o() 



NATUHK, FORMATION, AND ACTIVITIES 



tioii of highly active cuhures. When tliis 

 method is combined with other procedures, 

 such as radiation treatments, highly active 

 substrains can be obtained. Strain selection 

 of streptomycin-producing organisms has 

 been studied further l)y Savage (1949), I)vi- 

 laney et al. (1949), Dulaney (1953), and 

 Pittenger and McCoy (19o;-5). 



Exposui'e of antagonistic actinomycetes 

 to various mutagenic agents has been the 

 most successful method of obtaining better 

 antibiotic-forming strains. In this area, ultra- 

 violet light has played an important role, as 

 shown by the following procedure of Darken 

 d al. (1960), who discussed the methods used 

 to improve tetracycline-forming organisms. 

 An isolate was selected from among the 

 better produc(n-s. It gave an average of 125 

 ^g per ml of antibiotic. A spore suspension 

 of the culture was irradiated with ultraviolet 

 light at an exposure sufficient to kill 99.99 

 per cent of the spores, and 200 colonies were 

 transplanted to agar tubes. A total of 19 iso- 

 lates, or 9.5 per cent, showed various types 

 of gross morphological variation. When 

 X-ray treatment was used, an exposure of 

 100,000 roentgen units gave 99.5 per cent 

 kill. A total of 340 colonies was picked and, 

 of these, 149 (44 per cent) were gross mor- 

 phological variants. (3f 45 isolates obtained 

 by natural selection and 135 by X-ray treat- 

 ment, none showed a significant increase in 

 antibiotic production. Treatment with ultra- 

 violet irradiation, however, resulted in three 

 superior isolates. 



A culture giving a 20 per cent higher 

 tetracycline yield than the original isolate, or 

 an average of 150 /xg per ml, was selected for 

 medium development studies; 1250 deriva- 

 tives of this culture were examined. Of these, 

 160 (23 per cent) of 700 ultraviolet-treated 

 isolates and 190 (38 per cent) of 500 X-raj^- 

 treated isolates were morphological variants. 

 None of the 50 natural selections, however, 

 gave evidence of gross morphological change 



(see also Katagiri, 1954). The variability of 

 naturally occurring oxytetracycline-produc- 

 ing strains has been studied by Boi-enstain 

 and Wolf (1956). 



There was a greater numl)er of superior 

 antibiotic-producers as well as a higher 

 incidence of morphological types in the X- 

 ray-treated isolates than in either the ultra- 

 violet-treated or the untreated isolates. Of 

 640 ultraviolet-treated cultures, only three 

 gave significant increases in broth potency 

 over the parent. Of these, a culture was 

 selected as a control for the testing of 390 

 derivatives from the X-ray treatment of one 

 culture (TVS). The yields obtained with 

 these selected isolates from this culture 

 grown in a synthetic medium ranged from an 

 average of 1955 to 2150 ^g pei" nil. 



Goldat (1958) reported that the treatment 

 of spores of >S. aureofaciens with various 

 doses of ultra\'iolet rays causes an increase 

 in the mutation rate. A high dose of ultra- 

 \'iolet rays and a four-fold photoreactivation 

 gave the largest number of morphological 

 mutations; an exposure to smaller doses was 

 found to l)e more effective for increasing the 

 variation of activity. A single photoreactiva- 

 tion with daylight reduced both the lethal 

 and the mutational effects, whereas a re- 

 peated reactivation increased the latter. 



Alikhanian (1959b) discussed the results 

 obtained by various investigators in improv- 

 ing strains of S. griseus, S. rimosus, and >S. 

 aureofaciens in order to increase production 

 of streptomycin, oxytetracycline, and chlor- 

 tetracycline. liable 12 shows the predomi- 

 nant role played Ijy ultraviolet light and 

 X-rays. 



Some chemical mutagenic agents such as 

 nitrogen mustard have also found a place in 

 the searc'h for more active mvitants. Among 

 other mutagenic agents, actinophage may be 

 mentioned (Ilina and Alikhanian, 1957). 

 Further studies on the genetic interactions 

 among streptomycetes and the selection of 

 active strains is found in the work of Bradley 



