To obtain therapeutically effective new antibiotics, we first searched for bacterial culture supernatants with antimicrobial activity in vitro and then performed a secondary screening using the silkworm infection model. Through further purification of the in vivo activity, we obtained a compound with a previously uncharacterized structure and named it 'lysocin E'. Lysocin E interacted with menaquinone in the bacterial membrane to achieve its potent bactericidal activity, a mode of action distinct from that of any other known antibiotic, indicating that lysocin E comprises a new class of antibiotic. This is to our knowledge the first report of a direct interaction between a small chemical compound and menaquinone that leads to bacterial killing. Furthermore, lysocin E decreased the mortality of infected mice. To our knowledge, lysocin E is the first compound identified and purified by quantitative measurement of therapeutic effects in an invertebrate infection model that exhibits robust in vivo effects in mammals.

Porphyromonas gingivalis, a pathogen that causes inflammation in human periodontal tissue, killed silkworm (Bombyx mori, Lepidoptera) larvae when injected into the blood (hemolymph). Silkworm lethality was not rescued by antibiotic treatment, and heat-killed bacteria were also lethal. Heat-killed bacteria of mutant P. gingivalis strains lacking virulence factors also killed silkworms. Silkworms died after injection of peptidoglycans purified from P. gingivalis (pPG), and pPG toxicity was blocked by treatment with mutanolysin, a peptidoglycan-degrading enzyme. pPG induced silkworm hemolymph melanization at the same dose as that required to kill the animal. pPG injection increased caspase activity in silkworm tissues. pPGinduced silkworm death was delayed by injecting melanizationinhibiting reagents (a serine protease inhibitor and 1-phenyl-2-thiourea), antioxidants (N-acetyl-L-cysteine, glutathione, and catalase), and a caspase inhibitor (Ac-DEVD-CHO). Thus, pPG induces excessive activation of the innate immune response, which leads to the generation of reactive oxygen species and apoptotic cell death in the host tissue.The immune system is crucial for animal self-defense against pathogenic microorganisms. On the other hand, excessive activation of immune responses may cause serious damage to the host. Severely infected human patients die of septic shock and multiple organ failure, which seem to be caused by excessive activation of the host immune systems by the pathogens. The underlying pathologic mechanisms of sepsis, however, are not clear, and effective antiseptic remedies are yet to be established.In vertebrates, the immune system is divided into two categories: innate immunity and acquired immunity. Because innate immunity is the front line of host defense against pathogens and is involved in initiating acquired immune responses, it is a prominent target in studies of the pathology of diseases caused by immune system deregulation. Most of the major components of the innate immune system are conserved from vertebrates to invertebrates (1). Thus, invertebrate models with simple biologic systems are useful for studying the activation mechanisms of innate immune responses, but there are no reports of pathologic invertebrate models that are killed by excessive innate immune responses caused by bacterial invasion of the bloodstream.The silkworm, Bombyx mori, is an invertebrate model that can be administered quantitative injections of pathogens and drugs (2-4) and is suitable for the study of innate immunity (5). We recently reported that peptidoglycans and glucans, the cell wall components of bacteria and fungi, respectively, trigger the generation of reactive oxygen species (ROS) 2 from silkworm blood cells (hemocytes) followed by the activation of serine proteases and induce the activation of paralytic peptide (PP), an insect cytokine (6). At the same time, peptidoglycans and glucans induce melanization, a polymerization reaction of melanin that involves serine proteases (7,8), and phenoloxidase (a copper-conta...

Background: Serratia marcescens induced an acute increase in immune cell numbers in silkworms. Results: Serralysin metalloprotease was identified as a factor that inhibited immune cell adhesion and bacterial clearance. Conclusion: S. marcescens suppresses cellular immunity by degrading adhesion molecules. Significance: A novel mode of immune evasion by S. marcescens is proposed.

Background:The pathogenic mechanism of Serratia marcescens is poorly understood. Results: S. marcescens kills immune cells via an lipopolysaccharide-and flagella-dependent mechanism. Conclusion: S. marcescens suppresses innate immunity by killing immune cells.Significance: This is the first evidence to suggest that S. marcescens evades the immune system.

No abstract