Chapter 11 [Part 3/3] - Characterizing and Classifying Prokaryotes

Introduction to Gram-Negative Bacteria

• Gram-negative bacteria differ from gram-positive bacteria due to their thin peptidoglycan cell wall and the presence of a secondary cell membrane.

• The secondary membrane includes components like LPS (lipopolysaccharides) and lipid A, which are recognized by the immune system and contribute to pathogenicity.

Classification of Gram-Negative Bacteria

• Proteobacteria: Divided into six classes based on similarities in metabolism and other characteristics.

• Classes Include: Alpha, Beta, Gamma, Delta, Epsilon, and Zeta proteobacteria.

Alpha Proteobacteria

• Typically aerobic and thrive in low-nutrient environments.

• Notable metabolisms: nitrogen fixation, hydrogen gas production, and phototrophy.

• Key organisms:

  • Azospirillum: Associates with plant roots, stimulating growth through nitrogen fixation.

  • Rhizobium: Forms nodules in legumes, enhancing nitrogen supply to plants.

  • Rickettsia: Obligate parasite transmitted by arthropods; causes typhus and Rocky Mountain spotted fever.

  • Brucella: Infects mammals and causes fever and fatigue in humans.

  • Agrobacterium: Causes tumors in plants by inserting its DNA, beneficial for genetic modification techniques.

  • Acetobacter and Gluconobacter: Produce vinegar naturally.

  • Caulobacter: Noted for its strong adhesive qualities used for biofilm formation.

Beta Proteobacteria

• Diverse and found in various habitats, some with low nutrients.

• Key organisms:

  • Neisseria: Includes N. gonorrhoeae (causes gonorrhea) and N. meningitidis (common cause of meningitis).

  • Bordetella: Causal agent of whooping cough.

  • Burkholderia: Known for recycling environmental chemicals and can be pathogenic in cystic fibrosis patients.

  • Thiobacillus: Involved in sulfur recycling.

  • Zoogloea and Sphaerotilus: Form flocs, which are crucial in sewage treatment but can also cause clogging.

Gamma Proteobacteria

• The largest and most diverse group, which can be categorized into several subgroups:

  • Sulfur bacteria: Form symbiotic relationships with sulfur worms.

  • Intracellular pathogens: Include Legionella and Coxiella, evading the immune system by living within host cells.

  • Methane oxidizers: Metabolize methane in aquatic systems.

  • Glycolytic facultative anaerobes: Key role in microbiome health; includes opportunistic pathogens like E. coli and true pathogens like Salmonella.

  • Pseudomonas: Decomposers capable of recycling various organic materials, but can spoil food and cause infections.

Delta Proteobacteria

• Includes Desulfovibrio, which reduces sulfate and recycles sulfur, often in polluted environments.

• Contains pathogens that infect other bacteria, utilizing unique reproductive methods beyond binary fission.

• Myxobacteria: Show cell differentiation and communication, meaning behavior similar to multicellular organisms.

Epsilon Proteobacteria

• Includes genera like Campylobacter (intestinal inflammation) and Helicobacter (stomach ulcers).

Zeta Proteobacteria

• Recently discovered, mainly marine organisms, many still unnamed.

• Includes Mariprofundus ferroxidans, which oxidizes iron, living in deep-sea vents.

Conclusion

• Discussion of various gram-negative bacteria highlights their diversity, ecological roles, and significance in health and disease.

• Emphasis on the importance of understanding these organisms for medical and environmental contexts.

Introduction to Gram-Negative Bacteria

Gram-negative bacteria are a major group of bacteria characterized by their unique cell wall structure, which includes a thin peptidoglycan layer surrounded by an outer membrane. This outer membrane is composed of lipopolysaccharides (LPS), which consist of lipid A, a core oligosaccharide, and O-antigens. LPS plays a crucial role in the immune response as it is recognized by immune cells, contributing significantly to the pathogenicity of these organisms. The presence of this secondary cell membrane provides an additional barrier that protects gram-negative bacteria from certain antibiotics, detergents, and harmful substances.

Classification of Gram-Negative Bacteria

Gram-negative bacteria are classified primarily under the phylum Proteobacteria, which is divided into several distinct classes based on metabolic properties, genetic characteristics, and ecological niches. The six recognized classes are: Alpha, Beta, Gamma, Delta, Epsilon, and Zeta proteobacteria.

Alpha Proteobacteria

Alpha proteobacteria are typically aerobic and are known for their metabolic diversity, often thriving in oligotrophic (low-nutrient) environments. Significant organisms include:

• Azospirillum: A nitrogen-fixing bacterium found in association with plant roots, enhancing plant growth and fertility through the fixation of atmospheric nitrogen.

• Rhizobium: Forms symbiotic relationships with legumes, inducing nodule formation on the roots and providing essential nitrogen for the plants while receiving carbohydrates in return.

• Rickettsia: An obligate intracellular parasite transmitted by arthropods, responsible for diseases such as typhus and Rocky Mountain spotted fever.

• Brucella: Infects various mammals and is linked to brucellosis in humans, causing recurrent fever and other systemic symptoms.

• Agrobacterium: Known for its ability to transfer DNA to plant cells, facilitating the genetic modification of plants for agricultural purposes.

• Acetobacter and Gluconobacter: Bacteria that are crucial in vinegar production through the fermentation of ethanol to acetic acid.

• Caulobacter: Noted for its unique life cycle and the ability to form biofilms through strong adhesive structures.

Beta Proteobacteria

Beta proteobacteria exhibit a wide range of metabolic capabilities and inhabit diverse ecological niches, particularly those with lower nutrient levels. Key organisms include:

• Neisseria: This genus encompasses species such as N. gonorrhoeae (the causative agent of gonorrhea) and N. meningitidis (a common cause of bacterial meningitis).

• Bordetella: Recognized for its role in human diseases, particularly whooping cough caused by Bordetella pertussis.

• Burkholderia: Known for its role in environmental remediation and also pathogenic behavior in immunocompromised individuals, particularly cystic fibrosis patients.

• Thiobacillus: Involves in the oxidation of sulfur compounds, playing a significant role in the sulfur cycle.

• Zoogloea and Sphaerotilus: Form gelatinous flocs essential for wastewater treatment processes but can also contribute to clogging issues in treatment facilities.

Gamma Proteobacteria

The largest and most diverse group within gram-negative bacteria, gamma proteobacteria can be categorized into several subgroups:

• Sulfur bacteria: These organisms form symbiotic relationships with marine invertebrates, such as tube worms, in deep-sea hydrothermal vents.

• Intracellular pathogens: Include Legionella and Coxiella, both capable of evading the immune system by living within host cells.

• Methane oxidizers: These specialized bacteria play a vital role in the carbon cycle by metabolizing methane in aquatic environments.

• Glycolytic facultative anaerobes: Such as Escherichia coli, which can thrive in both aerobic and anaerobic conditions, contributing to gut health but also acting as opportunistic pathogens. Salmonella, another member, is a well-known true pathogen.

• Pseudomonas: A versatile genus known for its role as a decomposer, capable of utilizing a wide range of organic substrates, although some species are pathogenic and can cause infections in immunocompromised patients and spoil food products.

Delta Proteobacteria

This class primarily includes bacteria that engage in sulfate reduction, such as Desulfovibrio, which plays an essential role in sulfur cycling, particularly in environments impacted by pollution. Notable features include:

• Pathogens that specifically infect other bacterial species through unique reproductive methods that deviate from traditional binary fission.

• Myxobacteria: Characterized by complex life cycles, including social behaviors and the ability to form fruiting bodies, showcasing traits akin to multicellular organisms.

Epsilon Proteobacteria

Genus representatives include:

• Campylobacter: Recognized for its role in gastrointestinal infections, often related to contaminated food and water sources, leading to acute intestinal inflammation.

• Helicobacter: Associated with various gastric diseases, including stomach ulcers and its potential role in gastric cancer.

Zeta Proteobacteria

A relatively recent discovery, zeta proteobacteria predominantly inhabit marine environments. Despite being less studied, they are significant in biogeochemical processes, with organisms like Mariprofundus ferroxidans known for iron oxidation, particularly in extreme environments such as deep-sea vents.

Conclusion

The diversity of gram-negative bacteria reflects their critical ecological roles, pathogenic potential, and significance in both health and disease. Understanding these organisms is crucial for developing new antibiotics, implementing effective environmental strategies, and managing public health concerns.