4.2 Proteobacteria

Characteristics and Significance of Proteobacteria Classes

Proteobacteria make up one of the largest and most diverse phyla of bacteria. All five classes are Gram-negative, but they differ widely in their oxygen requirements, metabolic strategies, and ecological roles. You'll find them everywhere: soil, oceans, deep-sea vents, and the human gut. Some fix nitrogen for plants, others cause serious infections, and still others help clean up environmental pollutants.

Understanding Proteobacteria matters across multiple fields. In medicine, several major human pathogens belong to this group. In agriculture, nitrogen-fixing species reduce the need for synthetic fertilizers. In environmental science, certain species break down toxic compounds through bioremediation.

Classes of Proteobacteria

Alphaproteobacteria are aerobic or facultatively anaerobic, with remarkably diverse metabolisms. Different species can be photoautotrophic (energy from light), chemoautotrophic (energy from inorganic chemical reactions), or chemoheterotrophic (energy from organic compounds). This class includes important nitrogen-fixing bacteria like Rhizobium, which convert atmospheric $N_2$ into forms plants can use.

Betaproteobacteria are aerobic or facultatively anaerobic and primarily chemoheterotrophic. Some species carry out denitrification, reducing nitrate ($NO_3^-$) back to nitrogen gas ($N_2$), which plays a major role in the nitrogen cycle. This class includes plant pathogens like Ralstonia solanacearum as well as symbionts.

Gammaproteobacteria are the largest and most metabolically varied class. They are aerobic or facultatively anaerobic and mostly chemoheterotrophic, though the class also includes purple sulfur bacteria (Chromatium) that use $H_2S$ as an electron donor for photosynthesis. Bioluminescent species like Vibrio fischeri belong here, along with many clinically important human pathogens (Salmonella, Yersinia, Escherichia coli).

Deltaproteobacteria are predominantly anaerobic. Key members include sulfate-reducing bacteria (Desulfovibrio) and iron-reducing bacteria (Geobacter), both of which play critical roles in anaerobic environments. Some species have unusual lifestyles: Bdellovibrio is a predatory bacterium that invades other Gram-negative cells, and Myxococcus forms multicellular fruiting bodies under starvation conditions.

Epsilonproteobacteria are microaerophilic (they need oxygen, but at lower levels than atmospheric concentration) or fully anaerobic. They are chemoheterotrophic and include the human pathogen Helicobacter pylori as well as deep-sea hydrothermal vent symbionts like Sulfurovum.

Representative Proteobacteria Species

• Alphaproteobacteria: Rhizobium leguminosarum forms symbiotic relationships with legumes (peas, beans, lentils). It colonizes root nodules and fixes atmospheric $N_2$ into ammonia ($NH_3$), providing bioavailable nitrogen directly to the plant.
• Betaproteobacteria: Burkholderia cepacia is an opportunistic pathogen that poses a serious threat to cystic fibrosis patients. It's also capable of degrading various pollutants, including pesticides and herbicides, making it useful in bioremediation.
• Gammaproteobacteria: Escherichia coli serves as the primary model organism in molecular biology and genetics. Most strains are harmless gut commensals, but pathogenic strains cause intestinal infections (diarrhea, hemorrhagic colitis) and extraintestinal infections (urinary tract infections).
• Deltaproteobacteria: Myxococcus xanthus exhibits cooperative social behavior, forming fruiting bodies when nutrients are scarce. It moves by gliding motility and produces bioactive compounds with potential pharmaceutical applications, including antitumor and antibiotic properties.
• Epsilonproteobacteria: Helicobacter pylori colonizes the acidic environment of the human stomach. It causes gastritis and peptic ulcers and is classified as 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 dependence on synthetic fertilizers and improving soil fertility. This is one of the most agriculturally important bacterial processes on Earth.
2. Bioactive compound production by Myxococcus xanthus offers new avenues for drug discovery. Its secondary metabolites include compounds with antitumor and antibiotic activity.
3. Bioremediation by Betaproteobacteria such as Burkholderia and Comamonas can break down environmental pollutants, aiding in cleanup of contaminated sites.

Pathogenic Interactions

1. Gastrointestinal infections: Pathogenic E. coli strains (EHEC, ETEC) cause diarrhea and hemorrhagic colitis, and EHEC can trigger life-threatening hemolytic uremic syndrome (HUS). Helicobacter pylori causes chronic gastritis, peptic ulcers, and increases gastric cancer risk.
2. Respiratory infections: Bordetella pertussis (Betaproteobacteria) causes whooping cough, while Legionella pneumophila (Gammaproteobacteria) causes Legionnaires' disease. Both can lead to severe respiratory complications.
3. Urinary tract infections: Uropathogenic E. coli (UPEC) is the most common cause of UTIs. Recurrent infections can lead to kidney damage if left untreated.
4. Opportunistic infections: Immunocompromised patients are especially vulnerable. Burkholderia cepacia causes lung infections in cystic fibrosis patients, and Pseudomonas aeruginosa (Gammaproteobacteria) is a leading cause of hospital-acquired (nosocomial) infections at various body sites. Both exploit weakened immune defenses.

Cellular Structures and Communication

Three structural and behavioral features are especially worth knowing for Proteobacteria:

• Flagella provide motility and enable chemotaxis, which is directed movement toward or away from chemical signals in the environment. This helps bacteria find nutrients or avoid toxins.
• Lipopolysaccharide (LPS) is a complex molecule embedded in the outer membrane of all Gram-negative bacteria, including Proteobacteria. LPS contributes to pathogenicity because it triggers strong immune responses (it's an endotoxin), and the outer membrane itself helps confer resistance to certain antibiotics.
• Quorum sensing is a cell-to-cell communication system in which bacteria release and detect signaling molecules. When the population reaches a threshold density, coordinated behaviors kick in, such as biofilm formation and virulence factor production. This is why some infections become much harder to treat once a biofilm has established.