Preliminary Observations on the Fine Structure of Species of 
Micromonospora (Actinomycetales) 
Diana D. Kenner 1 , Hans-Rudolf Hohl 2 , and Gladys E. Baker 3 
The genus Micromonospora was first described 
by 0rskov in 1923 as a member of the Acti- 
nomycetales characterized by unicellar mycelium 
and the production of single, terminal conidia. 
The genus was little known for some time 
(Jensen, 1930), but subsequently the frequent 
occurrence of micromonosporae in soil (Jensen, 
1932), lake bottoms (Erikson, 1941; Potter 
and Baker, 1956) and lake water (Potter and 
Baker, 1956), became well recognized. In spite 
of this wide acquaintance with the genus, the 
general morphology and cell structure is still 
not well understood and descriptions are some- 
times at variance. The chemical composition of 
the actinomycete wall, including micromono- 
sporae, (Erikson, 1947; Avery and Blank, 
1951; Yamaguchi, 1965) is better documented 
than is the general morphology of the cell 
structure. Ultrastructure studies are few to date, 
and provide little detail (Agre, 1962; Leude- 
mann and Brodsky, 1964). Both of these studies 
were concerned primarily with spores. A paper 
by Arai, Koyama, Kuroda, and Honda (1964) 
is more definitive for both mycelium and spores. 
It also includes some electron micrographs. 
1 1934 N. E. Ballinger Way, Seattle, Washington 
98155. 
2 Department of Microbiology, University of 
Hawaii, Honolulu, Hawaii 96822. 
3 Department of Botany, University of Hawaii, 
Honolulu, Hawaii 96822. Manuscript received Janu- 
ary 24, 1967. 
The small size of the mycelium and spores 
in the micromonosporae has made study with 
the light microscope difficult and, in part, ac- 
counts for the lack of critical morphological 
and cytological information. Waksman (1961) 
characterized the substrate or vegetative hypae 
as straight or curved, branching, and without 
cross-walls. The lack of cross-walls has been 
accepted widely (Jensen, 1930; Erikson, 1941; 
Krasil’nikov and Agre, 1965), although in 
1964 Arai et al. illustrated septa. 
There is increasing recognition of the eco- 
nomic importance of the micromonosporae as 
sources of antibiotics (Leudemann and Brodsky, 
1964; Weinstein, Leudemann, Oden, and Wag- 
man, 1965) and as etiologic agents of disease 
in man (Castellani, De Brito, and Pinto, 1959). 
In view of this and of the discrepancies among 
reports on their structure, there is need for a 
consistent comparative study of the general cell 
structure and the process of spore production in 
the group. Consequently, studies at the ultra- 
structural level were undertaken for three spe- 
cies: Micromonospora fusca Jensen (isolated 
from Flathead Lake water, Potter, No. M- 
1012) ; M. purpurea Leudemann and Brodsky 
(nrrl No. 2953); and M. sp., isolated from 
a foot lesion at the University of Miami. All 
cultures came from the collection of Dr. Louise 
F. Potter, Biology Department, Elmira College, 
Elmira, New York. 
Fig. 1 . Micromonospora fusca, showing the electron-dense cytoplasm and fibrillar nucleoid. The two 
roundish masses probably represent prespores. X 32,000. 
Fig. 2. Spore of Micromonospora fusca, showing the sculptured spore wall. X 26,000. 
Fig. 3. Micromonospora purpurea, with septum and Y-shaped branching. X 25,000. 
Fig. 4. Micromonospora purpurea, with prespore in form of a constricted and bulbous hyphal tip; Y-shaped 
branching as in Figure 3. X 22,500. 
Fig. 5. Micromonospora purpurea, showing general cell morphology and septa of various thicknesses. X 
22,500. 
Fig. 6. Example of septate hypha in pathogenic Micromonospora spp. X 22,500. 
Fig. 7. Longitudinal section through hypha of Micromonospora purpurea, demonstrating the frequent oc- 
currence of septa. X 25,000. 
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