66 STUDYING MOLDS, YEASTS, AND ACTINOMYCETES 



Temperature of Incubation. The optimum temperatures of molds, 

 yeasts, and aetinomycetes vary rather widely according to the species, 

 and extensive studies are not available for most groups. Fortunately, 

 with many species, the top of the temperature growth curve appears 

 again to be a plateau rather than a peak, so that one has more lati- 

 tude than with most bacteria. It is noteworthy that most of the 

 pathogenic species do not grow best at body temperature but at a 

 point considerably lower. Even the highly parasitic dermatophytes 

 find their optimum temperature at 27° C. according to Kadisch.^^ 

 In general, 30° C. falls near the optimum for most common patho- 

 genic and saprophytic fungi, and the bacteriological laboratory which 

 has frequent occasion to work with these microorganisms should be 

 equipped with a special incubator kept at or near this temperature. 

 Mrak and Bonar ~^ showed that the temperature of incubation has a 

 decided influence on the formation of yeast ascospores as well as on 

 their size. Lower temperatures tend to reduce the size of the asco- 

 spores in relation to the ascus and therefore the individual spores are 

 easier to delineate and study. In some groups, e.g., Rhizopus, the 

 temperature range is of taxonomic significance. 



Determination of Morphology. The identification of fungi de- 

 pends more upon morphological characters than that of bacteria 

 does. The appearance of the plant mass to the naked eye is fre- 

 quently quite characteristic, and in many cases the organism can be 

 recognized at a glance. It should be borne in mind, however, that 

 both gross morphology and microscopic characters may sometimes 

 vary not only with changes in the composition of the medium but 

 also with the age of the culture. 



For observing the morphology of molds a much better view can be 

 obtained if the fungus is grown in a Petri dish rather than in a cul- 

 ture tube. Certain precautions must be observed, however. Great 

 care must be exercised in handling plate cultures of pathogenic fungi, 

 and certain agents of pulmonary and generalized mycoses such as 

 Coccidioides immitis and Nocardia asteroides which produce large 

 numbers of easily dissociated air-borne spores and hyphal fragments 

 should never be planted in Petri, dishes. In cases of non-pathogenic 

 fungi it is likewise necessary to avoid dissemination of spores in the 

 laboratory when the Petri dish cover is removed since such spores 

 will be a troublesome source of contamination. Plate cultures can 

 usually be uncovered safely when the colony is young and has just 

 begun to form mature spores. Fortunately many of the morpho- 

 logical features of taxonomic interest ^re rnost easily seen in a young 

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