78 



THE ACTINOMYCETES, Vol. II 



The optimum reaction for the growth of 

 actinomycetes is pi I 6.8 to 7.5. When these 



organisms are grown on complex organic 

 media, and on many synthetic media, the 

 reaction usually becomes alkaline. Some 

 actinomycetes, however, are able to grow 

 at pH 4.5 to 6.5 and even at pH 3.0 to 4.5. 

 Such forms are not common, but the reaction 

 of the substrate has been recognized as a 

 potential diagnostic property. 



On the basis of their effects on dead resi- 

 dues and upon living forms of life, actino- 

 mycetes have been grouped as saprophytes 

 and parasites, the latter being further 

 grouped into plant and animal parasites. 

 Thus we speak of "actinomycosis," caused 

 by A. bovis and A. israelii, and "nocardio- 

 sis," caused by different species of Nocardia. 

 We associate S. scabies with the "scab" of 

 potato tubers, and S. ipomoeae with a dis- 

 cvise of sweet potato roots. 



Genetics 



Little is known about the genetic proper- 

 ties of actinomycetes and their possible 

 bearing upon problems of classification. 

 Certain observations have been made re- 

 cently, however, which offer rather promis- 

 ing leads in establishing species relationships. 

 The concept of vegetative hybridization of 

 Streptomyces cultures has been suggested. By 

 repeated growth of a culture in a sterile fil- 

 trate of sand-macerated mycelium of another 

 culture, the former undergoes morphological 

 and physiological changes. The significance 

 of this phenomenon and its potential utiliza- 

 tion for species characterization are still to 

 be elucidated. Sermonti and Spada-Ser- 

 monti (1956) demonstrated several types of 

 recombination among "wild" and mutanl 

 strains of S. coelicolor (most probably S. 

 violaeeoruber) . It has been brough.1 out in 

 Chapter 6 of Volume I thai true hybrids can 

 be obtained by mating two different mutant 

 strains of an act inomycete. Welsch (1958) 

 suggested thai mating may offer a conven- 



ient criterion for the practical definition of 

 a species. The assumption was thereby made 

 that a species is distinct if it does not cross, 

 or gives only unfertile crosses with other 

 similar species. 



Type Cultures 



An important, and often-used technique 

 in species characterization of actinomycetes 

 is that of comparing fresh isolates with 

 type cultures. For the higher forms of plant 

 life, species characterization is facilitated by 

 study of preserved herbarium specimens. For 

 microorganisms, special collections of named 

 cultures are available for study. These cul- 

 tures allow comparisons of living material, 

 since dead or dried cultures are of but little 

 significance. 



In establishing type cultures of actino- 

 mycetes it is important to keep in mind the 

 fact that such cultures undergo considerable 

 variation when grown for a long time upon 

 artificial media. Some of the cultures may 

 lose their ability to produce aerial mycelium 

 and are thus deprived of properties of major 

 diagnostic value. Unknown strains of Strep- 

 tomyces free from aerial mycelium may even 

 be considered as species of Nocardia. 



According to Pridham,* if reasonably fresh 

 isolates were maintained on the proper media 

 and preserved by lyophilization, the indi- 

 vidual laboratory would experience far fewer 

 difficulties than have been experienced in the 

 past. Pridham reported that since 1953, with 

 the use of these media and techniques, very 

 rarely have strains been found that produce 

 no aerial mycelium (which is generally well 

 sporulated) on the isolation media; a very 

 low incidence of si rain degeneration has been 

 noted in active cultures as determined by 

 the methods of assessment, and all isolates 

 have been routinely lyophilized. These 

 lyophil tubes, opened from time to time, 

 have been found to give cultures that are 



* Personal communication. 



