22 MISCELLANEOUS PUBLICATION 95 5, U.S. DEPT. OF AGRICULTURE 



vegetation, humus, and polluted water. For yeasts, f rass and exudates 

 of various sorts may also be good. Moist, tropical soil is probably the 

 best single source for isolating the greatest number and diversity of 

 species, although bacteria would be present in higher numbers in 

 dung. It has been estimated that the number of bacteria in fertile 

 soil is often as high as 100 million per gram, representing 6 to 12 genera 

 and perhaps twice that number of species. The numbers of acti- 

 nomycetes and fungi might total more than a million, including 

 mycelial fragments and spores, and the number of species represented 

 might easily reach 75 to 100. The greatest number of micro-organisms 

 are in the upper 2 inches of soil. A variety of fungi can be isolated 

 directly from bits of dead wood, leaves, or debris ; even more can be 

 found after incubating such materials in moist chambers. 



CULTURE COLLECTIONS 



Although many micro-organisms can be isolated rather readily 

 from nature by a competent microbiologist, often a particular strain 

 is desired because of its unique properties. It may be a strain used 

 for a particular assay, for the production of a particular product in 

 high yields, for a particular enzyme system, or for checking the re- 

 port of others on the behavior of a particular organism; or it may 

 be the type strain one wishes to study for comparative purposes. 

 Some organisms are geographically isolated, such as the fungus 

 Blakeslea trispora Thaxt., which can be found only in the Tropics. 

 Where specific strains are desired, it is best to acquire them from a 

 culture collection. 



Culture collections are of two kinds. The first is the large collec- 

 tion with hundreds of cultures of many different species and genera. 

 The second is the specialized collection with a few to many strains 

 usually limited to a single group or genus and maintained by an in- 

 vestigator who has spent a lifetime in isolating different forms and 

 assembling all the type cultures available from collections around the 

 world. 



There are several large general collections of micro-organisms in 

 the United States. Foremost is the American Type Culture Collec- 

 tion in Washington, D.C., which uses the letters ATCC preceding 

 its culture numbers. According to the sixth edition of its catalog, 

 the American Type Culture Collection has over 4,350 strains avail- 

 able for sale. This collection was established in 1924. It consists 

 of bacteria, fungi, yeasts, bacteriophages, viruses, and a few algae 

 and protozoa. It includes cultures of both animal and plant 

 pathogens. 



Two government collections that are more specialized are the ARS 

 (Agricultural Research Service) Culture Collection, U.S. Depart- 

 ment of Agriculture, located at Peoria, 111., and the Quartermaster 

 Culture Collection at Natick, Mass. 



The ARS Culture Collection has micro-organisms that are useful 

 or related to useful strains employed in industrial microbiology. Some 

 are strains that produce such products as penicillin; others are used 

 for the assay of vitamins and antibiotics, or serve as type strains for 

 comparative taxonomic purposes. No catalog is issued for this collec- 



