Classification and Diversity of Prokaryotes: Study Notes (Chapters 10 & 11)

Classification of Microorganisms

Bergey's Manual and Determinative Bacteriology

Bergey's Manual is a foundational reference for the identification and classification of bacteria. It organizes bacteria based on phenotypic characteristics and biochemical tests.

• Determinative Bacteriology: Focuses on identifying bacteria using observable traits and laboratory tests.

• Phylogenetic Classification: Uses genetic relationships, such as rRNA sequencing, to classify organisms.

Example: Biochemical tests like catalase or oxidase tests help differentiate bacterial species.

Phylogenetic Relationships

Phylogenetic classification reflects evolutionary relationships among organisms, often determined by comparing genetic material such as rRNA sequences.

• Genetic Methods: DNA hybridization, PCR, and sequencing are used to establish relationships.

• Environmental Relationships: Organisms are grouped based on shared ancestry rather than just physical traits.

Serological Testing

Serological tests use antibodies to detect specific antigens on microorganisms, aiding in identification and classification.

• Advantage: Rapid and specific identification of pathogens.

• Disadvantage: May not distinguish closely related species.

Example: ELISA and Western blot are common serological tests.

Biochemical Tests

Biochemical tests assess the metabolic capabilities of bacteria, such as their ability to ferment sugars or produce certain enzymes.

• Purpose: Differentiate bacterial species based on metabolic properties.

• Example: Lactose fermentation test distinguishes Escherichia coli from Salmonella.

Bacteriophage Typing

Bacteriophage typing uses viruses that infect bacteria (bacteriophages) to identify bacterial strains, especially during outbreak investigations.

• Application: Tracking sources of bacterial outbreaks.

Genetic Methods in Bacterial Identification

Genetic techniques, such as DNA hybridization and PCR, are used to identify bacteria by analyzing their genetic material.

• DNA Fingerprinting: Compares DNA patterns to distinguish between strains.

• rRNA Sequencing: Used to classify bacteria at higher taxonomic levels.

Three Domains of Life

All living organisms are classified into three domains based on genetic and biochemical differences:

• Bacteria

• Archaea

• Eukarya

Note: Not all bacteria are found in all three domains; only Archaea and Bacteria are prokaryotic.

Taxonomic Hierarchy

The taxonomic hierarchy organizes living organisms from broadest to most specific:

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

The Prokaryotes: Domains Bacteria and Archaea

Major Classes of Proteobacteria

Proteobacteria is a major phylum of Gram-negative bacteria, divided into several classes:

• Alpha (e.g., Rhizobium, Rickettsia)

• Beta (e.g., Bordetella, Neisseria)

• Gamma (e.g., Escherichia, Pseudomonas, Vibrio)

• Delta (important in sulfur cycle)

• Epsilon (e.g., Campylobacter, Helicobacter)

Nitrogen Fixation and the Nitrogen Cycle

Nitrogen-fixing bacteria convert atmospheric nitrogen (N2) into ammonia (NH3), a process essential for life.

• Symbiotic Nitrogen Fixers: Rhizobium in legume root nodules.

• Free-living Nitrogen Fixers: Azotobacter, Cyanobacteria.

Equation:

$\mathrm{N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i}$

Pathogenic Proteobacteria

Many Proteobacteria are important human pathogens:

• Bordetella pertussis: Whooping cough

• Neisseria gonorrhoeae: Gonorrhea

• Neisseria meningitidis: Bacterial meningitis

• Escherichia coli: Gastroenteritis, hemolytic uremic syndrome

• Salmonella: Typhoid fever, food poisoning

• Shigella: Dysentery

• Yersinia pestis: Bubonic plague

• Vibrio cholerae: Cholera

• Pseudomonas aeruginosa: Opportunistic infections, antibiotic resistance

• Haemophilus influenzae: Pneumonia, meningitis

Other Notable Bacterial Groups

• Chlamydia: Obligate intracellular parasites causing chlamydia.

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

Gram-Positive Bacteria

Gram-positive bacteria are divided into two main groups based on their DNA G+C content:

• Firmicutes (Low G+C): Includes Clostridium, Bacillus, Staphylococcus, Streptococcus, Listeria.

• Actinobacteria (High G+C): Includes Streptomyces (antibiotic producers), Mycobacterium (tuberculosis, leprosy).

Important Pathogenic Gram-Positive Bacteria

• Clostridium tetani: Tetanus

• Clostridium botulinum: Botulism

• Clostridium difficile: Antibiotic-associated diarrhea

• Bacillus anthracis: Anthrax

• Staphylococcus aureus: Opportunistic infections, MRSA

• Streptococcus pyogenes: Strep throat, scarlet fever

• Listeria monocytogenes: Listeriosis

General Features of Archaea

Archaea are prokaryotes distinct from bacteria, often found in extreme environments.

• Methanogens: Produce methane from CO2 and H2.

• Extreme Halophiles: Thrive in high-salt environments.

• Thermophiles: Live in very hot environments.

Summary Table: Major Bacterial Groups and Examples

Group	Example Genera	Key Features / Diseases
Alpha Proteobacteria	Rhizobium, Rickettsia	Nitrogen fixation, intracellular pathogens
Beta Proteobacteria	Bordetella, Neisseria	Whooping cough, gonorrhea, meningitis
Gamma Proteobacteria	Escherichia, Salmonella, Vibrio, Pseudomonas	Food poisoning, cholera, opportunistic infections
Firmicutes	Clostridium, Bacillus, Staphylococcus	Tetanus, anthrax, MRSA
Actinobacteria	Streptomyces, Mycobacterium	Antibiotic production, tuberculosis
Spirochetes	Borrelia, Treponema	Lyme disease, syphilis

Additional info: These notes cover the classification, identification, and diversity of prokaryotes, including both Bacteria and Archaea, as well as important pathogenic species and their clinical significance.