42 A MANUAL OF THE ASPERGILLI 



nies as soon as possible after sporulation begins. For freeing one species or 

 strain of Aspergillus from another, or from a contaminating Penicillium, 

 streaking upon Czapek's solution agar usually gives satisfactory results. 

 The method finds its greatest usefulness in separating comparatively slow- 

 growing molds, such as the Aspergilli, from such very rapidly growing forms 

 as the Mucoraceae, Trichodcrma, etc. The critical step in this procedure 

 is to isolate the slow growing Aspergilli during the first two to three days 

 before the whole plate is overrun by the spreading, faster growing forms. 

 If the contamination is bacterial in nature, malt extract agar or some other 

 medium of more acid reaction should be substituted. Diluting the spores 

 in water before streaking is not recommended since this often tends to dis- 

 perse the contaminating organism more than it does the desired form; this 

 is especially true with bacterial contaminations, actinomycetes, and other 

 minute forms. 



Single Spore Cultures 



The isolation of cultures from single spores is a time-honored technique 

 with many mycologists, and with many workers it is considered a "must." 

 When properly employed, there is much to recommend this practice. There 

 are also certain dangers inherent in this procedure, hence we believe it worth 

 while to consider the subject at some length and to analyze both its advan- 

 tages and its limitations. As generally employed, the primary objective of 

 single spore isolations is to secure cultures of unquestionable purity. If the 

 operation is skillfully performed and adequately verified, there can be no 

 doubt but that the resulting colony will represent a pure culture of a single 

 species or strain. The continued purity and physiological stability of such 

 cultures, however, cannot be taken for granted. There is no substitute for 

 vigilance, and the culture thus isolated must be kept under critical ob- 

 servation. 



As a means of purifying a culture, i.e., separating it from foreign forms, 

 the single spore method undoubtedly has its place ; but it cannot be recom- 

 mended as a means of preserving the morphological and physiological sta- 

 bility of a particular strain. One has only to study the reports of Stakman 

 and his associates to realize that monospore selection and isolation is not a 

 touchstone to strain stability. In fact, they have ably exploited the tech- 

 nique to show just the opposite. While we do not have an accumulation of 

 data on the Aspergilli and related genera which can compare with that on 

 the smuts and other pathogenic fungi, we do have enough information to 

 know that one of the best methods of obtaining change in some of the 

 Aspergilli is to make a series of successive monosporous isolations. For 

 every species and strain there is apparently a normal range of natural 

 variation, and by isolating single spores it is often possible to secure certain 



