Chapter 11: Characterizing and Classifying Prokaryotes – Study Notes

Characterizing and Classifying Prokaryotes

General Prokaryotic Characteristics

Prokaryotes are unicellular organisms lacking a membrane-bound nucleus and organelles. They include members of the domains Bacteria and Archaea. Understanding their basic features is essential for their identification and classification.

• Cell Shape: Common shapes include cocci (spherical), bacilli (rod-shaped), spirilla (spiral), and vibrio (comma-shaped).

• Cell Arrangement: Prokaryotes may exist as single cells, pairs (diplo-), chains (strepto-), clusters (staphylo-), or other arrangements depending on the species and division pattern.

• Endospores: Some bacteria (notably Bacillus and Clostridium species) can form highly resistant, dormant structures called endospores, allowing survival in harsh conditions.

Example: Bacillus anthracis forms endospores that can survive extreme heat and desiccation.

Binary Fission and Reproduction in Prokaryotes

Prokaryotes primarily reproduce asexually through binary fission, a process distinct from eukaryotic mitosis and meiosis.

• Steps of Binary Fission:

  1. Replication of the circular DNA molecule.

  2. Attachment of DNA molecules to the cell membrane.

  3. Elongation of the cell and separation of DNA copies.

  4. Formation of a septum (cross-wall).

  5. Division into two genetically identical daughter cells.

• Contrast with Eukaryotic Reproduction: Eukaryotes reproduce via mitosis (asexual) or meiosis (sexual), involving complex chromosome segregation and, in meiosis, genetic recombination.

Additional info: Some prokaryotes can also exchange genetic material via horizontal gene transfer mechanisms such as transformation, transduction, and conjugation.

Common Features of Archaea

Archaea are a distinct domain of prokaryotes with unique molecular and physiological traits.

• Cell Wall Composition: Archaea lack peptidoglycan; instead, their cell walls may contain pseudopeptidoglycan or other polymers.

• Membrane Lipids: Archaeal membranes contain ether-linked lipids (as opposed to ester-linked in bacteria and eukaryotes), often with branched isoprenoid chains.

• Genetic Machinery: Archaeal DNA replication, transcription, and translation machinery are more similar to eukaryotes than to bacteria.

• Habitats: Many archaea are extremophiles, thriving in high-salt, high-temperature, or acidic environments.

Example: Halobacterium species live in highly saline environments.

Key Features of Major Bacterial Phyla

Bacteria are classified into several phyla based on genetic, morphological, and physiological characteristics. Six major phyla are commonly discussed in introductory microbiology:

Phylum	Key Features	Medically Important Genera
Proteobacteria	Gram-negative; diverse metabolism; includes many pathogens	Escherichia, Salmonella, Neisseria
Firmicutes	Gram-positive; low G+C content; many form endospores	Bacillus, Clostridium, Staphylococcus
Actinobacteria	Gram-positive; high G+C content; often filamentous	Mycobacterium, Corynebacterium
Bacteroidetes	Gram-negative; anaerobic; common in gut microbiota	Bacteroides
Chlamydiae	Obligate intracellular pathogens; Gram-negative	Chlamydia
Spirochaetes	Gram-negative; spiral-shaped; motile via axial filaments	Treponema, Borrelia

Additional info: The above table summarizes the main features and examples of medically important genera within each phylum.

Key Etiologic Agents within Bacterial Phyla

Etiologic agents are organisms responsible for causing disease. Recognizing these agents within each phylum is crucial for clinical microbiology.

• Proteobacteria: Escherichia coli (urinary tract infections), Neisseria gonorrhoeae (gonorrhea)

• Firmicutes: Staphylococcus aureus (skin infections), Clostridium botulinum (botulism)

• Actinobacteria: Mycobacterium tuberculosis (tuberculosis)

• Bacteroidetes: Bacteroides fragilis (intra-abdominal infections)

• Chlamydiae: Chlamydia trachomatis (chlamydia)

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

Example: Mycobacterium tuberculosis is the causative agent of tuberculosis, a major global health concern.