156 INFECTIONS CAUSED BY MOLDS 



tophytes and once it has replaced the original type of growth the 

 latter cannot be recovered. Old laboratory cultures are thus fre- 

 quently quite atypical unless they have been properly cared for. 

 If a strain is to be conserved in typical condition a culture should 

 be allowed to grow for 10 days (or longer if required for sporulation 

 by slowly growing species) and then stored at 2° to 5° C. until it is 

 again subcultured. Transfer periods should not exceed 4 months. 

 Strains can be conserved by leaving cultures at room temperature 

 for several months and then, only after the culture is very dry, 

 transferring spores from the upper end of the slant. Under these 

 conditions the sterile overgrowth is usually dead and conidia which 

 have not mutated but have remained viable^ when transferred to new 

 slants, will reproduce the original colony type. This method cannot 

 be depended upon, however, and strains kept in this manner are apt 

 to be lost. From these considerations it will be seen that the precise 

 determination of species is a matter requiring some experience and 

 judgment. 



Morphology in Culture. In artificial cultures specific and char- 

 acteristic structures of several kinds are produced. Most important 

 of these are the conidia which are produced in large numbers by 

 many strains and species. The conidium of the dermatophytes shows 

 more clearly tban any other single structure the close relationship 

 between the different groups. It varies in size, shape, and abundance 

 to some extent, and in one genus is lacking, but in general it is char- 

 acteristic. The size varies from 2 by 2 microns to 3 by 5 microns. 

 It is spherical to egg-shaped or even clavate, and has a thin smooth 

 wall. The conidium has a broad attachment to the conidiophore 

 and when it breaks loose from this attachment the broad base, fringed 

 by the broken fragments of the end of the conidiophore, can be easily 

 seen if the spore is carefully examined under high magnification. 

 The shape of the spore in some strains of Trichophyton is almost 

 spherical except for this basal facet. In other strains, and in Micro- 

 sporum, the conidium is usually about 3 by 5 microns or larger, and 

 is distinctly clavate, but the basal facet is like that seen in Tricho- 

 phyton. 



The attachment of the conidium in some species of Trichophyton 

 is rather persistent and in some strains one can observe spores still 

 attached to a phantom hypha in which most of the cells are empty. 

 This characteristic has led the French taxonomists to designate these 

 spores aleuries. In the most common species, however, the name is 

 a misnomer because the conidia are easily detached and these conidia 

 are obviously of the same type in all species. A second definitive 



