Chemicals vary greatly in their ability to kill microorganisms. They act primarily by one of three mechanisms: (1) disruption of the lipid-containing cell membrane, (2) modification of proteins, or (3) modification of DNA. Each of the following chemical agents is classified into three categories, but some chemicals act by more than one mechanism.
DISRUPTION OF CELL MEMBRANES
Alcohol
Ethanol is widely used to clean the skin before immunization or venipuncture. It disrupts the lipid structure in membranes and denatures proteins. Ethanol requires the presence of water for maximal activity (i.e., it is far more effective at 70% than at 100%). Ethanol will not kill bacterial spores and therefore cannot be used for sterilization.
Detergents
Detergents are “surface-active” agents composed of a long chain, lipid-soluble, hydrophobic portion, and a polar hydrophilic group. These surfactants interact with the lipid in the cell membrane and disrupt it. Quaternary ammonium compounds (e.g., benzalkonium chloride) are cationic detergents widely used for skin antisepsis.
Phenols
Phenol was the first disinfectant used in the operating room (by Lister in the 1860s), but it is rarely used as a disinfectant today because it is too caustic, damaging membranes and denaturing proteins. Chlorhexidine is a chlorinated phenol that is widely used as a hand disinfectant prior to surgery (“surgical scrub”) and cleansing wounds.
MODIFICATION OF PROTEINS
Chlorine
Chlorine is used as a disinfectant to purify the water supply and to treat swimming pools. It is also the active component of hypochlorite (bleach, Clorox), which is used as a disinfectant. Chlorine is a powerful oxidizing agent that kills by cross-linking essential sulfhydryl groups in enzymes to form the inactive disulfide.
Iodine
Iodine is the most effective skin antiseptic used in medical practice and should be used prior to obtaining a blood culture and installing intravenous catheters because contamination with skin flora such as Staphylococcus epidermidis can be a problem. Iodine, like chlorine, is an oxidant that inactivates sulfhydryl containing enzymes.
Iodine is supplied in two forms: (1) Tincture of iodine (2% solution of iodine and potassium iodide in ethanol) is used to prepare the skin prior to blood culture. Because tincture of iodine can be irritating to the skin, it should be removed with alcohol.
(2) Iodophors are complexes of iodine with detergents that are frequently used to prepare the skin prior to surgery because they are less irritating than tincture of iodine. They are also used to disinfect the skin of healthcare personnel. Povidone-iodine (Betadine) is an iodophor commonly used as an antiseptic.
Heavy Metals
Mercury and silver have the greatest antibacterial activity of the heavy metals and are the most widely used in medicine. They bind sulfhydryl groups and block enzymatic activity. Thimerosal (Merthiolate) and merbromin (Mercurochrome), which contain mercury, are used as skin antiseptics. Silver nitrate drops are effective in preventing gonococcal neonatal conjunctivitis (ophthalmia neonatorum). Silver sulfadiazine prevents infection of burn wounds.
Hydrogen Peroxide
Hydrogen peroxide is an antiseptic used to clean wounds. It is an oxidizing agent that attacks sulfhydryl groups, inhibiting enzymatic activity. Its effectiveness is limited by the organism’s ability to produce catalase, an enzyme that degrades H2O2.
Formaldehyde & Glutaraldehyde
Formaldehyde, which is available as a 37% solution in water (formalin), denatures proteins and nucleic acids. Both proteins and nucleic acids contain essential –NH2 and –OH groups, which are the main sites of alkylation by the hydroxymethyl group of formaldehyde. Glutaraldehyde, which has two reactive aldehyde groups, is 10 times more effective than formaldehyde and is less toxic. In hospitals, it is used to sterilize respiratory therapy equipment, endoscopes, and hemodialysis equipment.
Ethylene Oxide
Ethylene oxide gas is used extensively in hospitals for the sterilization of heat-sensitive materials (surgical instruments and plastics). It kills by alkylating proteins and nucleic acids, and is classified as a mutagen and a carcinogen.
Acids & Alkalis
Strong acids and alkalis kill by denaturing proteins. Although most bacteria are susceptible, it is important to note that Myco bacterium tuberculosis and other mycobacteria are relatively resistant to 2% NaOH, which is used in the clinical laboratory to liquefy sputum prior to culturing the organism. Weak acids, such as benzoic, propionic, and citric acids, are frequently used as food preservatives because they are bacteriostatic.
MODIFICATION OF NUCLEIC ACIDS
A variety of dyes not only stain microorganisms but also inhibit their growth. One of these is crystal violet (gentian violet), an antiseptic used to treat fungal infections of the skin. Its action is based on binding of the positively charged dye molecule to the negatively charged phosphate groups of the nucleic acids. Malachite green, a triphenylamine dye like crystal violet, is a component of Löwenstein–Jensen medium, which is used to grow M. tuberculosis. The dye inhibits the growth of unwanted organisms in the sputum during the 6-week incubation period.
