Iron (ferric chloride) is widely used in water purifiers as a protein coagulant, and in veterinary medicine as a hemostat.
Inorganic mercuric bichloride, one of the early antiseptics, was later replaced by the less irritating and less toxic organic mercurials—eg, merbromin, thimerosal (49% mercury), nitromersol, and phenylmercuric nitrate.
Thimerosal is used as a preservative in vaccines. Thimerosal and its metabolite ethylmercury are known to have neurotoxic effects, and they are linked with neurodevelopmental disorders in children and rats. Therefore, thimerosal has been replaced by other vaccine preservatives, such as gentamicin, amphotericin B, or 2-phenoxyethanol, in some vaccines; and preservative-free vaccines have been trialed. However, some veterinary (and human) vaccines do still contain thimerosal.
At moderate concentrations, the organic mercurials are bacteriostatic, inhibiting bacterial enzymes through their affinity for sulfhydryl groups. This effect can be reversed by sulfur-containing compounds—eg, cysteine or glutathione. Mercurials are not effective against spores. Use of mercurial antiseptics or disinfectants has decreased, partly because of their environmental persistence and contaminant potential. Repeated application of topical mercurials can result in appreciable absorption and systemic toxicosis.
Silver compounds: Silver is a remarkably safe, nontoxic metal for humans and animals, and consumer uses of silver range from the disinfection of medical devices, textiles, cosmetics, and home appliances to the treatment of water. Silver compounds can have caustic, astringent, antimicrobial, antiviral, and antifungal effects. The antimicrobial action of silver or silver compounds is proportional to the amount of bioactive silver ion (Ag+) released and its availability to interact with bacterial or fungal cell membranes. Silver metal and inorganic silver compounds ionize in the presence of water, body fluids, and tissue exudates. Silver ions readily interact with proteins, amino acid residues, free anions, and receptors on mammalian and bacterial cell membranes. Silver ions precipitate proteins by combining with amino, phosphate, sulfhydryl, and carboxyl groups. Silver ions also interfere with essential metabolic activities of microbial cells.
Pure silver nanoparticles (AgNPs) are more efficient than silver ions. Pure silver is used against many different pathogens, especially those that are resistant to multiple antimicrobials and are thus particularly difficult to treat. AgNPs, it is thought, can interact with the disulfide bonds of glycoproteins/proteins in viruses, bacteria, and fungi. AgNPs have the advantage of being nontoxic at low concentrations and having a wide range of antimicrobial action against at least 12 bacterial species, including multiresistant bacteria like methicillin-resistant Staphylococcus aureus(MRSA), as well as multidrug-resistant Pseudomonas aeruginosa, ampicillin-resistant E coli O157:H7, and erythromycin-resistant Streptococcus pyogenes.
Silver has been used for centuries to disinfect cutlery and dishes, with an innate understanding of its antimicrobial action. In efforts to develop food packaging that has antimicrobial properties, silver ions have been incorporated into polylactide films that have been shown to have a dramatic antimicrobial effect on Salmonella enterica and feline calicivirus (FCV) activity in vitro, and greater effects at higher silver concentrations. (FCV is used as a surrogate for norovirus—a virus that causes vomiting and diarrhea in humans—because FCV is more easily grown in cell culture.) In vivo, the type of food available and the temperature of the environment strongly affect the antimicrobial activity of silver.
A 0.1% aqueous silver solution is bactericidal but somewhat irritating to tissue; a 0.01% solution is bacteriostatic. A 0.5% solution is sometimes applied as a dressing on burns to decrease infection and induce rapid eschar formation. Silver is frequently used for wound management in veterinary and human medicine: silver sulfadiazine cream and silver-impregnated wound dressings are widely available.
Colloidal silver compounds, which release silver ions slowly, are bacteriostatic and have a more sustained effect than other silver compounds. They do not irritate the tissues, and they have little astringent or caustic effect. They are generally used as mild antiseptics and in ophthalmic preparations.