Filamentous And Mycelial Growth

Although prokaryotes are typically considered unicellular, a number of groups exhibit filamentous growth. Some bacteria (streptococci) and cyanobacteria (e.g., Nostoc) grow filamentously, not as continuous hyphae, but as chains of cells. In some cases, this is because dividing cells do not separate completely but in others it provides some of the advantages of true mycelial organisms in allowing mycelia to compartmentalize and differentiate. For example, in N2-fixing cyanobacteria, anaerobic conditions required for N2 fixation can be localized in some cells, called heterocysts, while others carry out oxygenic photosynthesis. Chemical communication between cells allows two-way flow of nutrients and also signaling processes leading to regular distribution of N2-fixing cells along filaments.

The actinobacteria (previously termed the actinomycetes) exhibit the greatest variety of growth forms, ranging from single-celled rods and cocci to mycelial structures (Prosser and Tough, 1991). For example, arthrobacters and some rhodococci grow as single cells that do not completely separate and thus do not form true mycelia, but which may subsequently fragment. Others, such as Nocardia, are dimorphic and form true, branching hyphal structures during early growth, which then fragment as conditions become less favorable. The hyphal form facilitates colonization of soil particles and, potentially, movement across barren regions to new nutrient sources. Fragments may subsequently develop centers of mycelial growth and their major function may be in dispersal.

The most highly developed mycelial structures are formed by streptomycetes, which grow as branched hyphae forming a true mycelium, similar to those of filamentous fungi (Chap. 6). Hyphal fragmentation may occur under certain conditions, but the major means of dispersal is through exospores borne on aerial hyphae. Exospores do not exhibit the high levels of resistance to environmental extremes of endospores, but are very resistant to desiccation. This enables their survival in the soil, but is also an important factor in their very effective dispersal through the atmosphere. On solid medium, spores develop at the center of actinobacterial colonies fueled by lytic products of substrate mycelium, which stop growing as nutrients become limited. These organisms therefore achieve growth over short distances by extension and branching of hyphae and dispersal by producing high numbers of single-celled spores.

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