Antisepsis, Disinfection and Sterilization: Methods, Agents and Efficacy Tests

Antisepsis, Disinfection and Sterilization

Antisepsis: lowers organic surface microbes. Toxic disinfectants like those used on skin are antiseptics. Antisepsis substances inhibit the development of bacteria and are useful on skin. Antisepsis preserves sterility.

Disinfection: destruction of pathogenic microorganisms, classified by materials, inorganic in nature. Effectiveness levels:

1. High-level disinfection: kills all microorganisms.
2. Intermediate-level disinfection
3. Low-level disinfection: kills only some vegetative bacteria, fungi and lipid viruses.

Disinfectant: destroys non-sporulated organisms and their spores.

Sterilization: destruction of all forms of microorganisms, including their spores.

Sterilization cycle: heat generation over a defined time.

Methods of Sterilization

Physical: heat (moist or dry), filtration and radiation.

Chemical: sterilization with gaseous agents (organic or inorganic).

Mechanisms of Action

• Protein coagulation: proteins precipitate, enzymatic activity stops and cells die.
• Oxidation of free SH groups: SH groups are fixed by an oxidizing agent and the cell dies.
• Enzymatic antagonism: inhibits cell reproduction by occupying receptors.
• Membrane disruption: the cell membrane is broken.

Organic Agents

Alcohol: used as a skin antiseptic. Dehydrates and denatures proteins and is bactericidal. 70% (70 g alcohol per 30 mL H₂O). Examples: isopropyl and ethyl (ethanol).

Aldehydes: used for sterilization and disinfection.

• Formaldehyde: gas form, or 8% formaldehyde in 70% alcohol. Aqueous solution is a potent bactericide, sporicide and virucide (e.g., polio).
• Glutaraldehyde: aqueous alkaline pH; bactericide, sporicide, tuberculocide, virucide and fungicide. Activated 2% solution (CIDEX) is useful in endoscopes for 20 minutes.

Phenols: obtained by distillation of crude coal tar. Used as antiseptics (Lister), causing coagulation and denaturation of proteins through insoluble products.

Organic acids: mandelic acid and methenamine, used as urinary tract antiseptics (formaldehyde is released at acidic pH). Oral use may cause gastric irritation.

Anionic detergents: soaps of sodium and potassium, used to reduce surface tension.

Cationic detergents: quaternary ammonium compounds, bactericidal against Gram-positive bacteria but not Gram-negative. Ineffective against spores, tuberculosis bacilli and some viruses. Examples: cetrimonium bromide (cetrimide) and benzalkonium chloride (benzal).

• Benzalkonium chloride: 0.1% in non-alcoholic tincture and 0.5% in aqueous solution.

They damage the cell membrane and are effective at alkaline pH. They act as antiseptics or disinfectants, but sterilization fails with chemistry alone.

Special agents: terpenes and camphors (menthol).

Dyes: aniline or triphenylmethane dyes (brilliant green, crystal violet, gentian violet, malachite green), and thionine (methylene blue). Bactericidal against Gram-positive bacteria, not spores. Injected into tissues to define pathways, drawing incisions and flaps in plastic and reconstructive surgery.

Nitrofurans:

• Nitrofurantoin: urinary tract antiseptic against Enterobacteria.
• Furazolidone: treats enteric infections caused by Gram-negative bacteria. Nitrokinoline + nitrofurantoin are formulated as ointments.

Inorganic Agents

Halogens:

• Iodine (iodophor): Betadine or povidone-iodine (polyvinylpyrrolidone-iodine). Antiseptic solution used before surgery. 8-11 g/100 mL (0.8-1.1 g iodine). Also known as Isodine and Germidin.
• Chlorine: used as a disinfectant for drinking water, in the form of sodium hypochlorite. Very bactericidal, including against HIV and hepatitis viruses. 1% causes membrane oxidation. Does not kill tuberculosis bacteria.

Oxidants: interfere with anaerobic metabolism (weak bactericides).

• Hydrogen peroxide (oxygenated water): releases O₂ through effervescence, aiding mechanical and bactericidal action.
• Potassium permanganate: purple crystals. Used at 1/1000 for irrigation; for soaks, 1 g per 10 L.

Metals and metal salts:

• Mercury, silver and zinc salts: disinfectants and antiseptics acting on SH groups and coagulating proteins.
• Thimerosal: thimerosal (merthiolate tincture 1:1000), bacteriostatic (not sporicidal). Used as a preoperative antiseptic and for second-intention healing.
• Merbromin (mercurochrome): slightly toxic, used in proctology.
• Cupric sulfate (copper sulfate): used in treatment of infections caused by Gram-positive bacteria.
• Silver nitrate: 0.5% bacteriostatic. Stains skin and tissues dark. Applied in 1% solution, used ophthalmically to prevent gonococcal ophthalmia.
• Zinc sulfate: ophthalmic antiseptic, used to treat conjunctivitis.

Inorganic acids: boric acid is soluble in water and sold as 10% boric powder. Used in dermatology and for ulcers. Used as an antiseptic for conjunctivitis.

Related Terms

• Fomites or fomes: objects that transmit substances or infection (not food).
• Septicemia (bloodstream infection): clinical syndrome caused by microorganism invasion. Microbes spread through the bloodstream from an infection focus.
• Germicides: inactivate viruses and protozoa, but do not affect spores of bacteria and fungi.
• Sanitizing: reduction of microorganisms to an accepted safe level, officially confirmed.
• Tyndallization: intermittent sterilization by heating at 80-100 °C for 24 hours, repeated over several days to kill spores that develop.

Disinfectant Testing

• Phenol coefficient: compares a disinfectant against phenol, using Salmonella typhi as the test organism.
• Inhibition coefficient: the percentage concentration of a disinfectant that completely inhibits bacterial or mold growth in a nutrient medium.
• Lower phenol coefficient: the dilution of disinfectant at which it kills the organism in a time shorter than the control.
• Upper phenol coefficient: the dilution of disinfectant at which it kills the organism in a time longer than the control.
• Phenol coefficient definition: the bactericidal power of a substance compared to pure phenol.