Surfactants lower the surface tension of an aqueous solution and are used as wetting agents, detergents, emulsifiers, antiseptics, and disinfectants. As antimicrobials, they alter the energy relationship at interfaces. The position of the hydrophobic moiety in the molecule determines whether a surfactant is classified as anionic or cationic.
Anionic Surfactants for Use With Animals
Soaps are dipolar anionic detergents with the general formula RCOONa/K, which dissociate in water into hydrophilic K+ or Na+ ions and lipophilic fatty acid ions. Because NaOH and KOH are strong bases (whereas most fatty acids are weak acids), most soap solutions are alkaline (pH 8–10) and may irritate sensitive skin and mucous membranes. Soaps emulsify lipoidal secretions of the skin and remove dirt, desquamated epithelium and bacteria, which are then rinsed away with the lather. The antimicrobial potency of soaps is often enhanced by inclusion of certain antiseptics—eg, phenols, carbanilides, or potassium iodide. Soaps are incompatible with cationic surfactants.
Cationic Surfactants for Use With Animals
Cationic detergents are a group of alkyl- or aryl-substituted quaternary ammonium compounds (QACs—eg, benzalkonium chloride, benzethonium chloride, cetylpyridinium chloride) with an ionizable halogen, such as bromide, iodide, or chloride. The major site of action of these compounds appears to be the cell membrane, where they are adsorbed and cause changes in permeability. The activity of older QACs is decreased by hard water and by porous or fibrous materials (eg, fabrics, cellulose sponges) that adsorb them. QACs are also inactivated by anionic substances (eg, soaps, proteins, fatty acids, phosphates). Therefore, they are of limited value in the presence of blood and tissue debris. However, newer dialkyl QACs (fourth generation, including dodecyl dimethyl ammonium bromide, dioctyl dimethyl ammonium bromide, etc) purportedly remain active in hard water and are tolerant of anionic residues.
Fifth-generation QACs are mixtures of the fourth generation with the second generation and demonstrate greater biocidal activity under conditions of high soil load, making them useful disinfectants in barns and foot baths. QACs are effective against enveloped viruses, some fungi (including yeasts), and protozoa (including Giardia cysts), but not against nonenveloped viruses, mycobacteria, and spores. QACs are commonly used for disinfecting noncritical instruments and cleaning hard surfaces.
Aqueous solutions at concentrations of 1:1,000 to 1:5,000 have antimicrobial activity, especially at slightly alkaline pH. However, extensive use of QACs at subinhibitory concentrations has led to the emergence of antimicrobial-resistant bacteria (especially Escherichia coli and Salmonella spp) and thus a public health risk; five QAC resistance genes (qacE, qacEΔ1, qacF, qacG, and sugE [p]gram-negative bacteria. Cross-resistance to phenicols (florfenicol and chloramphenicol) results from gradual increases in QAC concentrations.
When applied to skin, QACs may form a film under which microorganisms can survive. For that reason, QACs have limited reliability as antiseptics. Concentrations >1% are injurious to mucous membranes.
Octenidine dihydrochloride is a cationic surfactant used increasingly in Europe as an alternative to quaternaries, chlorhexidine, and iodophors for skin, mucous membrane, and wound antisepsis. It is effective against Dichelobacter nodosus (which causes footrot in sheep), even in the presence of dirt. Octenidine initially appeared promising against MRSA, but emerging resistance has subsequently been observed for this disinfectant as well as others.
