Streptomycete Spores

Abstract

Gram‐positive bacteria of the genus Streptomyces are major inhabitants of soil and grow as branching hyphae, strikingly like the filamentous fungi. Whereas their mycelium is multicellular, dispersal and long‐term survival are achieved by unicellular spores. Spore formation is part of a complex developmental cycle involving dedicated reproductive aerial hyphae. The long apical cells of the aerial hyphae are divided into chains of tens of prespore compartments, which in turn mature into thick‐walled spores with condensed nucleoids and low metabolic activity, primed for dispersal and survival. Aerial hyphae and spore surfaces are coated with a proteinaceous fibrous sheath. This highly hydrophobic sheath is required for aerial hyphal emergence, and may aid spore dispersal and interaction with various interfaces. When exposed to appropriate environmental conditions, spores rehydrate and swell. Cell polarisation is required for the emergence of a germ tube, which develops into a mycelium by apical growth and side‐branch formation.

Key Concepts:

  • Streptomyces spores are distinctly different from bacterial endospores.

  • Spores develop from dedicated reproductive structures called aerial hyphae.

  • Streptomyces spores are formed by division of long, multigenomic hyphal cells into unigenomic compartments.

  • A thick spore wall is assembled from within the maturing spore and contributes to quiescence and resilience.

  • Spores are dehydrated, accumulate trehalose and have condensed nucleoids.

  • An outer fibrous sheath confers surface hydrophobicity to aerial hyphae and spores.

  • Germination involves rehydration, symmetrical swelling and establishment of polar growth leading to germ tube emergence.

Keywords: differentiation; sporulation; germination; cell division; chromosome segregation; cell wall

Figure 1.

Schematic view of spore formation in Streptomyces. The vegetative mycelium is represented by hyphae growing down in the medium. On the surface of the colony, aerial branches emerge and extend away from the medium and into the air. Initially, the aerial hyphae form occasional crosswalls, similar to the crosswalls observed in vegetative hyphae. After extension into the air has ceased, the apical cell of the aerial hyphae is segmented into prespores by multiple and regularly spaced sporulation septa. Maturation and rounding up of the prespores result in the final chain of spores, ready for dispersal in the environment.

Figure 2.

Streptomyces spores. (a) Scanning electron micrograph of S. coelicolor spores (foreground) and aerial hyphae (background), encased within a fibrous sheath having a striking paired rodlet architecture. (b) Transmission electron micrographs of mature spore chain cross‐sections. The chains are held together by the fibrous sheath (indicated by arrows), which presumably ruptures to release free spores. Scale bars: (a) 200 nm and (b) 500 nm.

Figure 3.

Schematic view of sporulation septation and spore wall formation. (a) Aerial hyphae are encased within a fibrous sheath (FS), outside of the hyphal wall (HW). (b) Sporulation septa (SS) grow inwards and have a double annular appearance. (c) Thickening of the wall at the sporulation septa is the first stage of spore wall (SW) formation. (d) A uniformly thick spore wall is assembled on the inside of the original hyphal wall. (e) Spore maturation includes rounding up to an ovoid shape and detaching from each other. The fibrous sheath and the original hyphal wall tend to break up to release the spores. Some studies have reported a separation of the spore wall into distinct layers, as indicated in panel (e), but the significance of this observation remain unclear. Redrawn from Wildermuth and Hopwood .

Figure 4.

Micrographs showing different stages of spore germination in S. coelicolor. From left to right, the images show a phase‐bright spore, a spore that has become phase‐dark and partially swollen, a swollen spore in which the cell polarity determinant DivIVA (here visualised using a DivIVA‐EGFP fusion protein) has assembled at a specific location, and finally, a spore with emerging germ‐tube with DivIVA‐EGFP at the apex. Scale bar: 2 μm. Reproduced with permission from Flärdh ().

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Further Reading

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Dyson P (ed.) (2011) Streptomyces. Molecular Biology and Biotechnology. Norfolk, UK: Caister Academic Press.

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McCormick JR (2009) Cell division is dispensible but not irrelevant in Streptomyces. Current Opinion in Microbiology 12: 689–698.

Swiercz JP and Elliot MA (2012) Streptomyces sporulation. In: Abel‐Santos E (ed.) Bacterial Spores: Current Research and Applications. Norfolk, UK: Caister Academic Press.

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Elliot, Marie A, and Flärdh, Klas(Aug 2012) Streptomycete Spores. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000308.pub2]