General Characteristics of Prokaryotic Organisms

Prokaryotic Diversity and Abundance

Prokaryotes are the most numerous and diverse group of cellular microbes, thriving in a wide range of habitats. Despite their abundance, only a small fraction are capable of colonizing humans and causing disease.

Prokaryotic Morphology

Prokaryotes exhibit a variety of shapes, which are important for identification and classification:

• Coccus: Spherical

• Bacillus: Rod-shaped

• Vibrio: Curved rods

• Spirillum: Thick, stiff spiral

• Spirochete: Thin, flexible spiral

• Coccobacillus: Intermediate between cocci and bacilli

• Pleomorphic: Variable shapes and sizes

• Other forms: Star-shaped, triangular, rectangular

Endospore Formation

Endospores are highly resistant, dormant structures formed primarily by Gram-positive genera such as Bacillus and Clostridium. They serve as a defensive strategy against unfavorable conditions and are a significant concern in food safety and healthcare due to their resistance to destruction.

• Sporulation: The process of endospore formation, typically taking 8–10 hours.

• Endospore location: Can be central, subterminal, or terminal within the cell.

Prokaryotic Reproduction

Prokaryotes reproduce asexually through several mechanisms:

• Binary fission: Most common; cell replicates DNA, elongates, and divides into two daughter cells.

• Snapping division: A variation of binary fission seen in some Gram-positive bacilli, where tension in the cell wall causes it to snap, leaving daughter cells attached.

• Reproductive spores: Daughter cells with tough outer coatings, common in actinomycetes.

• Budding: Outgrowth from parent cell receives a copy of DNA and separates.

• Viviparity: Rare; live offspring are produced within a parent cell (e.g., Epulopiscium).

Cellular Arrangements

The arrangement of prokaryotic cells is determined by the planes of division and whether daughter cells remain attached:

• Cocci:

  • Diplococci: Pairs

  • Streptococci: Chains

  • Tetrads: Groups of four (two planes)

  • Sarcinae: Cubes of eight (three planes)

  • Staphylococci: Grape-like clusters (random planes)

• Bacilli:

  • Single bacilli

  • Diplobacilli: Pairs

  • Streptobacilli: Chains

  • V-shaped and coccobacilli: Variations

Modern Prokaryotic Classification

Three-Domain System

Modern classification of living organisms is based on genetic relatedness, particularly rRNA sequences. All life is divided into three domains:

• Archaea

• Bacteria

• Eukarya

Approximately 99.5% of prokaryotes have not been cultured and are known only by their rRNA fingerprints.

Bergey's Manual of Systematic Bacteriology

This manual provides a consensus-based classification scheme for prokaryotes, organizing them into 26 phyla (2 Archaea, 24 Bacteria) based on evolutionary relationships reflected in rRNA sequences.

Survey of Archaea

General Features

Archaea are prokaryotes distinct from bacteria, with unique characteristics:

• Lack true peptidoglycan in cell walls

• Branched hydrocarbon chains in membrane lipids

• Initiator amino acid is methionine (AUG codon)

• Two main phyla: Crenarchaeota and Euryarchaeota

• Reproduce by binary fission, budding, or fragmentation

• Not known to cause disease

Extremophiles

Many archaea are extremophiles, thriving in extreme temperature, pH, or salinity. Major groups include:

• Thermophiles: Require temperatures above 45°C; important genera include Geogemma and Pyrodictium. Their enzymes are used in biotechnology and industry.

• Halophiles: Inhabit highly saline environments (e.g., Dead Sea, Great Salt Lake), requiring >9% salt for cell wall integrity. Many produce red or orange pigments for sunlight protection.

Methanogens

Methanogens are obligate anaerobes that produce methane from CO2, H2, and organic acids. They are the largest group of archaea and play a key role in carbon cycling and sewage treatment.

Survey of Bacteria

Deeply Branching Bacteria

These bacteria have rRNA sequences and growth characteristics similar to the earliest bacteria. They are autotrophic and inhabit environments resembling early Earth. Examples include:

• Aquifex: Gram-negative, earliest branch

• Deinococcus: Gram-positive, highly radiation-resistant

Phototrophic Bacteria

Phototrophic bacteria use light energy for metabolism and are divided into five groups based on pigments and electron donors:

• Cyanobacteria (blue-green bacteria): Gram-negative, produce O2, some fix nitrogen using heterocysts.

• Green sulfur bacteria: Use H2S as electron donor, deposit sulfur outside cells.

• Green nonsulfur bacteria: Use organic compounds as electron donors.

• Purple sulfur bacteria: Deposit sulfur inside cells.

• Purple nonsulfur bacteria: Use organic compounds as electron donors.

Low G+C Gram-Positive Bacteria (Phylum Firmicutes)

These bacteria have less than 50% guanine-cytosine content in their DNA and include:

• Clostridia: Obligate anaerobes, endospore-forming, produce toxins (e.g., C. tetani, C. botulinum).

• Mycoplasmas: Lack cell walls, smallest free-living cells, cause respiratory and urogenital infections.

• Bacillus: Endospore-forming aerobes/facultative anaerobes, includes B. anthracis (anthrax) and B. thuringiensis (insecticide toxin).

• Listeria: Survives refrigeration, can cause meningitis and fetal death.

• Lactobacillus: Non-spore-forming, beneficial in human microbiota and food production.

• Streptococcus and Enterococcus: Cocci in chains, cause various diseases.

• Staphylococcus: Cocci in clusters, includes S. aureus, a common human inhabitant and pathogen.

High G+C Gram-Positive Bacteria (Phylum Actinobacteria)

These bacteria have more than 50% guanine-cytosine content and include:

• Corynebacterium: Pleomorphic rods, cause diphtheria.

• Mycobacterium: Acid-fast, slow-growing, cause tuberculosis and leprosy.

• Actinomycetes: Filamentous, produce antibiotics (e.g., Streptomyces), cause disease in immunocompromised patients.

Gram-Negative Proteobacteria

This is the largest and most diverse group of bacteria, divided into five classes:

• Alphaproteobacteria: Capable of growth at low nutrient levels, includes nitrogen-fixers (Azospirillum, Rhizobium), nitrifying bacteria, purple nonsulfur phototrophs, and pathogens (Rickettsia, Brucella).

• Betaproteobacteria: Includes Neisseria (gonorrhea, meningitis), Bordetella (whooping cough), and environmental recyclers.

• Gammaproteobacteria: Includes purple sulfur bacteria, intracellular pathogens (Legionella, Coxiella), methane oxidizers, glycolytic facultative anaerobes (e.g., Escherichia, Salmonella), and pseudomonads (Pseudomonas).

• Deltaproteobacteria: Includes Desulfovibrio (sulfur recycling), Bdellovibrio (predator of other bacteria), and myxobacteria (fruiting body formation).

• Epsilonproteobacteria: Includes Campylobacter (intestinal inflammation) and Helicobacter (ulcers).

Other Gram-Negative Bacteria

• Chlamydias: Intracellular pathogens, lack peptidoglycan, cause sexually transmitted infections.

• Spirochetes: Helical, motile, cause syphilis (Treponema) and Lyme disease (Borrelia).

• Bacteroids: Inhabit digestive tracts, some cause infections; Cytophaga degrades sewage.

Additional info: This guide summarizes the major features, classification, and diversity of prokaryotes, including both Archaea and Bacteria, as well as their ecological and medical significance. Tables and images are included to reinforce key concepts and aid in visual identification.