General Characteristics of Prokaryotic Organisms

Morphology of Prokaryotic Cells

Prokaryotes are the most diverse group of cellular microbes, thriving in a wide range of habitats. Their morphology varies greatly, with only a few species capable of colonizing humans and causing disease.

• Coccus: Spherical shape

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

• Spirochete: Flexible, spiral-shaped

• Vibrio: Curved rod

• Coccobacillus: Short, oval rod

• Pleomorphic: Variable shape

• Star-shaped: Rare, star-like morphology

Endospores

Endospores are highly resistant structures produced by certain Gram-positive bacteria, such as Bacillus and Clostridium. Each vegetative cell forms one endospore, which can germinate into a new vegetative cell. Endospores serve as a defensive strategy against unfavorable conditions and are a major concern in food processing and healthcare.

• Formation: Vegetative cell transforms into an endospore

• Germination: Endospore returns to vegetative state

• Importance: Resistance to heat, chemicals, and desiccation

Reproduction of Prokaryotic Cells

Prokaryotes reproduce exclusively by asexual methods. The three main methods are:

• Binary fission: Most common; cell divides into two identical daughter cells

• Snapping division: Variation of binary fission, where the cell wall ruptures

• Budding: New cell grows from the surface of the parent cell

Viviparity in Epulopiscium

• Viviparity: Unique method where live offspring emerge from the body of the dead mother cell

• Significance: First noted case of viviparous behavior in prokaryotes

Arrangements of Prokaryotic Cells

The arrangement of prokaryotic cells is determined by the planes in which cells divide and whether daughter cells remain attached.

• Cocci: Can form chains (streptococci), clusters (staphylococci), tetrads, or other groupings

• Bacilli: Can form pairs (diplobacilli), chains (streptobacilli), or palisades

Modern Prokaryotic Classification

Genetic Relatedness and Domains

Modern classification of prokaryotes is based on genetic relatedness, particularly rRNA sequences. The three domains are:

• Archaea

• Bacteria

• Eukarya

Survey of Archaea

General Features

Archaea are distinguished by several unique features:

• Lack of true peptidoglycan in cell walls

• Cell membrane lipids with branched hydrocarbon chains

• AUG codon codes for methionine

• Two phyla: Crenarchaeota and Euryarchaeota

• Reproduction: Binary fission, budding, or fragmentation

• Morphology: Cocci, bacilli, spirals, or pleomorphic

• Pathogenicity: Not known to cause disease

Extremophiles

Extremophiles are archaea that require extreme conditions for survival, such as high temperature, pH, or salinity.

• Thermophiles: Require temperatures above 45°C; hyperthermophiles need >80°C

• Halophiles: Require >9% NaCl; often contain red/orange pigments for sunlight protection

Methanogens

Methanogens are the largest group of archaea, converting carbon dioxide, hydrogen gas, and organic acids to methane. They play a significant role in environmental methane production and are found in sediments and animal colons.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

• Deeply branching bacteria: Believed to resemble the earliest bacteria; autotrophic; live in habitats similar to early Earth

• Phototrophic bacteria: Contain photosynthetic lamellae; divided into five groups based on pigments and electron sources: cyanobacteria, green sulfur, green nonsulfur, purple sulfur, purple nonsulfur

Low G + C Gram-Positive Bacteria

• Clostridia: Rod-shaped, obligate anaerobes; many form endospores; important in medicine and industry

• Mycoplasmas: Facultative or obligate anaerobes; lack cell walls; smallest free-living cells; colonize mucous membranes

• Bacillus: Common in soil; form endospores; Bacillus thuringiensis toxin used as insecticide; Bacillus anthracis causes anthrax

• Listeria: Contaminates food; can reproduce under refrigeration; crosses placenta

• Lactobacillus: Used in food production; rarely causes disease

• Streptococcus, Enterococcus, Staphylococcus: Cause numerous diseases; some strains are multi-drug resistant

High G + C Gram-Positive Bacteria

• Corynebacterium: Pleomorphic aerobes/facultative anaerobes; produce metachromatic granules

• Mycobacterium: Aerobic rods; slow growth due to mycolic acid in cell walls

• Actinomycetes: Form branching filaments; important genera include Actinomyces, Nocardia, Streptomyces

Gram-Negative Proteobacteria

Proteobacteria are the largest and most diverse group of bacteria, divided into six classes:

• Alphaproteobacteria: Nitrogen fixers (e.g., Azospirillum, Rhizobium), nitrifying bacteria, purple nonsulfur phototrophs, pathogenic genera (Rickettsia, Brucella), and others (Acetobacter, Caulobacter)

• Betaproteobacteria: Pathogenic genera (Neisseria, Bordetella, Burkholderia), environmental recyclers (Thiobacillus, Zoogloea, Sphaerotilus)

• Gammaproteobacteria: Purple sulfur bacteria, intracellular pathogens (Legionella, Coxiella), methane oxidizers, glycolytic facultative anaerobes, pseudomonads

• Deltaproteobacteria: Diverse metabolic types (Desulfovibrio, Bdellovibrio, Myxobacteria)

• Epsilonproteobacteria: Includes Campylobacter and Helicobacter

• Zetaproteobacteria: Recently discovered; DNA common in oceans

Other Gram-Negative Bacteria

• Chlamydias: Intracellular pathogens; some are smaller than viruses; cause sexually transmitted diseases

• Spirochetes: Motile, corkscrew motion; diverse metabolism; Treponema and Borrelia cause human diseases

• Bacteroids: Inhabit digestive tracts; some cause infections; Cytophaga are aquatic, gliding bacteria important in sewage degradation

Summary Table: Prokaryotic Classification

• Domain Bacteria: Includes deeply branching, phototrophic, Gram-positive, and Gram-negative bacteria

• Domain Archaea: Includes extremophiles and methanogens

Key Equations and Concepts

Binary Fission Equation

The process of binary fission can be represented mathematically as: Where:

• = Number of cells at time t

• = Initial number of cells

• = Number of generations

Nitrogen Fixation (Cyanobacteria)

Methanogenesis (Methanogens)

Conclusion

Prokaryotes exhibit remarkable diversity in morphology, metabolism, and ecological roles. Their classification is based on genetic, structural, and physiological characteristics, with significant implications for medicine, industry, and environmental science. Additional info: Tables and images were included only when directly relevant to the explanation of the paragraph, as per strict relevance requirements.