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To characterize foot odor, we analyzed its components by sensory tests, isolated microorganisms that produce it, and evaluated the mechanism of the occurrence of foot odor. As a result, foot odor was found to be derived from isovaleric acid, which is produced when Staphylococcus epidermidis, a resident species of the normal cutaneous microbial flora, degrades leucine present in sweat. In addition, Bacillus subtilis was detected in the plantar skin of subjects with strong foot odor, and this species was shown to be closely associated with increased foot odor. Therefore, we screened various naturally occurring substances and fragrant agents that inhibit microbial production of foot odor without disturbing the normal microbial flora of the human skin. As a result, we identified citral, citronellal, and geraniol as fragrant agents that inhibit the generation of isovaleric acid at low concentrations.

The cell walls of living bacteria were chemically modified by adding cell-wall precursors. As the precursors to be incorporated into the cell wall, UDP-MurNAc pentapeptide, lipid I, and lipid II derivatives were synthesized. The aimed compounds were attached to the amine residue of lysine at the pentapeptide moiety. Fluorescein-attached UDP-MurNAc pentapeptide was efficiently incorporated into both Gram-positive and Gram-negative bacteria. In the case of Gram-negative bacteria, such as Escherichia coli, the permeability of the outer membrane (lipopolysaccharide layer) was enhanced by EDTA treatment before the incorporation. For Gram-positive bacteria, UDP-MurNAc derivatives were incorporated in the cell wall without EDTA treatment due to the lack of the lipopolysaccharide layer. Furthermore, instead of dyes, a ketone group was attached to the UDP-MurNAc pentapeptide. The ketone group was also delivered to the bacterial cell wall of lactic acid bacteria, giving a platform to attach large molecules on the surface.

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