A kind of microorganisms was isolated from the homogenized fluid of silkworm larvae infected with so-called infectious flacherie. It was identified as Mycoplasma. The isolate was examined for morphological characteristics, biological properties, and infectivity. The microorganism isolated formed minute semispherical colonies on Hofstad's medium. A fine polymorphic structure was observed in it under the electron microscope. The microorganism required no serum for its growth, had no ability to decompose any carbohydrate or reduce methylene blue, and was gamma-hemolytic for horse erythrocytes. It was obscure if it reduced blue tetrazolium. The isolate was pathogenic for silkworm larvae, which showed a chronic symptom after a considerably long period of incubation.
The infectivity titer to newly-hatched larvae was 4 to 5. By per os infection, young larvae were susceptible to the isolated Mycoplasma. With the advance in development of the instar, however, the larvae decreased considerably in susceptibility; that is, the fourth instar larvae showed a markedly weaker susceptibility than any of the first to third instar larvae, but the fifth instar larvae and pupae failed to show any susceptibility.
In silkworm larvae clinically infected with this Mycoplasma, multiplication of the microorganism was observed in the epithelium of the alimentary canal. It was found that no repeated transmissions had lowered the pathogeniciey of this microorganism.