Sterilization is a process that kills all forms of microbial life, including bacterial spores. Disinfection is a process that destroys pathogenic organisms, but not necessarily all microorganisms or spores. Sterilization and disinfection may be accomplished by physical or chemical methods.

METHODS OF STERILIZATION

The physical methods of sterilization include:

• Incineration

• Moist heat

• Dry heat

 • Filtration

• Ionizing (gamma) radiation

Incineration is the most common method of treating infectious waste. Hazardous material is literally burned to ashes at temperatures of 870° to 980°C. Incineration is the safest method to ensure that no infective materials remain in samples or containers when disposed. Prions, infective proteins, are not eliminated using conventional methods. Therefore incineration is recommended. Toxic air emissions and the presence of heavy metals in ash have limited the use of incineration in most large U.S. cities.

Moist heat (steam under pressure) is used to sterilize biohazardous trash and heat-stable objects; an autoclave is used for this purpose. An autoclave is essentially a large pressure cooker. Moist heat in the form of saturated steam under 1 atmosphere (15 psi [pounds per square inch]) of pressure causes the irreversible denaturation of enzymes and structural proteins. The most commonly used steam sterilizer in the microbiology laboratory is the gravity displacement type (Figure 1). Steam enters at the top of the sterilizing chamber; because steam is lighter than air, it displaces the air in the chamber and forces it out the bottom through the drain vent. The two common sterilization temperatures are 121°C (250°F) and 132°C (270°F). Items such as media, liquids, and instruments are usually autoclaved for 15 minutes at 121°C. Infectious medical waste, on the other hand, is often sterilized at 132°C for 30 to 60 minutes to allow penetration of the steam throughout the waste and the displacement of air trapped inside the autoclave bag. Moist heat is the fastest and simplest physical method of sterilization.

Fig1. Gravity displacement type of autoclave. A, Typical Eagle Century Series sterilizer for laboratory applications. B, Typical Eagle 3000 sterilizer piping diagram. The arrows show the entry of steam into the chamber and the displacement of air. (Courtesy AMSCO International, a subsidiary of STERIS Corp., Mentor, Ohio.)

Dry heat requires longer exposure times (1.5 to 3 hours) and higher temperatures than moist heat (160° to 180°C). Dry heat ovens are used to sterilize items such as glassware, oil, petrolatum, or powders. Filtration is the method of choice for antibiotic solutions, toxic chemicals, radioisotopes, vaccines, and carbohydrates, which are all heat sensitive. Filtration of liquids is accomplished by pulling the solution through a cellulose acetate or cellulose nitrate membrane with a vacuum. Filtration of air is accomplished using high-efficiency particulate air (HEPA) filters designed to remove organ isms larger than 0.3 µm from isolation rooms, operating rooms, and biologic safety cabinets (BSCs). The ionizing radiation used in microwaves and radiograph machines is composed of short wavelength and high-energy gamma rays. Ionizing radiation is used for sterilizing disposables such as plastic syringes, catheters, or gloves before use. The most common chemical sterilant is ethylene oxide (EtO), which is used in gaseous form for sterilizing heat sensitive objects. Formaldehyde vapor and vapor-phase hydrogen peroxide (an oxidizing agent) have been used to sterilize HEPA filters in BSCs. Glutaraldehyde, which is sporicidal (kills spores) in 3 to 10 hours, is used for medical equipment such as bronchoscopes, because it does not corrode lenses, metal, or rubber. Peracetic acid, effective in the presence of organic material, has also been used for the surface sterilization of surgical instruments. The use of glutaraldehyde or peracetic acid is called cold sterilization.

METHODS OF DISINFECTION

Physical Methods of Disinfection

The three physical methods of disinfection are:

• Boiling at 100°C for 15 minutes, which kills vegetative bacteria

• Pasteurizing at 63°C for 30 minutes or 72°C for 15 seconds, which kills food pathogens without dam aging the nutritional value or flavor

• Using nonionizing radiation such as ultraviolet (UV) light

UV rays are long wavelength and low energy. They do not penetrate well, and organisms must have direct surface exposure, such as the working surface of a BSC, for this form of disinfection to work.

Chemical Methods of Disinfection

 Chemical disinfectants comprise many classes, including:

 • Alcohols

 • Aldehydes

• Halogens

• Heavy metals

• Quaternary ammonium compounds

 • Phenolics

Chemicals used to destroy all life are called chemical sterilants, or biocides; however, these same chemicals, used for shorter periods, act as disinfectants. Disinfectants used on living tissue (skin) are called antiseptics.

A number of factors influence the activity of disinfectants, including:

• Types of organisms present

 • Temperature and pH of process

• Number of organisms present (microbial load)

 • Concentration of disinfectant

• Amount of organics present (blood, mucus, pus)

• Nature of surface to be disinfected (e.g., potential for corrosion; porous or nonporous surface)

• Length of contact time

 • Type of water available (hard or soft)

Resistance to disinfectants varies with the type of microorganism. Bacterial spores, such as Bacillus spp., are the most resistant, followed by mycobacteria (acid fast bacilli); nonenveloped viruses (e.g., poliovirus); fungi; vegetative (nonsporulating) bacteria (e.g., gram negative rods); and enveloped viruses (e.g., herpes simplex virus), which are the most susceptible to the action of disinfectants. The Environmental Protection Agency (EPA) registers chemical disinfectants used in the United States and requires manufacturers to specify the activity level of each compound at the working dilution. Therefore, microbiologists who must recommend appropriate disinfectants should check the manufacturer’s cut sheets (product information) for the classes of microorganisms that will be killed. Generally, the time necessary for killing microorganisms increases in direct proportion to the number of organisms (microbial load). This is particularly true of instruments contaminated with organic material such as blood, pus, or mucus. The organic material should be mechanically removed before chemical sterilization to decrease the microbial load. This is analogous to removing dried food from utensils before placing them in a dishwasher, and it is important for cold sterilization of instruments such as bronchoscopes.

The type of water and its concentration in a solution are also important. Hard water may reduce the rate of killing of microorganisms. In addition, 70% ethyl alcohol is more effective as a disinfectant than 95% ethyl alcohol because the increased water (H2O) hydrolyzing bonds in protein molecules make the killing of microorganisms more effective.

Ethyl or isopropyl alcohol is nonsporicidal (does not kill spores) and evaporates quickly. Therefore, its use is limited to the skin as an antiseptic or on thermometers and injection vial rubber septa as a disinfectant.

Because of their irritating fumes, the aldehydes (form aldehyde and glutaraldehyde) are generally not used as surface disinfectants.

The halogens, especially chlorine and iodine, are frequently used as disinfectants. Chlorine is most often used in the form of sodium hypochlorite (NaOCl), the com pound known as household bleach. The Centers for Disease Control and Prevention (CDC) recommends that tabletops be cleaned after blood spills with a 1 : 10 dilution of bleach.

Iodine is prepared either as a tincture with alcohol or as an iodophor coupled to a neutral polymer (e.g., povidone-iodine). Both iodine compounds are widely used antiseptics. In fact, 70% ethyl alcohol, followed by an iodophor, is the most common compound used for skin disinfection before drawing blood specimens for culture or surgery.

Because mercury is toxic to the environment, heavy metals containing mercury are no longer recommended, but an eye drop solution containing 1% silver nitrate is still placed in the eyes of newborns to prevent infections with Neisseria gonorrhoeae.

Quaternary ammonium compounds are used to disinfect bench tops or other surfaces in the laboratory. However, surfaces grossly contaminated with organic materials, such as blood, may inactivate heavy metals or quaternary ammonium compounds, thus limiting their utility.

Finally, phenolics, such as the common laboratory dis infectant Amphyl, are derivatives of carbolic acid (phenol). The addition of detergent results in a product that cleans and disinfects at the same time, and at concentrations of 2% to 5%, these products are widely used for cleaning bench tops.

The most important point to remember when working with biocides or disinfectants is to prepare a working solution of the compound exactly according to the manufacturer’s package insert. Many think that if the manufacturer says to dilute 1 : 200, they will be getting a stronger product if they dilute it 1 : 10. However, the ratio of water to active ingredient may be critical, and if sufficient water is not added, the free chemical for surface disinfection may not be released.

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المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

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باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Staining Techniques for Light Microscopy

Selection of Culture Media

Biochemical Assays for Metabolic Diseases

Laboratory Diagnosis of Urinary Tract Infections

Identification of Mycoplasma Infections

Laboratory Diagnosis of Infections of the Eyes

Laboratory Diagnosis of Central Nervous System Infections

Diagnosis and Differential Diagnosis of Paroxysmal Nocturnal Hemoglobinuria

Differential Diagnosis of Pancytopenia

Diagnosis of Aplastic anemia

Diagnosis of Upper Respiratory Tract Infections

tuberculosis testing (TB testing, Interferon gamma release assay [IGRA, T-Spot.TB], QuantiFERON-TB Gold [QFT, QFT-G, TB Gold test, TB blood test], Nucleic acid amplification for TB [NAAT], TB antibody)