16 PHYSIOLOGY OF THE FUNGI 



there is a tendency to fail to associate experimental results with the 

 composition. 



Effect of autoclaving. Media are commonly and effectively sterilized 

 by autoclaving. It should be noted, however, that such high tempera- 

 tures may cause destruction or alteration of some constituents in the 

 media. These changes are not serious for many uses; at least media pre- 

 pared in this way are satisfactory. Sugars are among the substances 

 most easily altered by autoclaving. The extent of decomposition depends 

 upon the specific sugar used, the other constituents of the medium, and 

 the time of autoclaving. It is desirable to adopt a uniform schedule for 

 autoclaving media. An increase in the amount of caramelization occurs 

 as the time of heating is increased. Maillard (1912) showed that a brown 

 color results when reducing sugars (glucose, fi-uctose, etc.) are autoclaved 

 with amino acids. Hill and Patton (1947) have shown that growth of 

 Streptococcus faecalis is reduced when tryptophane is autoclaved with 

 sugars. Margolin (1942) found that no one method of sterilization 

 resulted in best growth for all of the 14 species tested. Phythophthora 

 erythroseptica made three times the amount of growth on glucose sterilized 

 by filtration as when the entire medium was autoclaved. Syncephalastrum 

 racemosum, however, made more growth on autoclaved than on sterile- 

 filtered glucose (Table 2). The organisms most sensitive to heated 

 glucose appear to be various species of Cytophaga, which failed to grow 

 on glucose which had been heated to 50°C. (Stanier, 1942). These 

 organisms utilized glucose which had been sterilized by filtration. Phos- 

 phates, a universal constituent of media, are active in converting glucose 

 into ketoses and other products (Englis and Hanahan, 1945) during 

 autoclaving. 



Complex sugars and polysaccharides undergo some hydrolysis during 

 autoclaving. The amount of hydrolysis is dependent upon the carbo- 

 hydrate, the time and temperature of autoclaving, and the pH of the 

 medium. Sucrose, when autoclaved in acidic media, may undergo suffi- 

 cient hydrolysis to support some growth of species unable to utilize 

 sucrose. This possibility must be guarded against in experiments on the 

 availability of complex sugars. 



Other substances used in media may be destroyed during autoclaving. 

 To minimize or avoid such effects, heat-sensitive substances may be auto- 

 claved separately, or they may be sterilized using special bacteriological 

 filters. The Berkefeld and Chamberland filters are less used than 

 formerly, while at present Seitz and fritted-glass filters are widely used. 

 Fritted-glass filters are best for most purposes, inasmuch as the asbestos 

 pad used in the Seitz filter may adsorb active compounds. All methods 

 of sterilization w^hich depend upon filtration are slow and can be used only 

 with liquid media. Various volatile chemical sterilization agents such as 



