Various views are held about the antibacterial mechanisms operating on the skin surface (for reviews see Burtenshaw, 1945; Naylor and Rook, 1968). Substances resembling long-chain fatty acids extracted from normal skin by Burtenshaw (1945) had a marked bactericidal action on Streptococcus pyogenes. Ricketts, Squire and Topley (1951) suggested that a chemical mechanism was largely responsible for the destruction of Str. pyogenes on the skin, whereas drying is responsible for the destruction of Escherichia coli and Pseudomonas aeruginosa; both factors appeared to contribute to the elimination of Staphylococcus aureus. They concluded that unsaturated fatty acids, particularly oleic acid, are the active chemical agents. These fatty acids may be produced on the skin surface as a result of the splitting of esters in the sebum by the commensal flora (Scheimann et al., 1960; Davidson, 1965; Naylor, 1970).
Many workers have studied the antibacterial action of lipids in vitro (Walker, 1924; Dubos, 1947; Pollock, 1948; Nieman, 1954; Galbraith et al., 1971; Butcher, King and Dyke, 1976), usually in fluid media in which suspensions of bacteria had been added to emulsions of various lipids. Burtenshaw (1945) was unable to demonstrate an antibacterial effect with lipid substances placed in punched-out holes in an agar medium containing haemolytic streptococci; he concluded that antibacterial lipids did not diffuse in agar medium. Dubos (1947) described the influence of various lipids incorporated in solid media on the number and diameter of bacterial colonies.
Because the lipids on the skin probably exist as a water-in-lipid emulsion (Herrmann, Prose and Sulzberger, 1953), experimental procedures with a solid medium probably imitate the physical conditions on the skin better than studies of suspensions of bacteria in aqueous solutions containing fine droplets of emulsified lipid (Milyani and Selwyn, 1978). The following qualitative and quantitative studies take account of this view. A preliminary report of the qualitative method was given by Naylor (1970).
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