Characterizing and Classifying Prokaryotes: Structure, Reproduction, and Diversity

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. Only a small subset is capable of colonizing humans and causing disease. They exhibit a variety of cell shapes, which are important for identification and classification.

• Coccus: Spherical-shaped cells.

• Bacillus: Rod-shaped cells.

• Spirillum: Spiral-shaped cells.

• Spirochete: Flexible, corkscrew-shaped cells.

• Vibrio: Comma-shaped cells.

• Coccobacillus: Short, oval rods.

• Pleomorphic: Cells that vary in shape.

• Star-shaped: Rare, star-like morphology.

Endospores

Endospores are highly resistant, dormant structures produced by certain Gram-positive bacteria, such as Bacillus and Clostridium. Each vegetative cell forms one endospore, which can later germinate into a new vegetative cell. Endospores serve as a defensive strategy against unfavorable environmental conditions and are of significant concern in food safety and healthcare due to their resistance to heat, desiccation, chemicals, and radiation.

Reproduction of Prokaryotic Cells

All prokaryotes reproduce asexually. The main methods include:

• Binary Fission: The most common method, involving the replication of DNA, elongation of the cell, and division into two identical daughter cells.

• Snapping Division: A variation of binary fission where the inner cell wall layer forms a cross wall, and the outer layer snaps, releasing the daughter cells.

• Budding: A new cell develops from the surface of an existing cell and eventually detaches.

Some prokaryotes, such as Epulopiscium, reproduce by viviparity, where live offspring emerge from the body of the dead mother cell—a unique form of reproduction among prokaryotes.

Arrangements of Prokaryotic Cells

The arrangement of prokaryotic cells results from the planes in which cells divide and whether the daughter cells remain attached. Common arrangements include:

• Diplococci: Pairs of cocci.

• Streptococci: Chains of cocci.

• Tetrads: Groups of four cocci in a square.

• Sarcinae: Cuboidal packets of eight or more cocci.

• Staphylococci: Irregular clusters of cocci.

Modern Prokaryotic Taxonomy

Classification Based on rRNA Sequences

Modern taxonomy of prokaryotes is primarily based on genetic relatedness, especially rRNA sequences. Most prokaryotes have never been isolated or cultured and are known only from their rRNA fingerprints. The three domains of life are:

• Archaea

• Bacteria

• Eukarya

Survey of Archaea

General Features

Archaea are prokaryotes that lack true peptidoglycan in their cell walls and have unique membrane lipids with branched or ring-form hydrocarbon chains. The AUG codon codes for methionine in archaea. They reproduce by binary fission, budding, or fragmentation and are not known to cause disease. The two main phyla are Crenarchaeota and Euryarchaeota.

Extremophiles

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

• Thermophiles: Require temperatures above 45°C; hyperthermophiles thrive above 80°C. Examples include Thermococcus and Pyrodictium.

• Halophiles: Inhabit environments with >9% NaCl, often producing red or orange pigments for protection from sunlight. Halobacterium salinarium is a well-studied example.

Methanogens

Methanogens are the largest group of archaea, producing methane gas from carbon dioxide, hydrogen, and organic acids. They play a significant role in the carbon cycle and are a primary source of environmental methane, especially in anaerobic environments such as sediments and animal colons.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

Deeply branching bacteria are considered similar to the earliest forms of bacteria, often autotrophic and living in extreme environments. Examples include Aquifex and Deinococcus.

Phototrophic bacteria contain photosynthetic lamellae and are divided into five groups based on pigments and electron donors:

• Blue-green bacteria (cyanobacteria)

• Green sulfur bacteria

• Green nonsulfur bacteria

• Purple sulfur bacteria

• Purple nonsulfur bacteria

Cyanobacteria are Gram-negative phototrophs that contributed to the oxygenation of Earth's atmosphere and can fix nitrogen in specialized cells called heterocysts.

Low G+C Gram-Positive Bacteria

Low G+C Gram-positive bacteria include several medically and industrially important genera:

• Clostridia: Obligate anaerobes, many form endospores.

• Mycoplasmas: Lack peptidoglycan, smallest free-living cells, pleomorphic, colonize mucous membranes.

• Bacillus: Endospore-forming, includes B. thuringiensis (insecticide) and B. anthracis (anthrax).

• Listeria: Can reproduce under refrigeration, contaminates food, can cross placenta.

• Lactobacillus: Used in food production, rarely pathogenic.

• Streptococcus and Enterococcus: Cause various diseases, some are multi-drug resistant.

• Staphylococcus: Common human inhabitant, produces toxins and enzymes.

High G+C Gram-Positive Bacteria

High G+C Gram-positive bacteria include:

• Corynebacterium: Pleomorphic, produces metachromatic granules.

• Mycobacterium: Slow-growing, acid-fast due to mycolic acid in cell walls, includes pathogens causing tuberculosis and leprosy.

• Actinomycetes: Form branching filaments, important in soil ecology and antibiotic production (e.g., Streptomyces).

Gram-Negative Proteobacteria

Overview and Classification

Proteobacteria are the largest and most diverse group of Gram-negative bacteria, classified into six classes: Alpha-, Beta-, Gamma-, Delta-, Epsilon-, and Zetaproteobacteria.

Alphaproteobacteria

• Nitrogen fixers: Azospirillum and Rhizobium associate with plant roots and fix atmospheric nitrogen.

• Nitrifying bacteria: Nitrobacter oxidizes nitrogenous compounds, important in agriculture.

• Pathogens: Rickettsia (arthropod-borne diseases), Brucella (brucellosis).

• Other genera: Acetobacter, Gluconobacter, Caulobacter.

Betaproteobacteria

• Pathogens: Neisseria (meningitis, gonorrhea), Bordetella (pertussis), Burkholderia (respiratory infections).

• Environmental bacteria: Thiobacillus (sulfur recycling), Zoogloea (sewage treatment), Sphaerotilus (impedes waste flow).

Gammaproteobacteria

• Purple sulfur bacteria: Phototrophic, deposit sulfur internally.

• Intracellular pathogens: Legionella (Legionnaires’ disease), Coxiella (Q fever).

• Methane oxidizers: Use methane as a carbon and energy source.

• Glycolytic facultative anaerobes: Includes Yersinia pestis (plague), Vibrio cholerae (cholera), Haemophilus influenzae (meningitis, pneumonia).

• Pseudomonads: Pseudomonas (opportunistic infections), Azotobacter, Azomonas.

Deltaproteobacteria

• Desulfovibrio: Sulfate-reducing bacteria.

• Bdellovibrio: Predatory bacteria that attack other Gram-negative bacteria.

• Myxobacteria: Exhibit complex life cycles, forming fruiting bodies and myxospores.

Epsilonproteobacteria

• Campylobacter: Causes gastroenteritis and blood poisoning.

• Helicobacter pylori: Major cause of gastric ulcers and associated with stomach cancer.

Zetaproteobacteria

• Discovered through DNA sequencing; Mariprofundus ferrooxydans is the only formally named species.

Other Gram-Negative Bacteria

Chlamydias

Obligate intracellular bacteria that infect mammals, birds, and some invertebrates. Some species are smaller than viruses and are the most common sexually transmitted bacteria in the United States.

Spirochetes

Motile bacteria with a corkscrew motion, diverse in metabolism and habitat. Notable pathogens include Borrelia burgdorferi (Lyme disease) and Treponema pallidum (syphilis).

Bacteroids

• Bacteroides: Common in the digestive tracts of humans and animals; some species are opportunistic pathogens.

• Cytophaga: Aquatic, gliding bacteria important in the degradation of raw sewage.