Antimicrobial components in the cocoon silk of silkworm, Bombyx mori

Silkworm spins silk fibers to make the cocoon to protect the pupa from predators and pathogenic microbes. To understand the defense mechanism of the cocoon, many antimicrobial proteins are currently identified. The functionality of these proteins is studied, including protease inhibitors and seroins. Protease inhibitors are incredibly variable in sequences and domains, and most of them contain multiple pairs of disulfide bonds. Thereby they have stable structures and activities. Seroins have two motifs: the proline-rich N-terminal motif and the sequence conserved C-terminal motif. Protease inhibitors mainly play antifungal roles, whereas seroins have broad-spectrum antimicrobial activities against bacteria, fungi and viruses. These antimicrobial proteins show higher abundance in the sericin layers than in the fibroin layer and are more abundant in the outer cocoon layer than in the inner cocoon layer. Besides silk proteins, the silkworm cocoon also contains small amounts of non-protein antimicrobial components such as organic acids, alkaloids, flavonoids, and heterocyclic compounds. This review describes the extraction methods, expression pattern, microbiostatic mechanism, application fields and advantages of the antimicrobial components in the silkworm cocoon. The in-depth understanding of antimicrobial silk components will help us improve the processing technology of cocoons and expand the application fields of the cocoons.