The pityrosporon of Malassez, the flask bacillus of Unna or the "bottle" bacillus, is an organism the true nature and proper classification of which it is impossible to determine in the light of present knowledge. It appears in the scales of dandruff, in seborrheic dermatitis and frequently in psoriasis. It is part of the flora of comedones. Probably it is not a bacillus at all, but belongs to the fungi. It is usually an oval or roundish body with one "bud" which gives it a flasklike or gourdlike appearance. It is fairly regular in size (about 7 microns in length). No mycelian forms have been observed in scales. It is easily seen in scales soaked in a 20 per cent solution of potassium hydroxide or when stained with ordinary aniline dyes. Knowledge of the organism is practically limited to its morphologic characteristics. Its pathogenicity has not been determined.
ENGMAN MF, HAGEBUSCH OE. LXV.—PITYROSPORON OF MALASSEZ. Arch Derm Syphilol. 1933;28(6):855–858. doi:10.1001/archderm.1933.01460060092016