Possible antimicrobial defense by free sugars on the epidermal surface of aquatic vertebrates

Abstract

Biochemical, lectin histochemical, and densitometrical methods demonstrated contents and spectra of free sugars in the skin of aquatic vertebrates (fishes: cod, marine trout, freshwater trout, perch; amphibians: Koa-Tao-Island caecilian, smooth newt, edible frog, South-African clawed toad; mammals: northern fur seal, common seal, otter, Capybara). Total free sugar contents, particu- larly sialic acids, were relatively higher in freshwater species (perch, Capybara) than in marine species. In amphibians, the caecilian and the newt showed higher amounts of free sugars than the anurans; strong reactions indicated a specific involvement of α-L-fucose in mucus functions. Lectin histochemistry demonstrated positive reactions in the stratum superficiale, the outer stratum spinosum, and specific cells of the fish epidermis. The integumental free sugar spectrum of the amphibians was narrow, with generally lower reaction intensities, except for mucus glands. In the mammals, the epidermal cells exhibited rather weak lectin histochemical staining, except for the northern fur seal and the Capybara, where the upper vital epidermis and corneal cells reacted strongly. The mammalian apocrine skin glands, particularly, exhibited positive reaction staining in their secretory endpiece. The free sugar spectrum was similar in all species: α-D-mannose, β-D-N-acetylglucosamine, α-D-N-acetylgalactosamine, α-D-galactose, β-D-galactose, α-L-fucose, sialic acids (mainly in fish epidermis and skin glands). High concentrations of free sugars on the skin surface possibly impede attacks of commensal skin micro-inhabitants (bacteria, fungi) against the integrity of the epidermis. This features a basic or general biological mechanism that operates before the specific immune system is activated.