Identification of a Serratia marcescens virulence factor that promotes hemolymph bleeding in the silkworm, Bombyx mori

•S. marcescens culture supernatant caused continuous bleeding in injected silkworms.•Serralysin metalloprotease was purified as the bleeding–promoting factor.•Recombinant Serralysin protein induced silkworm bleeding.•Disruption of the ser gene attenuated the bleeding–promoting activity.•Serralysin contributes to S. marcescens virulence via inhibition of wound healing. Injection of culture supernatant of Serratia marcescens, a Gram-negative bacterium pathogenic to a wide range of host animals including insects and mammals, into the hemolymph of silkworm (Bombyx mori) larvae led to continuous flow of the hemolymph (blood of insects) from the injection site. The amount of hemolymph lost within 60min reached 15–20% of the total larval weight. Using a bioassay with live silkworms, we purified Serralysin, a metalloprotease that requires divalent cations for its activity, as the factor responsible for the promotion of hemolymph bleeding from the culture supernatant of S. marcescens. Recombinant protein also induced hemolymph bleeding in silkworms. Moreover, the culture supernatant of an S. marcescens disruption mutant of the ser gene showed attenuated ability to promote hemolymph bleeding. In addition, this bleeding–promoting activity of the S. marcescens culture supernatant was attenuated by disruption of the wecA gene, which is involved in the biosynthesis of the lipopolysaccharide O-antigen. These findings suggest that Serralysin metalloprotease contributes to the pathogenesis of S. marcescens by inhibiting wound healing, which leads to a massive loss of hemolymph from silkworm larvae.