In the routine identification of fungi, minute structural changes such as the size and arrangement of spores are seldom important. Undue emphasis on points of microscopic differentiation might even be misleading, as for instance in certain fluffy colonies suspected of being Trichophyton gypseum or Trichophyton purpureum, for with both organisms the fuseaux are infrequent, spiral forms are uncertain but may be present and microconidia and mycelium are similar. In other words, the microscopic picture is not definitive. The gross cultural appearance by itself is not always significant unless one includes the pigment produced in the substrate. This production of pigment is not generally appreciated as a constant characteristic of certain fungi. Our observations indicate uniform pigment formation with both T. purpureum and Microsporum lanosum. With T. purpureum a rose purple pigment is typical, and we have never observed a similar hue in the environment of any other species of pathogenic
LEWIS GM, HOPPER ME. PIGMENT PRODUCTION BY FUNGI: I. NUTRITIVE REQUIREMENTS. Arch Derm Syphilol. 1941;44(3):453–462. doi:10.1001/archderm.1941.01500030135015
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