Chapter Capsule

A. Moist Heat: Use of hot water or steam in disinfection to kill vegetative cells or in sterilization to kill spores. Mode of action: Denaturation of proteins, destruction of membranes and DNA.

1. For sterilization:

a. Autoclave uses steam under pressure (15 psi/121°C/10–40 min). Applied on heat-resistant materials that steam can penetrate. Not recommended for oils, powders, heat-sensitive materials.

b. Intermittent sterilization uses free-flowing, unpressurized steam at 100°C applied for 30 to 60 minutes on 3 successive days. Used for substances that cannot be autoclaved, especially media and certain canned foods.

2. For disinfection:

a. Pasteurization is application of heat less than 100°C to liquids; employs the flash method to heat liquid to 71°C for 15 seconds. Targets non-spore-forming milk-borne pathogens, such as Salmonella and Listeria, and reduces spoilage by lowering the overall microbial count; also applied to beer and wine. Only reduces microbial content; many thermodurics survive.

b. Boiling water at 100°C used to destroy pathogens (not spores) on household supplies; materials may become recontaminated.

B. Dry Heat: Use of hot air with low moisture content to sterilize.

1. Mode of action: Depending on the temperature, combusts, dehydrates, or coagulates proteins.

2. Incineration:

a. Open flame: Materials are placed for a few seconds in an 800° to 1,800°C flame of Bunsen burner.

b. Furnace: Incinerator chamber equipped with 600°–1,200°C flame that burns materials to ashes. Used for decontamination of hospital and industrial wastes; may add pollutants to atmosphere.

3. Dry oven: Closed chamber heated to 150°–180°C for 2 to 4 hours. Used for sterilizing metals and glass, but too vigorous for liquids, plastics, paper, cloth.

C. Cold Temperatures

1. Refrigeration (0°–15°C) or freezing (below 0°C) is not a reliable mode of disinfection. It is only microbistatic, meaning it slows the growth rate of most microbes.

2. Refrigeration is used for preserving foods, media, and cultures.

D. Drying/desiccation is the gradual withdrawal of water from cells by exposure to room air, leading to metabolic inhibition. Effective in preservation, but not reliable for pathogens, and not really an effective method for infection control.

E. Radiation/Irradiation

1. Use of energy in the form of waves (electromagnetic) and particles that can be transmitted through space. A method of cold sterilization.

2. Ionizing radiation uses high-energy, short waves and particles that can dislodge electrons from atoms. This causes direct damage to DNA by the formation of breaks and mutations. Types are gamma rays, X rays, cathode rays (high-speed electrons) used in irradiation and used to sterilize heat-sensitive medical materials in packages; also to sterilize and increase storage time of fresh products; very penetrating. Somewhat more expensive and dangerous than other methods.

3. Nonionizing radiation uses moderate energy, medium-length ultraviolet waves that excite, but do not ionize, atoms.

a. Rays act on DNA molecule, forming dimers between adjacent pyrimidines, and produce toxic products.

b. UV lamps disinfect air in medicine and industry; treat water, sera, vaccines, drugs; disinfect solid surfaces.

c. Does not penetrate most solids and can damage human tissues (skin, eyes).

F. Filtration involves the physical removal of microbes from liquids and air by trapping them in fine filters. The fluid passes through tiny pores into a container. Can be used to remove viruses. Filters can sterilize heat-sensitive liquids (vaccines, serum, drugs, media, water). Can remove microbes from air in hospital rooms, isolation units, and “clean rooms.’’

A. General uses: Disinfectants, antiseptics, sterilants, preservatives, sanitizers, degermers.

B. Physical states: Solutions come in aqueous form (chemicals dissolved in water) and tincture form (chemicals dissolved in alcohol); solutions expressed as dilutions, percents, and parts per million.

C. Halogens

1. Chlorine: Forms are Cl₂, hypochlorites, chloramines.

a. Denaturation of proteins by disrupting disulfide bonds; can be sporicidal with adequate time.

b. Elemental chlorine (1–2 ppm) disinfects water by destroying pathogenic vegetative pathogens.

c. Hypochlorites (chlorine bleach) are used extensively for disinfection and sanitization.

d. Chloramines are clinical disinfectants and antiseptics and alternative water disinfection agents.

e. Chemical action may be retarded by high levels of organic matter.

2. Iodine: Forms are free iodine (I₂) and iodophors (iodine complexed to organic polymers).

a. Interfere with protein interchain bonds, causing denaturation; can be sporicidal.

b. Weak solutions are topical antiseptics; iodine tincture is for a high degree of surgical asepsis and disinfection.

c. Iodine solutions stain and corrode. Stronger solutions are too irritating and toxic to use on tissue.

d. Iodophors are aqueous solutions with 2–10% iodine (Betadine, Povidone); they are milder medical and dental degerming agents, disinfectants, and ointments.

D. Phenolics

1. Phenols act on microbes by disrupting cell membranes and precipitating proteins. They are bactericidal, fungicidal, and virucidal, but not sporicidal.

2. Phenol is an older toxic disinfectant; 1–3% cresol in soap is a common housekeeping disinfectant and cleaner. Orthophenyl phenol (Lysol) is a milder phenolic used in air sprays and hospital disinfectants. Triclosan is an antibacterial additive to soaps, detergents, and numerous household products.

3. Most phenolics are too toxic for use as antiseptics and are relatively insoluble.

E. Chlorhexidine is available as Hibiclens and Hibitane.

1. Is a surfactant and protein denaturant, with bactericidal, some antiviral and antifungal effects, but not sporicidal.

2. An aqueous or alcohol solution is a skin degerming agent for preoperative scrubs, skin cleaning, and burns.

3. Product is relatively mild, nontoxic, and fast-acting.

F. Alcohols include ethyl and isopropyl, usually in solutions of 50–95%.

1. Act as a surfactant to dissolve membrane lipids and coagulate proteins; work on bacterial vegetative cells and fungi, but are not sporicidal.

2. Ethyl alcohol (70–95%) is a germicide in the clinic, laboratory; used at home for degerming, low-level disinfection, and sanitization.

3. Isopropyl alcohol has similar uses, though it is more toxic and less safe.

G. Hydrogen peroxide is available in weak (3%) to strong (25%) solutions that produce highly active hydroxyl-free radicals and damage proteins and DNA molecules; also decomposes to water and O₂ gas, which can be toxic to anaerobes; strong solutions are sporicidal.

1. 3% hydrogen peroxide is used for skin and wound antisepsis, care of mucous membrane infections; and disinfection of utensils.

2. 6–25% can be used to sterilize equipment.

3. Decomposes in presence of light and catalase.

H. Detergents and Soap

1. Cationic detergents are known as quaternary ammonium compounds or quats. They are surfactants that alter cell permeability. Their action is limited to some bacteria and fungi; they are not sporicidal.

2. Benzalkonium and cetylpyridinium chlorides are quats used in environmental disinfection, as cleansers in clinics and the food industry, and as preservatives.

3. Soaps are not very microbicidal but function in the mechanical removal of grease and soil on skin, utensils, and environmental surfaces.

4. Soaps do not destroy the tubercle bacillus or hepatitis B virus and are inactivated by large quantities of organic matter.

I. Heavy metal compounds include mercury and silver solutions and tinctures; work in high dilutions (oligodynamic action) and precipitate proteins; are germicidal but not sporicidal.

1. Organic mercurial tinctures (thimerosal, nitromersol) are skin antiseptics and preservatives.

2. Silver nitrate solutions are used as a topical antiseptic; silver sulfadiazine ointment can prevent burn infections; colloidal silver preparations are used for mouth and eye rinses.

3. Metal solutions are highly toxic, cause allergies, and are neutralized by organic substances.

J. Aldehydes include glutaraldehyde and formaldehyde or formalin solutions. They kill cells by alkylation of amino and nucleic acids, with wide microbicidal action.

1. Glutaraldehyde in 2% solutions (Cidex) is used as a sterilant for heat-sensitive instruments in the medical and dental office, and for some types of environmental disinfection.

2. Formalin in 1–8% is limited to disinfection of some instruments, rooms, and as a preservative.

3. Glutaraldehyde is somewhat unstable; formaldehyde is toxic and irritating.

K. Gases and aerosols include ethylene oxide (ETO), propylene oxide, and betapropiolactone, which inactivate nucleic acids and proteins. All are sporicidal, but only ETO is an approved sterilant.

1. Ethylene oxide is added to a special chamber for sterilizing plastic supplies and to treat spices and dried foods.

2. Propylene oxide can disinfect food (nuts, starch).

3. Betapropiolactone is used to disinfect rooms and some biological materials.

4. ETO is somewhat dangerous because of its explosiveness and toxicity, and it is slow-acting. Betapropiolactone is toxic and has poor penetration.