no 



THE ACTINOMYCETES, Vol. I 



may become not only resistant to, l)ut also Often, colonies of intermediate character are 



dependent on, this agent, as was found to be produced. This is particularly true of cases 



true for bacterial strains requiring strepto- where the mutation rate is high and where 



mycin for their growth. reverse mutation occurs. 



Variation and the Action of Phage 



The lytic properties of actinomycetes, 

 especially under the influence of phage, may 

 also undergo a variety of changes, depending 

 largely upon the development of strains re- 

 sistant to phage action. When an actino- 

 mycete culture is attacked by a phage, the 

 culture will clear up after a few hours as a 

 result of destruction of the sensitive cells. 

 After further incubation, which may some- 

 times require days, the culture will begin to 

 grow again as a result of development of a 

 variant which is resistant to the action of the 

 phage. This variant can be isolated and 

 freed from the phage and will in many cases 

 retain its resistance to the action of the 

 phage even if subcultured through many 

 generations. Though the sensitive strain 

 adsorbed the phage readily, the resistant 

 variant will generally not show any affinity 

 to it. 



The variant may differ from the original 

 strain in morphological or metabolic charac- 

 teristics, in serological properties, or in 

 colony type. Most often, however, no such 

 correlated changes are apparent, and the 

 variant may be distinguished from the orig- 

 inal strain by its resistance to the inciting 

 strain of phage. It has been suggested that 

 the resistance to phage is due to a heritable 

 change of the microbial cell, which oc(;urs 

 independently of the action of the phage. 

 The mechanism may be more complex when 

 the resistant cultun; does not develop until 

 several days after lysis of the sensiti\'e cells. 



The proportion of mutants in a culture and 

 the mutation rate are detected by changes 

 in the colony type produced by the mutant, 

 either in its pigmentation, or in the character 

 of the surface or the edge of the colony. 



Genetic Recombinations 



Recent genetic studies on Streptomyces 

 have proceeded along two main lines: (a) 

 the radiation genetic studies (Xewcombe), 

 for which the uninucleate status of Strepto- 

 myces spores offers an advantage; (b) 

 studies on genetic interaction among Strepto- 

 myces. Anastomosis, or the fusion of hyphae, 

 is frequently encountered, with heterokaryo- 

 sis resulting when both types of parental 

 rmclei persist and multiply in a common 

 cytoplasm. Recombination, based on the 

 exchange of genetic characters between 

 nuclei, is a much rarer phenomenon, re- 

 ported for *S'. coelicolor, S. griseus, and S. 

 fradiae. 



Sermonti and Spada-Sermonti demon- 

 strated a parasexual process, leading to 

 genetic recombination in S. coelicolor. Three 

 types of recombination between strains oc- 

 curred: (a) strains carrying all the "wild" 

 characters of the original organism; (b) 

 some "wild" characters of one strain and 

 some mutant characters of another strain; 

 (c) mutant characters from both strains. 



Bradley and Lederberg used nutritional 

 and resistance markers to establish two 

 parental types of S. griseus. They demon- 

 strated that fusion occurs between the hy- 

 phae of the two parents, giving rise to hetero- 

 karyotic mycelium in which nuclei from both 

 parents were contained in the same hyphae. 

 The spores produced by the heterokaryon 

 were of only one parental type; they were, 

 therefore, considered as homokaryotic. The 

 recombinations did not pro^'e to be stable, 

 howe\-er. 



According to Bradley, various species of 

 Streptomyces can form heterokaryons, i.e. 

 associations showing genetic interaction 

 between diverse nuclei in a connnon cyto- 



