Scanning electron microscopy of some endophytic streptomycetes in snakevine-Kennedia nigricans

Abstract

Soils of all types and locations have generally served as the major sources of streptomycetous bacteria. These organisms are the source of nearly 80% of the world's antibiotics. Now, it is realized that Streptomyces spp. (within the group of prokaryotic filamentous bacteria known as actinomycetes) can exist as endophytes within the interstices of some higher plants. While it is sometimes possible to isolate one or two different streptomycetes from certain plants, most plants are free of these organisms. However, the snakevine (Kennedia nigricans) of the Northern Territory of Australia has yielded at least 39 different endophytic actinomycetes (95% of them being Streptomyces spp.) Most of these isolates possessed no detectable antibiotic properties, while at least seven had antibacterial and antifungal activities. Examination of eight selected cultures by scanning electron microscopy (SEM) as well as environmental scanning electron microcopy (FEI ESEM FEG) (FEI Company, Hillsobro, Ore., USA) revealed unusual patterns, structures, and features of the spores and hyphae of these microorganisms. For instance, as revealed by ESEM FEG for the first time, it has become obvious that extremely fine hair-like structures (average 25-49 nm with gold-coated specimens) exist on the spores and hyphae of some endophytic streptomycetes. The biological purpose of these hair-like protrusions is unknown. Both SEM and ESEM FEG can be effectively used as tools in identification and elucidation of the biology of these organisms. In addition, unusual colony morphology, observed with the unaided eye can very easily be used to distinguish some of these isolates since characteristic donut and pseudo-horn shaped colonies appeared in culture.