Both live and dead microorganisms can be removed from liquids by positive- or negative-pressure filtration. Membrane filters, usually composed of cellulose esters (eg, cellulose acetate), are available commercially with variable pore sizes (0.005-1 μm). For removal of bacteria, a pore size of 0.2 μm is effective for disinfection of large volumes of fluid, especially fluid containing heat-labile components such as serum. Filtration is not considered effective for removing viruses.
Membrane filters remove bacteria
Pasteurization involves exposure of liquids to temperatures in the range 55°C to 75°C to remove all vegetative bacteria. Spores are unaffected by the pasteurization process. Pasteurization is used commercially to render milk safe and to extend its storage quality. With the outbreaks of infection due to contamination with enterohemorrhagic Escherichia coli (see Chapter 33); this has been extended (reluctantly) to fruit drinks. To the dismay of some of his compatriots, Pasteur proposed application of the process to wine-making to prevent microbial spoilage and vinegarization. Pasteurization in water at 70°C for 30 minutes has been effective and inexpensive when used to render plastics, such as those used in inhalation therapy equipment, free of organisms that may, otherwise, multiply in mucus and humidifying water.
✺ Kills vegetative bacteria but not spores
Used for foods and fragile medical equipment
The use of microwaves in the form of microwave ovens or specially designed units is another method of disinfection. These systems are not under pressure, but they can achieve temperatures near boiling if moisture is present. In some situations, they are being used as a practical alternative to incineration for disinfection of hospital waste. These procedures cannot be considered sterilization only because heat-resistant spores may survive the process.
Microwaves kill by generating heat
Given access and sufficient time, chemical disinfectants cause the death of pathogenic vegetative bacteria. Most of these substances are general protoplasmic poisons and are not used in the treatment of infections other than very superficial lesions, having been replaced by antimicrobial agents. Some disinfectants such as the quaternary ammonium compounds, alcohol, and the iodophors reduce the superficial flora and can eliminate contaminating pathogenic bacteria from the skin surface. Other agents such as the phenolics are valuable only for treating inanimate surfaces or for rendering contaminated materials safe. All are bound and inactivated to varying degrees by protein and dirt, and they lose considerable activity when applied to other than clean surfaces.
Most agents are general protoplasmic poisons
✺ Disinfectants are variably inactivated by organic material
Chemical disinfectants are classified on the basis of their ability to sterilize. High-level disinfectants kill all agents, except the most resistant of bacterial spores. Intermediate-level disinfectants kill all agents, but not spores. Low-level disinfectants are active against most vegetative bacteria and lipid-enveloped viruses.
Activity against spores and viruses varies
The alcohols are protein denaturants that rapidly kill vegetative bacteria when applied as aqueous solutions in the range of 70% to 95% alcohol. They are inactive against bacterial spores and many viruses. Solutions of 100% alcohol dehydrate organisms rapidly but fail to kill, because the lethal process requires water molecules. Ethanol (70-90%) and isopropyl alcohol (90-95%) are widely used as skin decontaminants before simple invasive procedures such as venipuncture. Their effect is not instantaneous, and the traditional alcohol wipe, particularly when followed by a vein-probing finger, is more symbolic than effective because insufficient time is given for significant killing. Isopropyl alcohol has largely replaced ethanol in hospital use because it is somewhat more active and is not subject to diversion to parties.
Alcohols require water for maximum effectiveness
Action of alcohol is slow
Iodine is an effective disinfectant that acts by iodinating or oxidizing essential components of the microbial cell. Its original use was as a tincture of 2% iodine in 50% alcohol, which kills more rapidly and effectively than alcohol alone. Tincture of iodine has now been largely replaced by preparations in which iodine is combined with carriers (povidone) or nonionic detergents. These agents, termed iodophors, gradually release small amounts of iodine. They cause less skin staining and dehydration than tinctures, and are widely used in preparation of skin before surgery.
Tincture of iodine in alcohol is effective
✺ Iodophors combine iodine with detergents
Chlorine exists as hypochlorous acid in aqueous solutions that dissociate to yield free chlorine over a wide pH range, particularly under slightly acidic conditions. In concentrations of less than one part per million, chlorine is lethal within seconds to most vegetative bacteria, and inactivates most viruses; this efficacy accounts for its use in rendering supplies of drinking water safe and in chlorination of water in swimming pools. Chlorine is the agent of choice for decontaminating surfaces and glassware that have been contaminated with viruses or spores of pathogenic bacteria. For these purposes, it is usually applied as a 5% solution called hypochlorite.
Chlorine oxidative action is rapid
Good for water and glassware
Hydrogen peroxide is a powerful oxidizing agent that attacks membrane lipids and other cell components. Although it acts rapidly against many bacteria and viruses, it kills bacteria that produce catalase and spores less rapidly. Hydrogen peroxide has been useful in disinfecting items such as contact lenses, which are not susceptible to its corrosive effect.
Hydrogen peroxide oxidizes cell components
Surfactants are compounds with hydrophobic and hydrophilic groups that attach to and solubilize various compounds or alter their properties. Anionic detergents such as soaps are highly effective cleansers, but they have little direct antibacterial effect, probably because their charge is similar to that of most microorganisms. Cationic detergents, particularly the quaternary ammonium compounds (“quats”) such as benzalkonium chloride, are highly bactericidal in the absence of contaminating organic matter. Their hydrophobic and lipophilic groups react with the lipid of the cell membrane of the bacteria, alter the membrane’s surface properties and its permeability, and lead to loss of essential cell components and death. These compounds have little toxicity to skin and mucous membranes, and thus they have been used widely for their antibacterial effects in a concentration of 0.1%. They are inactive against spores and most viruses. Care is needed in the use of quats because they adsorb to most surfaces as well as cotton, cork, and even dust. As a result, their concentration may be lowered to a point at which certain bacteria, particularly Pseudomonas aeruginosa, can grow in the quat solutions and serve as a source of infection.
Hydrophobic and hydrophilic groups of surfactants act on lipids of bacterial membranes
Little activity against viruses
Quats adsorb to surfaces and cotton
Phenol is a potent protein denaturant and bactericidal agent. Substitutions in the ring structure of phenol have substantially improved activity and have provided a range of phenols and cresols that are the most effective environmental decontaminants available for use in hospital hygiene. Concern about their release into the environment in hospital waste and sewage has created some pressure to limit their use. Phenolics are less “quenched” by protein than are most other disinfectants, have a detergent-like effect on the cell membrane, and are often formulated with soaps to increase their cleansing property. They are too toxic to skin and tissues to be used as antiseptics, although brief exposures can be tolerated. They are the active ingredient in many mouthwash and sore throat preparations.
Relatively stable to protein
Environmental contamination limits use
Chlorhexidine is used as a routine hand and skin disinfectant and for other topical applications. It has the ability to bind to the skin and produce a persistent antibacterial effect. It acts by altering membrane permeability of both gram-positive and gram-negative bacteria. It is cationic, and thus its action is neutralized by soaps and anionic detergents.
Chlorhexidine persists in skin
Glutaraldehyde and Formaldehyde
Glutaraldehyde and formaldehyde are alkylating agents highly lethal to essentially all microorganisms (Figure 3–3). Formaldehyde gas is irritative, allergenic, and unpleasant—properties that limit its use as a solution or gas. Glutaraldehyde is an effective high-level disinfecting agent for apparatus that cannot be heat-treated, such as some lensed instruments and equipment for respiratory therapy. Formaldehyde vapor, an effective environmental decontaminant under conditions of high humidity, is sometimes used to decontaminate laboratory rooms that have been accidentally and extensively contaminated with pathogenic bacteria.
Glutaraldehyde is useful for decontamination of equipment
Action of glutaraldehyde. Glutaraldehyde polymerizes and then interacts with amino acids in proteins (left) or in bacterial peptidoglycan (right). As a result, they are alkylated and inactivated. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)