Chapter 21: Gram-Negative Bacteria Beyond Proteobacteria

The survey includes unique extremophiles such as the Deinococci, which exhibit extraordinary resistance to ionizing radiation and desiccation due to their robust genome repair systems, and the Mollicutes, or mycoplasmas, a phylum defined by the complete lack of a cell wall, resulting in pleomorphic morphology, small genomes, and a dependence on host-derived sterols for osmotic stability. Significant metabolic adaptations are explored in the PVC superphylum, featuring Planctomycetes that perform the globally crucial anammox reaction (anaerobic ammonia oxidation) within specialized anammoxosomes, along with Chlamydiae, which are obligate intracellular parasites with an energy-parasitic metabolism and a distinct life cycle utilizing infectious elementary bodies and reproducing reticulate bodies. Photosynthetic bacteria are categorized by their light-harvesting complexes, differentiating oxygenic Cyanobacteria—which utilize thylakoids, phycobilisomes, and specialized nitrogen-fixing heterocysts—from anoxygenic groups like the green sulfur and green nonsulfur bacteria, which rely on chlorosomes. Finally, the chapter details bacteria noted for their unusual motility mechanisms, including the flexible, helical Spirochaetes that use enclosed periplasmic flagella for corkscrew movement (e.g., agents of syphilis and Lyme disease), and the Bacteroidetes, essential chemoheterotrophs in the human gut that degrade complex substrates and employ a unique Type IX Secretion System to facilitate rapid gliding motility across surfaces, a mechanism also found in the spindle-shaped Fusobacteria, which are significant members of oral biofilms and are linked to certain human cancers.