THE ACTIXOMYCETES, Vol. II 



those of bacteria. Couch emphasized the re- 

 semblance of the mycelium and sporangia of 

 Actinoplanes to those of the chytrids; he con- 

 cluded thai this genus may represent :i con- 

 necting link between the bacteria and the 

 lower fungi. 



The Generic Problem with Actinomv- 

 cetes 



Prior to 1943, several systems of classifi- 

 cation of actinomycetes had been proposed. 

 In most instances, all the species were in- 

 cluded in a single genus, which was fre- 

 quently designated by different names. The 

 most common of these names were the two 

 oldest, Streptothrix and Actinomyces. Al- 

 though occasional efforts had been made to 

 separate the actinomycetes into several gen- 

 era, such attempts usually failed to receive 

 more than passing attention. The work of 

 Waksman (1919), 0rskov (1923), Jensen 

 (1931), and Erikson (1935) finally led Waks- 

 man and Henrici to suggest, in 1943, the divi- 

 sion of the actinomycetes into four genera. A 

 new genus, Streptomyces, was proposed to in- 

 clude those forms that are characterized by 

 the production of an aerial mycelium with 

 catenulate spores. Most of the important an- 

 tibiotic-producing organisms subsequently 

 have been found to belong to this genus. 



Unfortunately, this generic separation 

 brought with it a number of new problems, 

 which can be briefly summarized as follows: 



1. There is considerable overlapping 

 among the different genera, notably between 

 certain forms of Streptomyces that have lost 

 the capacity to produce aerial mycelium and 

 species of Nocardia, as brought out in a re- 

 cent paper by Gordon and Smith (1955); 

 there is also overlapping between certain 

 nocardiae and mycobacteria. 



2. The formation by species of Strepto- 

 myces and by certain forms of Nocardia of 

 two different types of mycelium, substrate 

 and aerial, and the influence of previous con- 

 ditions of cultivation upon the growth and 



biochemical activities of these organisms 

 served to confound the existing confusion. 



The nomenclatural status of the genera of 

 Actinomycetales has recently been discussed 

 by Lessel (1960). 



Lechevalier et al. (1961) described a new 

 genus Micropolyspora (type species M. bre- 

 vicatena), an organism that fragments like 

 the members of the family Actinomycetaceae 

 and sporulates like a member of the Strepto- 

 mycetaceae, by forming chains of conidia on 

 aerial hyphae; it also forms chains of conidia 

 on the substrate mycelium. These authors 

 suggested that the family Strcptomycetaceae 

 be dropped and the family Actinomycetaceae 

 be enlarged to include the genera Actinomy- 

 ces, Micromonospora, Thermoactinomyces, 

 Waksmania, Micropolyspora, Nocardia, and 

 Stri ptomyces. 



What Is a Microbial Species? 



In the study of the taxonomy of any group 

 of living organisms, including microorgan- 

 isms, one is faced sooner or later with the 

 problem of defining what is meant by a 

 species. With microorganisms, in usual prac- 

 tice, a microbial culture is designated by 

 a name, sometimes qualified with a strain 

 number; its morphological and cultural 

 properties, and frequently its ecological and 

 etiological characteristics, are described suf- 

 ficiently so that anj'one who finds this or- 

 ganism in nature will be able to recognize it 

 from the description. If possible, the type 

 form of the species is preserved in a type 

 culture collection, to aid in the future identi- 

 fication of the species. 



Unfortunately, microbial forms and types 

 of organisms are not fixed in nature or even 

 in culture. Some strains, even those closely 

 related to the fixed type, may differ enough 

 to raise a question as to their exact or specific 

 identity. This frequently leads, often on the 

 basis of only minor differences, to the crea- 

 tion of new species that are given new epi- 

 thets. This is particularly true of those mic- 



