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Web Review of Todar's Online Textbook of Bacteriology. "The Good, the Bad, and the Deadly"

Tag words: bacterial growth, antibiotic, chemotherapeutic agent, disinfectant, antiseptic, preservative, control of growth, sterilization, pasteurization.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Control of Microbial Growth (page 2)

(This chapter has 6 pages)

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Irradiation: usually destroys or distorts nucleic acids. Ultraviolet light is commonly used to sterilize the surfaces of objects,  although x-rays, gamma radiation and electron beam radiation are also used.

Ultraviolet lamps are used to sterilize workspaces and tools used in microbiology laboratories and health care facilities. UV light at germicidal wavelengths (two peaks, 185 nm and 265 nm) causes adjacent thymine molecules on DNA to dimerize, thereby inhibiting DNA replication (even though the organism may not be killed outright, it will not be able to reproduce). However, since microorganisms can be shielded from ultraviolet light in fissures, cracks and shaded areas, UV lamps should only be used as a supplement to other sterilization techniques.

An ultraviolet sterilization cabinet.

Gamma radiation and electron beam radiation are forms of ionizing radiation used primarily in the
health care industry. Gamma rays, emitted from cobalt-60, are similar in many ways to microwaves and x-rays. Gamma rays delivered during sterilization break chemical bonds by interacting with the electrons of atomic constituents. Gamma rays are highly effective in killing microorganisms and do not leave residues or have sufficient energy to impart radioactivity.

Electron beam (e-beam) radiation, a form of ionizing energy, is generally characterized by low penetration and high-dose rates. E-beam irradiation is similar to gamma radiation in that it alters various chemical and molecular bonds on contact. Beams produced for e-beam sterilization are concentrated, highly-charged streams of electrons generated by the acceleration and conversion of electricity.

e-beam and gamma radiation are for sterilization of items ranging from syringes to cardiothoracic devices.

Filtration involves the physical removal (exclusion) of all cells in a liquid or gas. It is especially important for sterilization of solutions which would be denatured by heat (e.g. antibiotics, injectable drugs, amino acids, vitamins, etc.). Portable units can be used in the field for water purification and industrial units can be used to "pasteurize" beverages. Essentially, solutions or gases are passed through a filter of sufficient pore diameter (generally 0.22 micron) to remove the smallest known bacterial cells.

This water filter for hikers and backpackers is advertised to "eliminate Giardia, Cryptosporidium and most bacteria." The filter is made from 0.3 micron pleated glass fiber with a carbon core.

A typical set-up in a microbiology laboratory for filtration sterilization of medium components that would be denatured or changed by heat sterilization. The filter is placed (aseptically) on the glass platform, then the funnel is clamped and the fluid is drawn by vacuum into a previously sterilized flask. The recommended size filter that will exclude the smallest bacterial cells is 0.22 micron.

Chemical and gas

Chemicals used for sterilization include the gases ethylene oxide and formaldehyde, and liquids such as glutaraldehyde. Ozone, hydrogen peroxide and peracetic acid are also examples of chemical sterilization techniques are based on oxidative capabilities of the chemical.

Ethylene oxide (ETO) is the most commonly used form of chemical sterilization. Due to its low boiling point of 10.4ºC  at atmospheric pressure, EtO) behaves as a gas at room temperature. EtO chemically reacts with amino acids, proteins, and DNA to prevent microbial reproduction. The sterilization process is carried out in a specialized gas chamber. After sterilization, products are transferred to an aeration cell, where they remain until the gas disperses and the product is safe to handle.

ETO is used for cellulose and plastics irradiation, usually in hermetically sealed packages.  Ethylene oxide can be used with a wide range of plastics (e.g. petri dishes, pipettes, syringes, medical devices, etc.) and other materials without affecting their integrity.

An ethylene oxide sterilization gas chamber.

Ozone sterilization has been recently approved for use in the U.S. It uses oxygen that is subjected to an intense electrical field that separates oxygen molecules into atomic oxygen, which then combines with other oxygen molecules to form ozone.

Ozone is used as a disinfectant for water and food. It is used in both gas and liquid forms as an antimicrobial agent in the treatment, storage and processing of foods, including meat, poultry and eggs. Many municipalities use ozone technology to purify their water and sewage. Los Angeles has one of the largest municipal ozone water treatment plants in the world. Ozone is used to disinfect swimming pools, and some companies selling bottled water use ozonated water to sterilize containers.

An ozone fogger for sterilization of egg surfaces. The system reacts ozone with water vapors to create powerful oxidizing radicals. This system is totally chemical free and  is effective against bacteria, viruses and hazardous microorganisms which are deposited on egg shells.

An ozone sterilizer for use in the hospital or other medical environment.

Low Temperature Gas Plasma (LTGP) is used as an alternative to ethylene oxide. It uses a small amount of liquid hydrogen peroxide (H2O2), which is energized with radio frequency waves into gas plasma. This leads to the generation of free radicals and other chemical species, which destroy organisms.

An LTGP sterilizer that pumps vaporized H2O2 into the chamber.

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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