Chapter 11: Characterizing and Classifying Prokaryotes

General Characteristics of Prokaryotes

Prokaryotes are the most diverse group of organisms on Earth, occupying a wide range of habitats and displaying varied morphologies and metabolic capabilities. Understanding their classification and characteristics is fundamental in microbiology.

• Diversity: Prokaryotes include both Bacteria and Archaea, with remarkable adaptability to extreme and moderate environments.

• Cellular Structure: Prokaryotes lack a membrane-bound nucleus and organelles, and their genetic material is typically a single circular chromosome.

• Reproduction: Most reproduce asexually by binary fission, but some can also reproduce by budding or other mechanisms.

• Habitats: Found in soil, water, extreme environments (hot springs, salt lakes), and as symbionts or pathogens in plants and animals.

• Medical Importance: Many prokaryotes are beneficial, but some are significant human pathogens.

Key Terms and Definitions

• Binary Fission: A form of asexual reproduction in which a cell divides into two genetically identical daughter cells.

• Budding: A process where a new organism grows from a bud due to cell division at one particular site.

• Endospores: Highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus, Clostridium) to survive adverse conditions.

• Archaea: A domain of prokaryotes distinct from bacteria, often found in extreme environments.

• Morphology: The study of the form and structure of organisms; in bacteria, refers to cell shape (coccus, bacillus, spirillum, etc.).

• Arrangement: The pattern in which bacterial cells are grouped (chains, clusters, pairs, etc.).

• Psychrophile: Microorganisms that thrive at low temperatures (typically below 15°C).

• Spirochete: A group of spiral-shaped bacteria with flexible cell walls and unique motility mechanisms.

Table: Bacterial Morphology and Arrangements

Modern Prokaryote Classification

Classification of prokaryotes is based on a combination of morphological, physiological, and genetic characteristics, with increasing emphasis on molecular data such as rRNA gene sequences.

• Bergey's Manual of Systematic Bacteriology: The authoritative reference for bacterial taxonomy, using rRNA sequence similarity and other traits.

• Domains: Bacteria and Archaea are the two main prokaryotic domains.

• Phylogenetic Relationships: Determined by comparing genetic sequences, especially 16S rRNA.

Survey of Domain Bacteria

• Gram-Positive Bacteria: Thick peptidoglycan cell wall, retain crystal violet stain.

• Gram-Negative Bacteria: Thin peptidoglycan layer, outer membrane, do not retain crystal violet stain.

Survey of Bacteria – Low G+C Gram-Positives

Low G+C Gram-positive bacteria have a lower proportion of guanine and cytosine nucleotides in their DNA. Many are medically important.

• Clostridium: Obligate anaerobes, form endospores. Includes pathogens such as C. botulinum (botulism), C. tetani (tetanus), C. perfringens (gas gangrene), and C. difficile (antibiotic-associated diarrhea).

• Bacillus: Common in soil, endospore-formers. B. anthracis causes anthrax; B. thuringiensis produces an insecticidal toxin.

• Endospores: Provide resistance to extreme conditions such as heat, desiccation, and chemicals.

Example: Bacillus anthracis forms endospores that can survive in soil for decades, causing disease in humans and animals through cutaneous, gastrointestinal, or inhalational routes.

Survey of Bacteria – High G+C Gram-Positives

High G+C Gram-positive bacteria have a higher proportion of guanine and cytosine in their DNA. They include important genera such as Corynebacterium and Mycobacterium.

• Corynebacterium: Pleomorphic, part of normal flora in the upper respiratory tract. C. diphtheriae causes diphtheria, a serious upper respiratory infection in children.

• Pathogenesis: C. diphtheriae produces an exotoxin that kills cells in the trachea and bronchi, leading to severe illness.

• Prevention: DTaP vaccine for children, TDaP for adults.

Example: The 1925 diphtheria epidemic in Nome, Alaska, was controlled by delivering antiserum via dog sled, commemorated by the Iditarod race.

Endospore Formation and Medical Relevance

• Endospore-Forming Genera: Bacillus and Clostridium are the primary endospore-formers among Gram-positive bacteria.

• Medical Importance: Endospores are highly resistant to environmental stress and are a major concern in sterilization and infection control.

Table: Comparison of Low G+C and High G+C Gram-Positive Bacteria

Summary of Key Pathogens and Diseases

• Clostridium botulinum: Causes botulism via neurotoxin (botulinum toxin).

• Clostridium tetani: Causes tetanus via tetanospasmin toxin.

• Bacillus anthracis: Causes anthrax; endospores survive in soil.

• Corynebacterium diphtheriae: Causes diphtheria; exotoxin damages respiratory tract.

Additional info:

• Prokaryotic taxonomy is continually updated as new genetic and molecular data become available.

• Understanding the structure and classification of prokaryotes is essential for identifying pathogens and developing treatments.