Factors Affecting the Efficacy of Disinfection and Sterilization

The efficacy of germicides against microorganisms depends on several factors, such as intrinsic qualities of the organism and the chemical and external physical environment. By knowing of these factors will lead to better use of disinfection and sterilization.

        When all other conditions remaining constant, the larger number of microbes, a germicide needs more time to destroy all of them. This emphasizes the importance of cleaning medical instruments before disinfection and sterilization. The location of microorganisms also must be considered. Since penetration of the disinfectant is difficult, medical instruments with multiple pieces must be disassembled before disinfection. In addition, only surface that directly contact the germicide will be disinfected, so the equipment must be completely immersed for the entire exposure period.

        Each microorganism has greatly different in their resistance to chemical germicides and sterilization process. The most resistant microbial subpopulation controls the sterilization or disinfection time.

        When all other variables remaining constant, the more concentrated the disinfectant has the greater efficacy and the shorter the time necessary to kill microbial. So, the potency of the germicide will decide the length of the disinfection time.

Several physical and chemical factors, such as temperature, pH, relative humidity, and water hardness, also influence disinfectant procedures. In most of the cases, as the temperature increases, the activity of most disinfectants increases. However, too much increase in temperature will causes the disinfectant to degrade and weakens its germicidal activity. The pH influences the antimicrobial activity by altering the disinfectant molecule or the cell surface. The water hardness reduces the rate of kill of certain disinfectants because divalent cations in the hard water interact with the disinfectant to form insoluble precipitates.

This further emphasizes the importance of cleaning of medical devices before any sterilization of disinfection procedure because both organic and inorganic soils are easily removed by washing. In general, longer contact times are more effective than shorter contact times.

Biofilms, thick masses of cells and extracellular materials, can protect microorganism from disinfectants. Biofilms are microbial communities that are tightly attached to surfaces and cannot be easily removed. Once these masses form, microbes within them can be resistant to disinfectants by multiple mechanisms. Some enzymes and detergents can degrade biofilms or reduce numbers of viable bacteria within a biofilm, however, no products are EPA-registered or Food and Drug Administration (FDA)-cleared for this purpose.

Hypochlorites are the most widely used of chlorine disinfectants. The most prevalent products are aqueous solution of 5.25-6.15% sodium hypochlorite, usually called household bleach. The exact mechanism is by free chlorine destroys microorganism has not been elucidated. There are a number of mechanisms bring about inactivation by chlorine, such as oxidation of sulfhydryl enzymes amino acids; ring chlorination of amino acids; loss of intracellular contents; decreased uptake of nutrients; inhibition of protein synthesis; decrease oxygen uptake; oxidation of respiratory components; decreased adenosine triphosphate production; breaks in DNA; and depressed DNA synthesis. Low concentrations of free available chlorine have a biocidal effect on mycoplasma and vegetative bacteria in seconds in the absence of an organic load. Higher concentrations of chlorine are required to kill M. tuberculosis. Hypochlorites are widely used in healthcare facilities in a variety of settings. Inorganic chlorine solution is used for disinfecting tonometer heads and for spot-disinfection of countertops and floor. For decontaminating blood spills, a 1:10-1:100 dilution of 5.25-6.15% household bleach or an EPA-registered tuberculocidal disinfectant has been recommended. For small blood spills on noncritical surfaces, the area can be disinfected with a 1:100 dilution of 5.25-6.15% sodium hypochlorite or an EPA-registered tuberculocidal disinfectant. Furthermore, since hypochlorites and other germicides are substantially inactivated in the presence of blood, the surface with large spills of blood require cleaning before an EPA-registered disinfectant or a 1:10 solution of household bleach is applied. Clinicians should not alter their use of chlorine on environmental surface on basis of testing methodologies that do not simulate actual disinfection practices. Other uses in health care include as an irrigating agent in endodontic treatment and as a disinfectant for manikins, laundry, dental appliances, hydrotherapy tanks, regulated medical waste before disposal, and the water distribution system in hemodialysis centers and hemodialysis.

Glutaraldehyde is a saturated dialdehyde, which as a wide acceptance high-level disinfectant and chemical sterilant. When the alkalinating agents activate the aqueous solutions of glutaraldehyde, these become sporicidal. However, after activated, these solutions have a shelf-life of minimally 14 days because the polymerization blocks the active sites of the glutaraldehyde molecules that are responsible for its biocidal activity. The use of glutaraldehyde-based solutions in health-care facilities is widespread because of their advantages including excellent biocidal properties; activity in the presence of organic matter; and noncorrosive action to endoscopic equipment, thermometers, rubber, or plastic equipment. The biocidal activity of glutaraldehyde results from its alkylation of sulfhydryl, hydroxyl, and amino groups of microorganisms, which alters RNA, DNA, and protein synthesis. 2% or less aqueous solutions of glutaraldehyde buffered to pH 7.5-8.5 with sodium bicarbonate effectively killed vegetative bacteria in 2 minutes or less; M. tuberculosis, fungi, and viruses less than 10 minutes; and spores of Bacillus and Clostridium species in 3 hours. FDA has cleared a glutaraldehyde-phenol/phenate concentrate as a high-level disinfectant that contains 1.2% glutaraldehyde with 1.93% phenol/phenate at its use concentration. Glutaraldehyde is used most commonly as a high-level disinfectant for medical equipment. Glutaraldehyde is noncorrosive to metal and does not damage lensed instruments, rubber, and plastics. However, glutaraldehyde is too toxic and expensive for cleaning noncritical surfaces. Area using glutaraldehyde should be monitored to ensure a safe environment. Healthcare staff members can be exposed to elevated levels of glutaraldehyde vapor when equipment is processed in poor fresh air rooms. Acute or chronic exposure can result in dermatitis, skin or mucous membrane irritation, and pulmonary symptoms.