326 MUTATION AND PLANT BREEDING 



present but dormant and the correct conditions, usually a substrate 

 or "precursor", or a "challenge" with a competing organism or dele- 

 terious environment must be chosen to "induce" production; or (b) 

 a culture may be considered to lack the immediate potential to pro- 

 duce but may be "forced" to evolve antibiotic production in order to 

 survive by subjecting it to an artificial selection system with 

 competing microorganisms. 



The second proposal can be disposed of by some calculations on 

 the time necessary for the evolution of an estimated 30 enzymes in 

 the biosynthesis scheme of an antibiotic. The first proposal, restated 

 in operational terminology and stripped of unpalatable and uncritical 

 nonmechanistic allusions, is more attractive on our limited time scale. 

 The difficulty arises in devising a method for "inducing" and recover- 

 ing such microorganisms from a complex soil population or in deter- 

 mining which, of a large number of pure cultures, is capable of 

 "induction". 



Attempts to isolate new antibiotic-producing microorganisms 

 from soil seeded with pathogenic bacteria have failed (111). Various 

 mutagenic methods that have been proposed are probably selective 

 rather than inductive in mechanism (69). Examples of gain of func- 

 tion are cited as supporting evidence, but critical evaluation of these 

 weigh against such proposals (45, 49, 50, 51, 56, 79). 



Any effective change in screening methods in the near future 

 will probably come by way of increased efficiency of the screen 

 coupled with a wider range of sources of organisms. Specific changes 

 in screening methods can be tested by "reconstruction" experiments. 

 Soil samples can be salted with a known number of spores of an 

 antibiotic-producing actinomycete, productive at the same low levels 

 typical of new isolates, and the efficiency of recovery of the added 

 organism determined. This form of proofing of a new method would 

 be most effective when the introduced strain is genetically marked 

 to allow its identification and recovery at various stages of the 

 screen (54). 



Success in uncovering new antibiotics and strains with improved 

 yields has been dependent upon empirical methods. When continued 

 application of routine and standardized methods does not produce 

 results, the usual solution has been to increase the scope of the opera- 

 tion. Any approach but this empirical one has been dosed by lack 



