General Microbiology - Schlegel H. 1987

Prokaryotes. Systematic Review
Actinomycetes in the strict sense (Group 17)

In the strict sense, actinomycetes should be classified as Bacteria that form a mycelium-like Structure. They are found predominantly in the soil. These bacteria are Gram-positive and are linked to coryneform bacteria and mycobacteria by an almost continuous series of transitional forms. Almost all of them are aerobes.

Actinomycetes get their name from the first described species—the anaerobe Actinomyces bovis («ray fungus»), which causes actinomycosis, a disease in cattle. It forms large sulfur granules with a radiate structure in Tissues and jawbones.

Actinomycetes grow well on simple media and differ from one another in their surface and submerged Agar growth, The formation of aerial and substrate mycelium, as well as The production of spores, sporangia, etc. (Figs. 3.3 and 3.4). Proactinomycetes (genus Nocardia) form both substrate and aerial mycelium, which breaks down into rod-shaped Cells in older colonies; true spores are absent (Figs. 3.3 and 3.5).

In contrast, in streptomycetes—members of the large genus Streptomyces—the mycelium is retained. The aerial and substrate mycelium is often strongly developed; there are specialized aerial hyphae (sporophores) from which conidia are abstricted to serve for the dissemination of the species. The structure of these sporophores (straight, wavy, spiral, clustered, verticillate, etc.), colony Morphology, color, size, and odor make it possible to distinguish numerous species and strains. Streptomycetes are also responsible for the characteristic earthy smell of freshly plowed soil in the spring. An oil responsible for this odor was isolated from Streptomyces griseus; it was named «geosmin» and turned out to be 1,10-dimethyl-9-decalol. As the industrial importance of streptomycetes as antibiotic producers grew, knowledge about them gradually expanded. Out of the hundreds of isolated Antibiotics, the most effective proved to be streptomycin (produced by S. griseus), chloromycetin (produced by S. venezuelae), aureomycin, and Tetracyclines (produced by S. aureofaciens).

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Fig. 3.3. Mycobacteria, nocardiae, and actinomycetes: a — colony morphology characteristic of the given genus; b — cross-section through an agar surface overgrown with bacteria. Typical growth forms of substrate mycelium (SM) and aerial mycelium (AM), sporophores (Sph) and sporangia (Spn), as well as non-flagellated and flagellated spores (Spo) are shown.

Fig. 3.4. Colony forms of streptomycetes. A. Colonies of Streptomyces with aerial mycelium after 5 weeks of growth on glycerol-nitrate agar. B — onset of aerial mycelium formation by a colony of a Streptomyces strain. (Photo by P. Hirsch.)

Many streptomycetes utilize Cellulose, Chitin, and other recalcitrant natural substances. The cellulose-degrading actinomycete Micromonospora is widespread in soil and in the anaerobic sludge of bodies of Water. It forms flat colonies lacking an aerial mycelium; spores are situated singly at the ends of weakly branched sporophores. Microbispora looks similar, but forms an aerial mycelium and produces two conidia at a time.

Fig. 3.5. Substrate mycelium hyphae of Nocardia saturnea. Bright-field light micrograph of a smear stained with carbol fuchsin. (Photo by P. Hirsch.)

Some actinomycetes (Actinoplanes, Streptosporangium, Ampullariella) form spores not directly on aerial hyphae, but within sporangia. Streptosporangium is an aerobic, cellulose-degrading streptomycete. On solid substrates, it grows by first forming a substrate mycelium and subsequently an aerial mycelium. The tips of the aerial hyphae swell and transform into spherical sporangia 5–8 µm in diameter, which can reach 18 µm upon maturation. The supporting hypha grows into the spherical terminal Cell, coils into a spiral, and abstricts sporangiospores. The latter are non-motile in Streptosporangium. Actinoplanes grows beneath The surface of plant debris and also forms spores inside capitate sporangia. When the sporangia open, the sporangiospores become motile owing to tufts of flagella.

Another motile-spore-forming actinomycete is Dermatophilus congolensis. This is the CAUSATIVE AGENT OF dermatosis affecting the back Skin in sheep and horses; it grows on solid substrates, forming rough or smooth colonies with a dense substrate mycelium. The hyphae divide both longitudinally and transversely, resulting in the formation of up to eight parallel rows of coccoid cells, which can be released As a result of gelatinization and lysis of the hyphal wall. The coccoid spores are capable of locomotion via flagella.

Actinomycete spores are not heat-resistant, but they withstand desiccation. The only heat-resistant spore-forming actinomycete is Thermoactinomyces vulgaris. It is thermophilic and is one of the microorganisms responsible for the self-heating of stacked damp hay and organic waste. In terms of its structure and dipicolinic acid content, the spores of this species resemble the endospores of Bacillus and Clostridium.