Chapter 22: Proteobacteria – Major Groups & Metabolic Diversity

The immense and highly diverse phylum Proteobacteria is professionally categorized into five major classes, which collectively represent numerous crucial environmental species and significant human and animal pathogens. The initial class, Alphaproteobacteria, is characterized by many oligotrophs, including the obligate intracellular parasites of the genus Rickettsia, which rely heavily on host ATP and nutrients due to their highly reduced genomes, and the purple nonsulfur bacteria, known for their exceptional metabolic flexibility as photoorganoheterotrophs. Phylogenetically, this class is linked to the ancestral protomitochondrion, which gave rise to eukaryotic organelles. Important life cycles featuring unique prosthecae and budding are demonstrated by genera like Caulobacter and Hyphomicrobium, while the order Rhizobiales encompasses both the nitrogen-fixing plant symbiont Rhizobium and the plant pathogen Agrobacterium. The Betaproteobacteria class includes key pathogens such as Neisseria and Bordetella, alongside vital environmental chemolithotrophs; for example, Nitrosomonas specializes in ammonia oxidation during nitrification, and acid-tolerant, colorless sulfur bacteria like Thiobacillus oxidize sulfur compounds to generate energy, contributing to industrial processes and phenomena like acid mine drainage. Gammaproteobacteria is the largest class, spanning anaerobic purple sulfur bacteria that internally store sulfur granules, to the ecologically indispensable Pseudomonas species recognized for their expansive catabolic abilities and role in mineralization. This group also contains Shewanella, a facultative anaerobe celebrated for its metabolic adaptability and unique ability to perform dissimilatory metal reduction against insoluble extracellular metals using specialized electron transport strategies like nanowires. The Deltaproteobacteria primarily consists of two functional groups: anaerobic sulfate-reducing bacteria, such as Desulfovibrio, which are integral to the sulfur cycle, and highly specialized predators, including the periplasm-invading Bdellovibrio and the social, gliding Myxobacteria that form complex, spore-filled fruiting bodies when nutrients are scarce. Finally, the smallest class, Epsilonproteobacteria, features important pathogens like Campylobacter and Helicobacter pylori, the latter of which utilizes urease enzyme activity to neutralize the acidic gastric mucosa, alongside various chemolithoautotrophs adapted to extreme habitats like deep-sea hydrothermal vents.