Proteobacteria are diverse bacteria with five main classes. They're found everywhere, from soil to deep-sea vents. Some fix nitrogen for plants, while others cause diseases. Their varied metabolisms and structures make them crucial in ecosystems and human health.

These bacteria interact with humans in both good and bad ways. Some help clean up pollution or produce useful compounds. Others cause infections like E. coli or H. pylori. Understanding Proteobacteria is key for medicine, agriculture, and environmental science.

Characteristics and Significance of Proteobacteria Classes

Classes of Proteobacteria

• Alphaproteobacteria are Gram-negative, aerobic or facultatively anaerobic bacteria with diverse metabolic capabilities including photoautotrophy (obtaining energy from light), chemoautotrophy (obtaining energy from chemical compounds), or chemoheterotrophy (obtaining energy from organic compounds) and includes important nitrogen-fixing bacteria that convert atmospheric nitrogen into bioavailable forms for plants (Rhizobium)
• Betaproteobacteria are Gram-negative, aerobic or facultatively anaerobic bacteria with chemoheterotrophic metabolism, some species capable of denitrification (reducing nitrate to nitrogen gas), and includes plant pathogens (Ralstonia solanacearum) and symbionts
• Gammaproteobacteria are Gram-negative, aerobic or facultatively anaerobic bacteria with chemoheterotrophic metabolism, includes bioluminescent bacteria (Vibrio fischeri) and purple sulfur bacteria (Chromatium), and contains many human pathogens (Salmonella, Yersinia)
• Deltaproteobacteria are Gram-negative, anaerobic bacteria that includes sulfate-reducing bacteria (Desulfovibrio) and iron-reducing bacteria (Geobacter), some species are predatory (Bdellovibrio) or form fruiting bodies (Myxococcus)
• Epsilonproteobacteria are Gram-negative, microaerophilic (require low oxygen levels) or anaerobic bacteria with chemoheterotrophic metabolism, includes human pathogens (Helicobacter pylori) and symbionts found in deep-sea hydrothermal vents (Sulfurovum)

Representative Proteobacteria species

• Alphaproteobacteria: Rhizobium leguminosarum forms symbiotic relationships with legumes (peas, beans) and fixes atmospheric nitrogen in root nodules, providing bioavailable nitrogen for the plant
• Betaproteobacteria: Burkholderia cepacia is an opportunistic pathogen in cystic fibrosis patients and is capable of degrading various pollutants (pesticides, herbicides)
• Gammaproteobacteria: Escherichia coli is a model organism for molecular biology and genetics research, while some strains are pathogenic, causing intestinal (diarrhea) and extraintestinal infections (urinary tract infections)
• Deltaproteobacteria: Myxococcus xanthus forms fruiting bodies and exhibits gliding motility, and produces bioactive compounds with potential pharmaceutical applications (antitumor, antibiotic properties)
• Epsilonproteobacteria: Helicobacter pylori colonizes the human stomach and causes gastritis, peptic ulcers, and is a risk factor for gastric cancer

Proteobacteria-human interactions

• Beneficial interactions:
  1. Nitrogen fixation by Rhizobium in legume root nodules provides bioavailable nitrogen for plants, reducing the need for synthetic fertilizers and improving soil fertility
  2. Production of bioactive compounds with pharmaceutical potential by Myxococcus xanthus, offering new avenues for drug discovery and development
  3. Bioremediation of pollutants by some Betaproteobacteria (Burkholderia, Comamonas) can aid in environmental cleanup and restoration
• Pathogenic interactions:
  1. Gastrointestinal infections caused by Escherichia coli strains (EHEC, ETEC) leading to diarrhea, hemorrhagic colitis, and potentially life-threatening hemolytic uremic syndrome, and Helicobacter pylori causing gastritis, peptic ulcers, and increasing the risk of gastric cancer
  2. Respiratory infections such as whooping cough caused by Bordetella pertussis (Betaproteobacteria) and Legionnaires' disease caused by Legionella pneumophila (Gammaproteobacteria), both leading to severe respiratory symptoms and complications
  3. Urinary tract infections commonly caused by uropathogenic Escherichia coli (UPEC), resulting in painful and recurrent infections that can lead to kidney damage if untreated
  4. Opportunistic infections in immunocompromised individuals, such as lung infections in cystic fibrosis patients caused by Burkholderia cepacia (Betaproteobacteria) and nosocomial infections in various body sites caused by Pseudomonas aeruginosa (Gammaproteobacteria), both exploiting weakened immune systems to establish infection

Cellular structures and communication

• Many Proteobacteria possess flagella, which are important for motility and chemotaxis (directed movement in response to chemical gradients)
• The outer membrane of Gram-negative Proteobacteria contains lipopolysaccharide, a complex molecule that contributes to pathogenicity and antibiotic resistance
• Some species use quorum sensing, a cell-to-cell communication mechanism, to coordinate group behaviors such as biofilm formation and virulence factor production