POLYSACCHARIDES 79 



Although early work, reviewed by Waksman (224), suggested that 

 most species of the Actinomycetales are able to attack cellulose, the 

 evidence must be considered inadequate. It is certain that some species 

 of Streptomyces and at least one of Micromonospora decompose cellu- 

 lose (56, 93, 143, 162, 222), but whether this is true of most species 

 and whether the actinomycetes as a group play an important role in 

 the decomposition of cellulose in nature remain to be determined. 



Generalizations on the distribution of cellulolytic capacity among 

 the different taxonomic groups are, it should be remembered, based on 

 limited data, not only in the sense that often only a few isolates of a 

 taxon have been tested, but also in the sense that the conditions of 

 test may not be favorable for all potentially cellulose-utilizing forms. 

 The utilization of cellulose is affected by such factors as the available 

 nitrogen (108), the temperature (126), the acidity (162), and the pres- 

 ence of other carbon compounds (85, 142). It seems likely, therefore, 

 that cellulose is used by more fungi than present information would 

 indicate. 



The widespread occurrence of cellulolytic fungi in soil and the fact 

 that addition of cellulosic materials to soil results in an increase in the 

 number of fungi isolated have led most workers to believe that, at 

 least under aerobic conditions and at moderate temperatures, the 

 fungi play a more important part in the initial decomposition of cellu- 

 lose in soil than do the cellulolytic bacteria. The procedures of soil 

 microbiology are, however, so inexact, and the total soil microflora is 

 so poorly known, that this supposition, attractive as it is, is not ade- 

 quately supported by the evidence. 



The fact of occurrence in nature on a cellulosic substrate, even very 

 frequent occurrence, is distinctly not acceptable evidence of ability to 

 utilize cellulose. Thus, Pulhdaria pallulans is one of the organisms 

 most frequently isolated from exposed cotton cloth, but it is not cel- 

 lulolytic (186). 



Chitin. Chitin, the structural polysaccharide of higher invertebrates, 

 is also a constituent of many fungi (Chapter 2). Chemically, it is a 

 linear polymer of N-acetyl-D-glucosamine, the units being linked by 

 /?-l,4-glycosidic bonds. Like cellulose, it is stable and difficult to purify 

 without alteration of structure. 



Fungi are often found growing on the exoskeletons of living or 

 dead Arthropoda. Although it is often assumed, perhaps correctly, 

 that these fungi attack chitin, this evidence by itself cannot establish 

 the point. Visible growth on or erosion of chitin has been reported 

 in studies on soil fungi and actinomycetes (189) and on aquatic fungi 



