Halogen-Based Biocides: (Chlorine Compounds and Iodophores)
Chlorine Compounds
Chlorine compounds are good disinfectants on clean surfaces, but are quickly inactivated by organic matter and thus reducing the biocidal activity. They have a broad spectrum of antimicrobial activity and are inexpensive and fast acting. Hypochlorites, the most widely used of the chlorine disinfectants, are available in liquid (e.g., Sodium hypochlorite), household bleach and solid (e.g., calcium hypochlorite, sodium dichloroisocyanurate) forms. Household bleach has an available chlorine content of 5.25%, or 52,500 ppm. Because of its oxidizing power, it loses potency quickly and should be made fresh and used within the same day it is prepared. The free available chlorine levels of hypochlorite solutions in both opened and closed polyethylene containers are reduced to 40% to 50% of the original concentration over a period of one month at room temperature.
There are two potential occupational exposure hazards when using hypochlorite solutions. The first is the production of the carcinogen bis-chloromethyl ether when hypochlorite solutions come in contact with formaldehyde. The second is the rapid production of chlorine gas when hypochlorite solutions are mixed with an acid. Care must also be exercised in using chlorine – based disinfectants which can corrode or damage metal, rubber, and other susceptible surfaces. Bleached articles should never be autoclaved without reducing the bleach with sodium thiosulfate or sodium bisulfate.
Chloramine T which is prepared from sodium hypochlorite and p-toluenesulfonamide is a more stable, odorless, less corrosive form of chlorine but has decreased biocidal activity in comparison to bleach.
Mode of action: They are oxidizing agents and cause damage by oxidation of essential sulfydryl groups of enzymes. Chlorine reacts with water to form hypochlorous acid, which is microbicidal. Examples: Chlorine compounds (chlorine, bleach, hypochlorite) and iodine compounds (tincture iodine, iodophores)
Applications: Tincture of iodine (2% iodine in 70% alcohol) is an antiseptic. Iodine can be combined with neutral carrier polymers such as polyvinylpyrrolidone to prepare iodophores such as povidone-iodine. Iodophores permit slow release and reduce the irritation of the antiseptic. For hand washing iodophores are diluted in 50% alcohol. 10% Povidone Iodine is used undiluted in pre and postoperative skin disinfection. Chlorine gas is used to bleach water. Household bleach can be used to disinfect floors. Household bleach used in a stock dilution of 1:10. In higher concentrations chlorine is used to disinfect swimming pools. 0.5% sodium hypochlorite is used in serology and virology. Used at a dilution of 1:10 in decontamination of spillage of infectious material. Mercuric chloride is used as a disinfectant.
Disadvantages: They are rapidly inactivated in the presence of organic matter. Iodine is corrosive and staining. Bleach solution is corrosive and will corrode stainless steel surface
Phenolics: (O-phenophenoate-base Compounds)
Phenolics are phenol (carbolic acid) derivatives. These biocides act through membrane damage and are effective against enveloped viruses, rickettsiae, fungi and vegetative bacteria. They also retain more activity in the presence of organic material than other disinfectants. Cresols, hexachlorophene, alkyl- and chloro derivatives and diphenyls are more active than phenol itself. Available commercial products are Lysol, Pine-Sol, Amphyl, O-syl, Tergisyl, Vesphene, L- Phase and Expose.
Mode of action
Act by disruption of membranes, precipitation of proteins and inactivation of enzymes. Examples: 5% phenol, 1-5% Cresol, 5% Lysol (a saponified cresol), hexachlorophene, chlorhexidine, chloroxylenol (Dettol)
Applications
Joseph Lister used it to prevent infection of surgical wounds. Phenols are coal-tar derivatives. They act as disinfectants at high concentration and as antiseptics at low concentrations. They are bactericidal, fungicidal, mycobactericidal but are inactive against spores and most viruses. They are not readily inactivated by organic matter. The corrosive phenolics are used for disinfection of ward floors, in discarding jars in laboratories and disinfection of bedpans. Chlorhexidine can be used in an isopropanol solution for skin disinfection, or as an aqueous solution for wound irrigation. It is often used as an antiseptic hand wash. 20% Chlorhexidine gluconate solution is used for pre-operative hand and skin preparation and for general skin disinfection. Chlorhexidine gluconate is also mixed with quaternary ammonium compounds such as cetrimide to get stronger and broader antimicrobial effects (eg. Savlon). Chloroxylenols are less irritant and can be used for topical purposes and are more effective against gram positive bacteria than gram negative bacteria. Hexachlorophene is chlorinated diphenyl and is much less irritant. It has marked effect over gram positive bacteria but poor effect over gram negative bacteria, mycobacteria, fungi and viruses. Triclosan is an organic phenyl ether with good activity against gram positive bacteria and effective to some extent against many gram negative bacteria including Pseudomonas. It also has fair activity on fungi and viruses. © Sridhar Rao P.N (www.microrao.com) Disadvantages: It is toxic, corrosive and skin irritant. Chlorhexidine is inactivated by anionic soaps. Chloroxylenol is inactivated by hard water.
Heavy Metals:
Soluble salts of mercury, silver lactate, mercuric chloride and mercurous chloride are efficient bactericidal agents. Silver nitrate and mercuric chloride are commonly used as 1:1000 aqueous solutions. Action is through attack on protein sulfhydryl groups and disruption of enzyme functions. Organic matter can reverse the disinfectant properties of mercurials.
Mode of action: Act by precipitation of proteins and oxidation of sulfydryl groups. They are bacteriostatic. Examples: Mercuric chloride, silver nitrate, copper sulfate, organic mercury salts (e.g., mercurochrome, merthiolate)
Applications: 1% silver nitrate solution can be applied on eyes as treatment for opthalmia neonatorum (Crede’s method). This procedure is no longer followed. Silver sulphadiazine is used topically to help to prevent colonization and infection of burn tissues. Mercurials are active against viruses at dilution of 1:500 to 1:1000. Merthiolate at a concentration of 1:10000 is used in preservation of serum. Copper salts are used as a fungicide. Disadvantages: Mercuric chloride is highly toxic, are readily inactivated by organic matter.
Surface active agents: Mode of actions: They have the property of concentrating at interfaces between lipid containing membrane of bacterial cell and surrounding aqueous medium. These compounds have long chain hydrocarbons that are fat soluble and charged ions that are water-soluble. Since they contain both of these, they concentrate on the surface of membranes. They disrupt membrane resulting in leakage of cell constituents. Examples: These are soaps or detergents. Detergents can be anionic or cationic. Detergents containing negatively charged long chain hydrocarbon are called anionic detergents. These include soaps and bile salts. If the fat-soluble part is made to have a positive charge by combining with a quaternary nitrogen atom, it is called cationic detergents. Cationic detergents are known as quaternary ammonium compounds (or quat). Cetrimide and benzalkonium chloride act as cationic detergents. Application: They are active against vegetative cells, Mycobacteria and enveloped viruses. They are widely used as disinfectants at dilution of 1-2% for domestic use and in hospitals. Disadvantages: Their activity is reduced by hard water, anionic detergents and organic matter. Pseudomonas can metabolise cetrimide, using them as a carbon, nitrogen and energy source.
Dyes
Mode of action
Acridine dyes are bactericidal because of their interaction with bacterial nucleic acids. Examples: Aniline dyes such as crystal violet, malachite green and brilliant green. Acridine dyes such as acriflavin and aminacrine. Acriflavine is a mixture of proflavine and euflavine. Only euflavine has effective antimicrobial properties. A related dye, ethidium bromide, is also germicidal. It intercalates between base pairs in DNA. They are more effective against gram positive bacteria than gram negative bacteria and are more bacteriostatic in action.
Applications: They may be used topically as antiseptics to treat mild burns. They are used as paint on the skin to treat bacterial skin infections. The dyes are used as selective agents in certain selective media.
Hydrogen peroxide
Hydrogen peroxide is a liquid chemical sterilizing agent which is a strong oxidant and can destroy a wide range of microorganisms. It is usedful in the sterilization of heat or temperature sensitive equipment like endoscopes. In medical applications, a higher concentration (35-90%) is used. It has a short sterilization cycle time as these cycles are as 28minutes where ethylene oxide has cycles that as long as 10-12hours. However, H2O2 drawbacks like low material compatibility, lower capacity of penetration and associated health risks. Vapourized hydrogen peroxide is used to sterilize largely enclosed and sealed areas such as entire rooms and aircraft interior.
Mode of action
It acts on the microorganisms through its release of nascent oxygen. Hydrogen peroxide produces hydroxyl-free radical that damages proteins and DNA. Application: It is used at 6% concentration to decontaminate the instruments, equipment such as ventilators. 3% Hydrogen Peroxide Solution is used for skin disinfection and deodorising wounds and ulcers. Strong solutions are sporicidal.
Disadvantages: Decomposes in light, broken down by catalase, proteinaceous organic matter drastically reduces its activity.
Gaseous method of sterilization
This involves the process of exposing equipment or devices to different gases in closed heated or pressurized chamber. It is a more effective technique as gases can pass through a tiny orfice and provide more effective results. Beside, gases are commonly used along with heat treatment which also facilitates the functioning of the gases. However, there is an issue of release of some toxic gases during the process which needs to be removed regularly from the system. The mechanism of action is different for different types of gases. Some of the common gases used for gaseous sterilization are:
Ethylene oxide (EO)
This gas is a common gas used for chemical treatment applied to sterilize, pasteurize or disinfect different types of equipment and surfaces because of its wide range of compatibility with different materials. Ethylene oxide treatment often replaces other sterilization techniques like heat, radiation and even chemicals in cases where the objects are sensitive to these techniques. This method is a widespread method used for almost 79% of all sterilization and around 50% for disposable medical devices.
Mechanism of action
The mechanism of antimicrobial action of this gas is assumed to be through the alkylation of sulphydryl, amino, hydroxyl and carboxyl groups on protein and amino groups of nucleic acids.
Ethylene oxide treatment is usually conducted at bthe range of 30-60oc for several hours which aids in the activity of the gas. The efficacy of the gas depends on the concentration of gas available for each article which is greatlyassisted by the good penetrating nature of the gas, which diffuses readily into many packaging materials including rubber, plastics and paper. Thisb gas kills microorganisms such as bacteria(including spores), viruses and fungi and is compatible with almost all materials even when repeatedly applied. The process however, is not without drawback as the level of gas in the stsrilizer goes on decreasing due to absorption, and the treated articles need to undergo a process of desorption to remove the the toxic residual wastes. Organisms are more resistant to ethylene oxide treatment in a dried state, as are those protected from the gas by inclusion in crystalline or dried organic deposits.
Disadvantages: It is highly toxic, irritating to eyes, skin, highly flammable, mutagenic and carcinogenic.
Formaldehyde
This is another important highly reactive gas which is used for sterilization This gas is obtained by heating formalin to a temperature of 70-80Oc It possesses broad-spectrum biocidal activity and has found application in the sterilization of reusable surgical instruments, specific medical, diagnostic and electrical equipment, and the surface sterilization of powders Formaldehyde does not have the same penetrating power of ethylene oxide but works on the same principle of modification of protein and nucleic acid As a result of the low penetrating power, its use is often limited to paper and cotton fabrics. It can generallybe detected by smell at concentrations lower than those permitted in the atmosphere and thus can be detected during leakage or other such accident
Nitrogen dioxide
This is a rapid and effective sterilant that can be used for removal of common bacteria, fungi and even spores. It has a low boiling point (20oC) which allows a high vapour pressure at standard temperature. This property of nitrogen dioxide enables the use of gas at standard temperature and pressure. The biocidal action of this gas involves the degradation of DNA by the nitration of phosphate backbone, which results in lethal effects on the exposed organism as it absorbs NO2. An advantage of this gas is that no condensation of the gas occurs on the surface of the devices because of the low level of gas used and high vapour pressure. This avoids the need for direct aeration after the process of sterilization.
Ozone
This is a highly reactive industrial gas that is commonly used to sterilize air and water and as a disinfectant for surfaces. Ozone is a potent oxidizing property that is capable of destroying a wide range of organisms including prions, without the use of hazardous chemical as ozone is usually generated from medical-grade oxygen. Similarly the high reactivity of ozone allows the removal of waste ozone by converting the ozone into oxygen by passing it through a simple catalyst. HOWEVER, because ozone is an unstable and reactive gas, it has to be produced on-site, which limits the use of ozone in different setting
Beta-propiolactone (BPL)
Mode of action
It is an alkylating agent and acts through alkylation of carboxyl- and hydroxyl- groups. Properties: It is a colorless liquid with pungent to slightly sweetish smell. It is a condensation product of ketane with formaldehyde. Application: It is an effective sporicidal agent, and has broad-spectrum activity. 0.2% is used to sterilize biological products. It is more efficient in fumigation that formaldehyde. It is used to sterilize vaccines, tissue grafts, surgical instruments and enzymes Disadvantages: It has poor penetrating power and is a carcinogen.