Overview:

This lecture will describe two additional developmental strategies found in the prokaryotic world that have profound influences on the metabolic states of the cells. First, the process of nitrogen fixation will be discussed (again). This time, the emphasis will be on N₂ fixation as carried out by symbiotic organisms, especially the rhizobia. Second, the lifestyle and developmental program of the gliding myxobacteria will be presented.

Symbiotic N₂ fixation and the Rhizobia

Many different species of bacteria participate in symbiotic relationships with eukaryotic hosts (see Table I on handout and Table 19.8). One well characterized example occurs between different closely related species of bacteria (the Rhizobia) and leguminous plants.

1. Rhizobia:

Obligately aerobic Gram negative bacteria capable of nitrogen fixation only in symbiotic association with legume roots. Differentiated bacterial cells that carry out the N₂ fixation are contained in nodules on the plants. Bacterial cells express genes important for causing the plant nodules to form (nod genes) and genes that code for the N₂ fixation enzymes (nif genes).

2. Root nodulation and establishment of symbiosis (see Fig 19.61)

• Attachment to root hairs via specific binding proteins such as rhicadhesins (bacterial) and lectins (plant).
• Host plant specificity determined (in part) by bacterial nod factors which detect plant flavonoids
• Nod factors induce root hair curling, growth of an infection thread, and proliferation of root cortical cells to form a nodule.
• Bacterial cells in infection thread are eventually enclosed within intracellular symbiosomes.
• Bacteroids are terminally differentiated, non-growing bacterial cells that express nif genes within symbiosomes.

3. Nodule metabolism (see Fig. 19.67)

• Organic acids from plant provide energy and reducing power for nitrogen fixation; fixed nitrogen is released from bacteroids as ammonia.
• Oxygen tension is regulated by the host plant.
• Boundary layer cells surround the nodule and act as a diffusion barrier.
• Leghaemoglobin is produced cooperatively by both partners to act as an O₂ buffer in nodules.

Gliding motility and sporulation in Myxobacteria

1. Myxobacteria

• Obligately aerobic, Gram-negative soil bacteria that feed primarily on lysed cells of other bacterial species.
• Undergo multicellular developmental cycle.
• Complex lifestyle seems to require a large genome (9.5 Mb for Myxococcus xanthus).

2. Vegetative growth

• Enzymes secreted by myxobacteria lyse other bacteria in environment and help to digest the released nucleic acids, proteins and lipids; these used by myxobacteria for growth.
• Lifestyle requires high cell density-- "the microbial wolf pack."
• Vegetative cells are motile by gliding motility (does not involve flagella), with individuals moving in "slime trails" left by other cells.

3. Fruiting body formation and formation of myxospores (Fig 12.47)

• Aggregation of cells leads to formation of fruiting body, and differentiation of individual cells into myxospores.
• Myxospores are fundamentally different from endospores, both in their formation and in their properties once formed.
• Myxospores form in the head of fruiting body.
• Germination of myxospores from a fruiting body leads directly to the formation of a new swarm of vegetative cells.

10/17/05