requires that irradiated foods be labeled as such so that consumers know what they are buying. A mandatory logo was added in 1986. It consists of a solid circle, representing an energy source, above two petals, which represent food. Like the FDA, the World Health Organization concludes that irradiation can substan­tially reduce food poisoning. It sees the use of irradia­tion as a means of reducing food cost because it can re­duce food spoilage. The first major food irradiation plants are currently under construction. The success of these plants will depend on consumer acceptance of the products. You can expect to see foods labeled with the FDA-required labels soon on the shelves of your food market.

Exposure to 0.3 to 0.4 Mrads (million units of radia­tion) is necessary to cause a tenfold reduction in the number of viable bacterial endospores. An exception is the bacterium Micrococcus radiodurans, which is particularly resistant to exposure to ionizing radia­tion. Vegetative cells of M. radiodurans tolerate as much as 1 Mrad of exposure to ionizing radiation with no loss of viability. It appears that efficient DNA repair mechanisms are responsible for the high de­gree of resistance to radiation exhibited by this bac­terium.

Ionizing radiation is used to pasteurize or sterilize some products. Most commercially produced plastic Petri plates are sterilized by exposure to gamma ra­diation. Foods also sometimes are sterilized in this manner. Most such sterilization procedures employ gamma radiation from ⁶⁰Co or ¹³⁷Ce. Bacon, for ex­ample, can be sterilized by using radiation doses of 4.5 to 5.6 Mrads.

Unlike gamma radiation, ultraviolet light (UV) does not have high penetrating power. It is useful for

killing microorganisms only on or near the surface of clear solutions. The strong germicidal wavelength of 260 nanometers coincides with the absorption maximum of DNA, suggesting that the principal mechanism by which ultraviolet light exerts its lethal effect is through the disruption of the DNA. Micro­organisms have several mechanisms that can repair the alterations in the DNA that are caused by ex­posure to ultraviolet light, limiting the effectiveness of using UV exposure to control microbial popula­tions. Exposure to ultraviolet light sometimes is used to maintain the sterility of some surfaces. In some hospitals, benchtops are maintained bacteria-free when not in use by using an ultraviolet lamp. The dangers involved in human exposure to excess ultra­violet radiation include blindness if light is viewed directly.