lec 21:Bacterial Diversity and Phototrophic Bacteria: Structure, Classification, and Ecological Roles

Bacterial Diversity

Overview of Bacterial Diversity

Bacteria are highly diverse microorganisms, classified based on genetic, structural, and metabolic characteristics. Most bacteria possess peptidoglycan cell walls, ester-linked membrane lipids, and circular DNA. Their diversity is reflected in their evolutionary groups, ecological roles, and metabolic strategies.

• Peptidoglycan cell wall: Provides structural support and shape.

• Ester-linked membrane lipids: Characteristic of bacterial membranes.

• Circular DNA: Most bacteria have a single, circular chromosome.

Major Groups of Bacteria

Bacteria are grouped into several major evolutionary lineages, each with unique structural and metabolic features.

• Proteobacteria: Gram-negative, diverse lifestyles, outer membrane with lipopolysaccharide (LPS).

• Firmicutes: Gram-positive, low G+C DNA, thick peptidoglycan wall, many form endospores.

• Actinobacteria: Gram-positive, high G+C DNA, often filamentous, antibiotic producers.

• Bacteroidetes: Gram-negative rods, common in the human gut, carbohydrate breakdown.

• Cyanobacteria: Oxygenic photosynthesis, ancestors of plant chloroplasts.

• Spirochetes: Spiral-shaped, move via internal flagella.

• Chlamydiae: Obligate intracellular, two-stage life cycle.

• Deep-Branching Thermophiles: Early lineages, thrive in extreme heat.

Proteobacteria (Gram-Negative)

Structural Features and Metabolic Diversity

Proteobacteria are the largest and most diverse group of bacteria. All members share an outer membrane containing lipopolysaccharide (LPS), a thin peptidoglycan layer, and an inner membrane.

• Metabolic diversity: Includes chemoorganotrophs, chemolithotrophs, phototrophs, methylotrophs, and methanotrophs.

• Examples: Escherichia coli, Rhizobium, Helicobacter pylori.

Key structural feature shared by all Proteobacteria: Outer membrane containing lipopolysaccharide (LPS).

Firmicutes (Gram-Positive, Low G+C DNA)

Cell Wall Structure and Endospore Formation

Firmicutes are Gram-positive bacteria with low G+C DNA content (<50%). They have thick peptidoglycan cell walls and many can form endospores, allowing survival in harsh conditions.

• Endospore-forming genera: Bacillus (aerobic), Clostridium (anaerobic).

• Non–spore-forming genera: Lactobacillus, Streptococcus, Staphylococcus.

Actinobacteria (Gram-Positive, High G+C DNA)

Filamentous Growth and Antibiotic Production

Actinobacteria are Gram-positive bacteria with high G+C DNA content (>50%). Many grow as filaments similar to fungi and produce important antibiotics.

• Key genera: Streptomyces (antibiotic production), Mycobacterium (thick, waxy cell wall with mycolic acids).

• Acid-fast staining: Required for Mycobacterium due to mycolic acids.

Bacteroidetes

Role in the Human Gut

Bacteroidetes are Gram-negative rods commonly found in the human gut, where they help break down complex carbohydrates.

• Example: Bacteroides fragilis.

Cyanobacteria

Oxygenic Photosynthesis and Ecological Importance

Cyanobacteria are the only bacteria that perform oxygenic photosynthesis, using two photosystems (PSI and PSII) to split water and release oxygen. They are major producers of oxygen and responsible for a significant portion of global carbon fixation.

• Examples: Prochlorococcus, Synechococcus, Anabaena, Nostoc.

• Specialized cells: Heterocysts for nitrogen fixation.

Spirochetes

Motility and Pathogenicity

Spirochetes are spiral-shaped bacteria that move in a corkscrew motion using internal flagella (endoflagella).

• Examples: Treponema pallidum (syphilis), Borrelia burgdorferi (Lyme disease).

Chlamydiae

Obligate Intracellular Lifestyle

Chlamydiae are obligate intracellular bacteria with a two-stage life cycle: infectious and reproductive forms.

• Example: Chlamydia trachomatis.

Deep-Branching Thermophiles

Adaptation to Extreme Environments

Deep-branching thermophiles represent some of the earliest bacterial lineages, thriving in extreme heat environments such as hot springs and hydrothermal vents.

• Examples: Aquificae, Thermotogae.

Phototrophic Bacteria

Classification and Metabolic Strategies

Phototrophic bacteria use light as an energy source and are divided into two main groups based on their ability to produce oxygen during photosynthesis.

• Oxygenic phototrophs: Cyanobacteria (produce oxygen).

• Anoxygenic phototrophs: Purple sulfur bacteria, purple non-sulfur bacteria, green sulfur bacteria, green non-sulfur bacteria, heliobacteria (do not produce oxygen).

Cyanobacteria (Oxygen-Producing Phototrophs)

Cyanobacteria use two photosystems to split water, releasing oxygen. They are major contributors to global carbon fixation and ocean productivity.

• Calvin Cycle: Used to fix carbon dioxide.

• Examples: Prochlorococcus, Synechococcus, Anabaena, Nostoc.

Anoxygenic Phototrophic Bacteria (Do NOT Produce Oxygen)

These bacteria use only one photosystem and do not split water, so they do not produce oxygen. Instead, they use molecules such as hydrogen sulfide (H₂S), hydrogen, or organic compounds as electron donors.

• Purple sulfur bacteria: Use H₂S as electron donor, store sulfur inside cells.

• Purple non-sulfur bacteria: Metabolically flexible, use organic compounds or light.

• Green sulfur bacteria: Strict anaerobes, thrive in low-light environments.

• Green non-sulfur bacteria: Often filamentous and thermophilic.

• Heliobacteria: Gram-positive, photoheterotrophic, found in soils.

Ecological Importance of Phototrophic Bacteria

Phototrophic bacteria are crucial for ecosystem productivity, nutrient cycling, and survival in extreme environments.

• Ocean productivity: Cyanobacteria dominate photosynthesis in oceans.

• Extreme environments: Phototrophs inhabit hot springs, hypersaline lakes, and polar regions.

• Nutrient cycling: Cyanobacteria fix nitrogen, enriching ecosystems.

Gram-Positive Bacteria: Firmicutes vs Actinobacteria

Comparison of Structural and Genetic Features

The two main groups of Gram-positive bacteria are Firmicutes (low G+C DNA) and Actinobacteria (high G+C DNA). The primary difference is the G+C content of their DNA.

• Firmicutes: Low G+C content, thick peptidoglycan wall, endospore formation.

• Actinobacteria: High G+C content, complex cell walls, filamentous growth, antibiotic production.

Firmicutes (Low G+C DNA)

• Endospore-forming: Bacillus (aerobic), Clostridium (anaerobic).

• Non–spore-forming: Lactobacillus, Streptococcus, Staphylococcus.

Mycoplasma

Mycoplasma are unusual bacteria lacking a cell wall, making them naturally resistant to penicillin-type antibiotics.

• Cell structure: Three-layered membrane, no peptidoglycan.

Actinobacteria (High G+C DNA)

• Streptomyces: Filamentous, produce antibiotics, responsible for earthy smell of soil.

• Mycobacterium: Thick, waxy cell wall with mycolic acids, acid-fast staining required.

Summary Table: Major Bacterial Groups

Group	Gram Stain	Key Features	Examples
Proteobacteria	Negative	Outer membrane with LPS, metabolic diversity	Escherichia coli, Rhizobium
Firmicutes	Positive	Thick peptidoglycan, endospore formation	Bacillus, Clostridium
Actinobacteria	Positive	High G+C DNA, filamentous, antibiotics	Streptomyces, Mycobacterium
Bacteroidetes	Negative	Rod-shaped, gut symbionts	Bacteroides fragilis
Cyanobacteria	Negative	Oxygenic photosynthesis	Anabaena, Prochlorococcus
Spirochetes	Negative	Spiral shape, internal flagella	Treponema pallidum
Chlamydiae	Negative	Obligate intracellular	Chlamydia trachomatis
Deep-Branching Thermophiles	Varies	Extreme heat adaptation	Aquificae, Thermotogae

Sample Exam Questions

• Which structural feature is shared by all members of Proteobacteria? B. Outer membrane containing lipopolysaccharide

• Which bacterial group is responsible for producing the majority of naturally derived antibiotics? B. Actinobacteria

• Which bacterial genera is known for forming endospores that survive extreme environmental conditions? A. Bacillus

• What is the primary difference between Firmicutes and Actinobacteria? C. G+C content of DNA

• Which bacterial group performs oxygenic photosynthesis? C. Cyanobacteria

• Why do anoxygenic phototrophic bacteria not produce oxygen? B. They use only one photosystem and do not split water

• Which phototrophic bacteria use hydrogen sulfide (H₂S) as an electron donor? B. Purple sulfur bacteria

• Which bacterial group contains organisms that move using internal flagella (endoflagella)? A. Spirochetes

• Why are Mycoplasma naturally resistant to penicillin? C. They lack a cell wall

• Which bacterial genus requires acid-fast staining due to a waxy cell wall containing mycolic acids? C. Mycobacterium

Additional info:

• Actinobacteria are responsible for the majority of naturally derived antibiotics due to their secondary metabolite production.

• Firmicutes and Bacteroidetes are dominant in the human gut and their balance is linked to health and disease.

• Cyanobacteria are crucial for global oxygen production and carbon fixation.