ISOLATION OF SAPROPHYTIC ACTINOMYCETES 59 



streaks the pus or sputum over a single slant. The use of pour plates 

 may increase the chances of isolating the fungus but is an unwise 

 procedure with such pathogens as Nocardia asteroides and Coccidio- 

 ides immitis in which there is grave danger of inhaling spores from 

 an uncovered Petri dish culture. 



Often fungi appear in a culture overcrowded with colonies of bac- 

 teria. In such cases the tuft of aerial mycelium may be free of 

 bacteria, or may contain only a few, so that if one carefully touches 

 the very surface of the mold colony he may obtain a relatively or 

 absolutely pure culture. If the fungus produces few or no conidia 

 it may be necessary to postpone subculturing until the fungus has 

 grown well beyond the bacteria or up on the side of the culture tube, 

 when a portion of the mycehum can be removed with a stiff, sharp 

 needle. 



Isolation of Saprophytic Actinomycetes. In studying actinomy- 

 cetes it must be borne in mind that most of them cannot tolerate an 

 acid medium. The acid-sugar media recommended for yeasts and 

 molds cannot therefore be used. They will for the most part grow 

 on the media used for bacteria, and their growth is generally stim- 

 ulated by the addition of sugar. 



In the isolation of these organisms the greatest difficulty is experi- 

 enced in separating them from bacteria, since the latter for the most 

 part grow more rapidly and, if they form spreading colonies, will 

 overgrow the plates and suppress the actinomycetes. But most of 

 the actinomycetes can grow with very small amounts of nutrient, and 

 one may use a very weak medium on which many bacteria will not 

 grow. Those that do will not spread much. 



For general isolation of saprophytic species of actinomycetes, per- 

 haps Czapek's solution agar has been most widely used. On it many 

 bacteria will not grow and those that do are not spreaders. Some 

 difficulty is experienced with spreading molds on this medium. 



Conn's ^ glycerin asparaginate medium is excellent for isolating 

 non-pathogenic actinomycetes. 



Soil extract agar is a "natural" medium which may be used for the 

 isolation of soil actinomycetes. It is made by heating 1000 grams 

 of rich garden soil with 1 liter of water in the autoclave for half an 



