Function of mucilaginous secretions in the antibacterial immunity system of Eisenia fetida

The mucus layer on the skin surface of Eisenia fetida consists of several antimicrobial agents that provide a first line of defense against invading pathogens. The function of skin secretions in the antibacterial immunity system of this earthworm was confirmed for the first time in the present study. E. fetida individuals were immersed in a bacterial bath of Escherichia coli to induce antibacterial skin secretions. The secretions were found to be selective. After induction with E. coli, bacteriostasis of earthworm skin secretions after exposure to E. coli and Staphylococcus aureus increased significantly, especially towards S. aureus, although no bacteriostatic effect was observed after exposure to Pseudomonas aeruginosa. In addition, the up-cuticle protuberances and microfloccus in the up-cuticle dissociation surface may have important functions in the mucilaginous secretions of E. fetida. Furthermore, we found that earthworm gland cells in the epidermis, such as small granular proteinaceous cells, large granular orthochromatic mucous cells, and reticulation mucous cells, are responsible for the mucilaginous secretions. Our results suggest that mucilaginous secretions are an important defense in the antibacterial immunity system of E. fetida.