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



intensively isolated and screened for new products. The rewards in 

 new commerical antibiotics have been tremendous, as seen in the 

 production of streptomycin, chloramphenicol, chlorotetracycline, 

 oxytetracycline, erythromycin, tetracycline, and in the subsequent 

 discoveries that some antibiotics stimulate animal growth, that they 

 may also be used in the preservation of food, and also that 

 actinomycetes are a source of vitamin B^. 



Micro-organisms, as they occur in their wild state, are restricted to 

 certain ecological habitats and some, at least, are restricted to geo- 

 graphical areas. An understanding of their growth requirements, 

 the substrates they occur on or in, and their activities in the wild state 

 gives clues as to which micro-organisms may best be suited to solve a 

 particular problem or to give a particular product. Fungi that grow 

 first on a substrate rich in sugars are the best organic acid producers. 

 Aspergillus and Rhizopus are usually the first and the most rapid- 

 growing forms on decaying plant material, and their members are the 

 best producers of citric, gluconic, and fumaric acids. Bacteria that 

 ferment unpasteurized milk produce the most lactic acid. On the 

 other hand, such organisms as algae tend to grow in water where 

 nutrients are low. Consequently, most media devised for algae have 

 small amounts of salts and a very low organic content. 



At the Northern Regional Research Laboratory much work was 

 done to develop dextran. It was known that the best organisms to 

 produce dextran gums were members of the bacterial genus Leuconos- 

 toc. There were many references that material high in sugar, when 

 contaminated with members of this genus, became thick and viscid. 

 Knowledge of this characteristic led to a survey of strains 

 of Leuconostoc and to the development of a process to make dextran 

 by using a culture of Leuconostoc mesenteroides (Cienk.) v. Tiegh. 

 Solvents, such as alcohol, acetone, and butanol, are produced by or- 

 ganisms, some of which live under anaerobic conditions. 



In any search for a new product or process a mere random screening 

 program should be tempered by a review of which micro-organism 

 might best offer a possible solution. When no clues are available, then 

 a random screening must be attempted. For example, a search for an 

 organism to destroy lignin would not include any of the sugar fungi, 

 but rather would concentrate on those forms that grow on and in 

 wood. Likewise, a search for an organism suitable for the assay of an 

 unknown growth factor would concentrate on organisms difficult to 

 grow on any medium and incapable of growing on a defined medium 

 in which all the known growth factors are added. 

 m Understanding the life cycle of micro-organisms should be con- 

 sidered in selecting an organism for fundamental studies. For ex- 

 ample, B. O. Dodge's pioneering work on Neurospora showed that this 

 organism was ideally adapted to genetical studies, and further work 

 was carried out by *G. W. Beadle and E. L. Tatum. On the other 

 hand^ a genus such as Mucor would offer extremely poor material for 

 genetical studies, since all the reproductive structures are multinucleate 

 and the diploid state, zygospore (the cell resulting from nuclear 

 fusion) , often resists germination. The same also applies to mutation 

 work. Organisms with a single nucleus and a single set of chromo- 

 somes are probably much easier to mutate than cells of an organism 

 with many nuclei. 



