VARIATION'S. MITATIOXS. AND Al )AI'TATJ()XS 



107 



( GRAV WYiELIUV 



|T[ f/YCEL'UMl 



ITE SECTOR) 



(GRflV MYCELIUM) 



PARENT ISOLATE 66 

 (DARK- GRAY mycelium: 



(GRAY MYCELIUM) 



(GRAY MYCELIUM) 



(GRAY MYCELIUM. 

 ONE SECTOR) 



(PARENT TYPE) 



(GRAY MYCELIUM) 



(GRAY MYCELIUM) 



(GRAY MYCELIUM) 



Figure 52. Diagrammatic relation of the sectors produced by single-cell cultures of a streptomyces 

 from a parent isolate (Reproduced from: Bchaal, L. A. J. Agr. Res. 69: 173, 1944). 



between pathogenicity and cultural (;harac- 

 teristics of the strains. 



Mutagenic Effects 



In recent years, extensive use has been 

 made of the mutagenic effects of irradiation 

 and of certain chemical agents. These ha\'e 

 found extensi^'e application in obtaining 

 special strains of antibiotic-producing or- 

 ganisms. It is difficult to state definitely 

 whether we are dealing here with true muta- 

 tions or with the elimination of certain 

 varieties in a highly variable microbial cul- 

 ture. 



Jensen reported that, under the influence 

 of ultraviolet rays, strains of Nocardia, iso- 

 lated from Australian soils, gave rise to new 

 forms; some of these resembled typical 

 species of Streptomyces and others were 

 closely related to the mycobacteria. Under 

 the influence of LiCl, mycobacteria gave rise 

 to forms that might be considered as species 

 of Nocardia. 



Savage reported that ultra \'iolet rays were 



less mutagenic, in the treatment of strepto- 

 myces, than were x-rays; 0.710 A and 0.210 

 A w^ave lengths were most efficient. Muta- 

 tion rates increased with killing rates up to 

 99.9 per cent of killing. When doses of 

 1,000,000 roentgens were used, as high as 50 

 per cent mutation rates were observed on 

 morphological properties and 40 per cent on 

 streptomycin production . 



By means of x-ray and ultraviolet light 

 irradiations, Kelner found that many cul- 

 tures not possessing any antibiotic properties 

 gave rise to antibicjtic-protlucing mutants. A 

 strain of S. griseus kept for a long time 

 (more than 30 years) in the culture collection 

 and which was inactive antibiotically was 

 induced to form a mutant that produced 

 streptomycin. The freciuency of active mu- 

 tants ranged from 0.01 to 1.2 per cent; mu- 

 tants obtained from th(> same parent cul- 

 ture varied in the nature of the antibiotic 

 they were able to produce or in their anti- 

 biotic spectra. The viability of spores ex- 

 posed to tiltraviolet irradiation could be 



