Evaluation of therapeutic effects and pharmacokinetics of antibacterial chromogenic agents in a silkworm model of Staphylococcus aureus infection.

The therapeutic effect of dye compounds with antibacterial activity was evaluated in a silkworm model of Staphylococcus aureus infection. Among 13 chromogenic agents that show antibacterial activity against S. aureus (MIC = 0.02 to 19 μg/mL), rifampicin had a therapeutic effect. The ED(50) value in the silkworm model was consistent with that in a murine model. Other 12 dyes did not increase survival of the infected silkworms. We examined the reason for the lack of therapeutic efficacy. Amidol, pyronin G, and safranin were toxic to silkworms, which explained the lack of therapeutic effects. Fuchsin basic and methyl green disappeared quickly from the hemolymph after injection, suggesting that they are not stable in the hemolymph. Although coomassie brilliant blue R250/G250, cresyl blue, and nigrosin showed no toxic effects or instability in the hemolymph, they also did not have a therapeutic effect. The in vitro antibacterial actions of these dyes were inhibited by silkworm plasma or bovine serum albumin and filtration experiments demonstrated that cresyl blue bound to plasma proteins in the silkworm, suggesting that plasma protein binding inhibited the therapeutic efficacy of these four dyes. These findings indicate that drug screening using the silkworm infection model is useful for evaluating toxicity and pharmacokinetics of potential antibiotics.