324 MUTATION AND PLANT BREEDING 



Elective culture methods, operating by reduction of populational 

 variability through selective growth, diminish the number of differ- 

 ent types of organisms originally present in a soil sample. Various 

 methods have been proposed to retain and separate physiological 

 and taxonomic groups in toto. Besides the physical difficulty in sepa- 

 ratino all oroanisms from their micro-environments in a heterogene- 

 ous soil suspension, the microorganisms will be in different physio- 

 logical states and growth phases from active metabolism and multi- 

 plication to dormancy. Separation of the various groups assumes a 

 greater difference between groups than between members within the 

 same group. Thus, various physical separation techniques may be 

 effective in resolving artificial mixtures of different organisms during 

 "reconstruction experiments" but fail when applied to soil samples. 



Besides methods favoring the growth of one group of micro- 

 organisms, there is the reverse technique of inhibiting all but one 

 group. Addition of antibiotics to plating media is common (42). This 

 method may be adequate for the recovery of new strains producing 

 known antibiotics but could not be expected to uncover organisms 

 producing new antibiotics. A variation could be used to isolate strains 

 producing unknown antibiotics in such low yield as to make their 

 initial identification impossible. Spent broths showing activity in 

 step (c) but too dilute to classify in step (d) could be concentrated 

 and added to plating media in step (a) to recover, by a system of 

 positive feedback, strains producing higher yields or similar anti- 

 biotics of greater specific activity. 



A separate intensive approach concentrates upon different 

 groups of microorganisms isolated without an initial regard to their 

 ability to produce antibiotics. The most intensively studied group 

 has been the aerobic sporulating actinomycetes since success breeds 

 success. Related groups, either less differentiated, such as the nocar- 

 dias, corynebacters, and propionic acid bacteria, or more complex 

 forms, such as the actinoplanes and streptosporangia, would be logical 

 departures. Other groups might be chosen on the basis of similar 

 habitat, growth forms, physiology, production of organic compounds, 

 or possible unbalanced growth. 



It is impossible to predict what group of organisms or set of 

 conditions will yield new antibiotics. Even macroorganisms may pro- 

 duce useful substances (70, 106). This is due to a lack of connection 



