PRESERVATION OF CULTURES 57 



can be recovered as required simply by streaking some of the soil particles on fresh 

 agar slants. 



"It is not in all cases advisable to depend on soil alone for the preservation of 

 valuable stocks, but reserve stocks may without difficulty be prepared on soil, and 

 the writers believe that in many instances soil will be found to be an excellent medium 

 for maintenance, with a minimum of change over long periods of time." 



During the past two years the soil method has been used at the Northern 

 Regional Research Laboratory with but minor modifications of the tech- 

 nique cited above (fig. 13 C). Its principal advantages lie in the fact that 

 (1) the viability of strains is apparently lengthened, and (2) from a single 

 stock tube, opened with proper care, repeated cultures can be started 

 simply by removing some of the soil particles to suitable substrata. 



Vegetable Substrata 



The oriental fermentation industries maintained their inoculating mate- 

 rial as selected rice or soybeans upon which the mold had been grown under 

 favorable conditions to produce maximum quantities of spores. This nu- 

 trient, dried and packaged, was stored and sold under the Japanese name 

 "Koji". Samples examined after several years showed excellent viability. 



Bainier was a pharmacist. He distributed licorice root in sections 5 to 10 

 mm. in diameter and 5 to 8 cm. in length in test tubes, sterilized them, and 

 kept his cultures regularly for years upon them. Tested by us, the method 

 was a very satisfactory laboratory practice. American mycologists have 

 successfully used bean stems for the purpose. Apparently any organic 

 material which provides frameworks of cellulose enmeshing sufficient nutri- 

 ents to support mold growth without complete breakdown of the mass may 

 be used. 



CONTAMINATION 



Mixed Strains 



In the routine conduct of cultural work, contaminations of cultures of one 

 species of Aspergillus by other species, or species of other genera, is very 

 common. The conidia of most molds are exceedingly light and are carried 

 freely in the air. Entire exclusion of such contamination is difficult. In 

 dealing with contaminations, several problems arise and different proce- 

 dures are possible. A colony of a single Aspergillus, well established, 

 usually inhibits the growth of other species developing in the immediate 

 vicinity. Even if invasion occurs, the effects are commonly so distinct as 

 to leave little doubt as to the limits of the different forms. When, however, 

 the contamination with spores or mycelium is carried in the inoculum and 

 so placed as to germinate in intimate contact with the organism desired, (1) 

 the species may sector out, and hence be easily recognized, or (2) the colony 



