Silkworm larvae as an animal model of bacterial infection pathogenic to humans

Silkworm larvae, Bombyx mori, were examined as an animal model of human infection with pathogenic bacteria. When 3 × 10⁷ cells of Staphylococcus aureus (S. aureus), Pseudomonas aeruginosa, or Vibrio cholerae were injected into the blood of fifth instar silkworm larvae, over 90% of the larvae died within 2 days, whereas over 90% survived for 5 days after injection of the same amount of Escherichia coli. Growth of S. aureus was observed in larvae blood and tissues. Immunostaining analysis revealed that S. aureus proliferated at the surface of the midgut. Infection of silkworm larvae by methicillin-sensitive S. aureus was cured by ampicillin, oxacillin, and vancomycin, whereas infection by methicillin-resistant S. aureus was not cured by ampicillin or oxacillin, although vancomycin was effective. Disinfectants were not effective because of toxicity against the larvae. Thus, silkworm larvae are useful for evaluating antibiotics for pathogenic bacterial infection in humans.