Sterilization

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Sterilization

Introduction

Sterilization refers to processes that eliminate microorganisms from surfaces, the interior of equipment, foods, and liquids. A catheter is one example of a device whose surface must be completely sterilized before it is inserted in a person to deliver food or medicine. A cardiac pacemaker is another example of a device whose surface must be sterilized before it is implanted inside the body to control the heart rate. A common example of a liquid requiring sterilization is the liquid-based nutrient medium used to grow bacteria in the laboratory.

Sterilization is intended to eliminate living microorganisms such as bacteria, fungi, and protozoa, along with nonliving microbes such as viruses that, given the appropriate host, can cause disease.

There are various methods of sterilization depending on the aim of the procedure. For example, surgical instruments are sterilized to guarantee the absence of pathogens (disease-causing organisms), while most laboratory growth media used for experimentation is sterilized to guarantee the absence of bacteria. Pharmaceutical medications also need to be free of all potential disease-causing agents.

WORDS TO KNOW

AUTOCLAVE: An autoclave is a device that is designed to kill microorganisms on solid items and in liquids by exposure to steam at a high pressure.

PATHOGEN: A disease causing agent, such as a bacteria, virus, fungus, etc.

History and Scientific Foundations

Heat has been used to sterilize objects for millennia. Until a few centuries ago, the flame of a fire was used. Today, the most common method of heat sterilization employs high temperature and pressure above atmospheric pressure. The combination of temperature and pressure most efficiently heats an object or a volume of liquid; exposure of the samples for a certain period of time has proved sufficient to kill even hardy microorganisms.

An autoclave is the most common instrument used for heat sterilization. Autoclaves range in size from small units that easily fit on the bench-top of a laboratory to large units that have the volume of an average kitchen refrigerator. An autoclave uses steam to sterilize the objects place inside it. Most typically, the steam is pumped into the autoclave chamber at a temperature of 250°F (121°C) and a pressure that is 15 pounds per square inch above atmospheric pressure. These conditions of temperature and pressure are maintained for at least 15 minutes (larger loads or greater volumes of liquid can be held longer). Then, the steam is released from the chamber in a controlled and safe manner. The chamber can be opened when it has returned to atmospheric pressure.

As a control over the process, indicator tape can be applied to the objects being autoclaved. Bands form on the tape when the proper conditions of temperature, pressure, and time have been attained. This helps the operator judge whether sterilization has been successful. To be even more certain, solutions that contain bacterial spores of Bacillus stearothermophilus can be included with the load being sterilized. Bacterial spores are very resistant to the killing action of heat. If the liquid containing the spores is incubated in a suitable liquid growth source and the medium remains clear, it indicates that sterilization was successful. In addition to the use of this so-called bio-indicator, many autoclaves record the temperature profile of each sterilization cycle, allowing the user to visually monitor whether the appropriate conditions were achieved.

Monitoring of autoclave performance is important. If, for example, too many items are autoclaved at once, the overcrowded conditions may not allow the steam to effectively penetrate the entire load, which can result in inadequate sterilization.

Although autoclaving is an effective method of sterilization with many applications, it is not foolproof. For example, it has been shown that prions—proteins whose abnormal folding stimulates damage of brain cells causing several similar diseases termed transmissible spongiform encephalopathies (an example is Creutzfeld-Jacob disease)—can remain potent following autoclaving of surgical instruments. Even the combination of autoclaving with chemicals has proven ineffective against prion contamination. The World Health Organization has recommended that surgery on patients suspected of having a prion-related disease be performed with disposable instruments, or that the instruments used be incinerated after the surgery is completed.

There are other methods of sterilization. Some metal objects can be surface-sterilized by holding them in an open flame. This is a common way of sterilizing a loop of metal called an inoculating loop that is used to transfer microorganisms from one place to another during experiments. Burning trash that contains medical waste is another way of sterilizing the residual ash. This ash can then be safety disposed of. Medical incinerators must be specially designed to retain the vapor given off, since infectious material can potentially be carried into the air during incineration. A common method for sterilizing drinking water is boiling. Boiling kills most bacteria and inactivates most viruses. However, agents like prions and some spore-forming bacteria can survive boiling for 15 minutes. However, in most situations, boiling is better than not treating water at all before drinking it.

Objects such as plastic, optical equipment, and electrical circuits that cannot stand heat can be sterilized using a chemical called ethylene oxide. This chemical is applied as a gas in a specialized machine. Another sporeforming bacterium, Bacillus subtilus, is used to monitor the success of ethylene oxide sterilization.

Ozone is another chemical sterilizer. Drinking water can be sterilized using ozone. A diluted solution of bleach (sodium hypochlorite) is an especially useful sterilizing agent for work surfaces. Other chemical sterilizers include glutaraldehyde and hydrogen peroxide.

Applications and Research

Sterilization is a necessity of everyday life in research, health care, and even the supermarket. Ongoing investigations seek to develop chemicals that sterilize more efficiently and quickly, while being safe to use.

IN CONTEXT: BACTERIA SPORES SPUR STERILIZATION RESEARCH

The discovery of bacteria that are resistant to sterilization could potentially contaminate experiments and environments studied by NASA and other space agency probes. One species was tentatively named B. odysseensis after being isolated from the surfaces of the Mars Odyssey spacecraft following routine sterilization. The method of spore formation is suspected of having a role in resistance of spore-forming bacteria to sterilization.

Earth-bound benefits of such research offer hope of improved methods of sterilization and prevention of unintentional contamination.

SOURCE: Jet Propulsion Laboratory, National Aeronautics and Space Administrations (NASA)

Impacts and Issues

Without the ability to sterilize growth media, many scientific experiments could not be accomplished, as it would be impossible to know if the results were due to the organism being studied or to a contaminant. In addition, surgeries would have a very poor survival rate, as was the case in the days before sterilization techniques were routinely employed.

In some instances, standard sterilization methods have not been sufficient to protect patients from developing disease as a result of contact with contaminated medical equipment. An example occurred in 2003 in the Canadian province of New Brunswick, where seven patients probably contracted Creutzfeld-Jacob disease from prion-contaminated instruments used during their neurosurgeries. The instruments became contaminated when used in an operation on a patient who was subsequently found to have the disease. The instruments were routinely sterilized, but it later became clear that this routine sterilization procedure did not remove the prions. It was this outbreak that resulted in the institution of hospital protocols mandating the use of disposable instruments during procedures on high-risk areas such as the brain and spinal cord involving patients suspected of having a prion-related disease.

Recently, scientists have found that prions can be digested by a particular enzyme. Enzymatic treatment of medical instruments has shown promise as sterilization technique effective for prions, but it is not yet in general use.

While important, the quest for sterilization can go too far. One example is the marketing of surface sterilizing products designed for use in the kitchen and bathroom. Bacteria that are not killed by these products have the potential to become resistant to the particular chemical, which can make these bacteria a more serious concern than before they developed this resistance.