Feather degradation in Stenotrophomonas maltophilia relies on conversion and internalization of keratin monomer

Feather keratin is the most abundant nitrogen source waste in nature. This insoluble material cannot be directly utilized as nutrition by most organisms, especially animals and plants. While feathers are naturally decomposed by some microorganisms through keratinase-based degradation which remains mechanistically not fully understood. In this study, we find that when feathers serve as the only nutrient source for Stenotrophomonas maltophilia culture, keratin monomer of about 10 kDa is present in the medium as a predominant hydrolyzed product. We further show that keratin monomers bind to cells and in turn enter cells via an undetermined pathway. The cell entry of keratin monomer elicits keratinase activity to promote the forward reaction in keratin hydrolysis. This study highlights the importance of keratin monomer production as the first step in feather biodegradation, in which the insoluble feather is converted into soluble intermediate to facilitate its internalization and complete hydrolysis. ImportanceA large amount of feathers have been produced in poultry breeding, which could cause environmental pollution if not processed in time. On the other hand, amino acids degraded from feathers can be used in many fields, such as feed, fertilizer, daily chemicals and other fields. Biodegradable feathers have the advantages of low energy consumption, mild conditions and no destruction of the amino acids obtained from degradation. We previously isolated and identified a highly efficient feather-degrading bacterium, Stenotrophomonas maltophilia DHHJ, which can grow well on the medium with feathers as the only nutrient and completely degrade feathers. We know, feather particles are much larger than bacterial cells. In what form and how they are absorbed into cells by bacteria are interesting and critical questions for feather degradation. In our research, feathers had been first degraded extracellularly by basically expressed keratinase into keratin monomers. The keratin monomers bound to cells and enter across the membrane and can induce further expression of keratinase. The understanding of keratin monomers provides new clues for the study of feather degradation mechanisms.