Characterizing and Classifying Prokaryotes: Structure, Reproduction, and Diversity

General Characteristics of Prokaryotic Organisms

Diversity and Habitats

Prokaryotes are the most diverse group of cellular microbes, inhabiting a wide range of environments from extreme cold (Antarctic glaciers) to extreme heat (thermal hot springs), and from animal colons to the cytoplasm of other prokaryotes. They can also be found in distilled water, supersaturated brine, disinfectant solutions, and even basalt rocks. However, only a small fraction of prokaryotes are capable of colonizing humans and causing disease.

Typical Prokaryotic Morphologies

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

• Coccus: Spherical-shaped cells

• Bacillus: Rod-shaped cells

• Coccobacillus: Short, oval rods resembling both cocci and bacilli

• Vibrio: Comma-shaped cells

• Spirillum: Rigid, spiral-shaped cells

• Spirochete: Flexible, spiral-shaped cells

• Pleomorphic: Cells that vary in shape and size

Reproduction of Prokaryotic Cells

Modes of Asexual Reproduction

All prokaryotes reproduce asexually, primarily through three main methods:

• Binary Fission: The most common method, involving the replication of DNA, elongation of the cell, formation of a cross wall, and separation into two daughter cells.

• Snapping Division: A variation of binary fission where the inner portion of the cell wall forms a cross wall, but the outer wall ruptures, causing the cells to snap apart.

• Budding: A process where a new cell develops from a bud due to the replication and migration of the nucleoid into the bud, which then separates as a daughter cell.

Binary Fission Process

1. Cell replicates its DNA.

2. The cytoplasmic membrane elongates, separating DNA molecules.

3. A cross wall forms and the membrane invaginates.

4. The cross wall completes formation.

5. Daughter cells may separate.

Snapping Division

Snapping division is characterized by the rupture of the outer cell wall, resulting in a hinge-like separation of daughter cells.

Budding

In budding, the nucleoid replicates and a new nucleoid is moved into a developing bud, which eventually separates as a new cell.

Other Reproductive Strategies

• Viviparity: Some prokaryotes, such as Epulopiscium, reproduce by releasing live offspring from the body of the dead mother cell, a unique form of viviparous behavior in the prokaryotic world.

Arrangement of Prokaryotic Cells

Cellular Arrangements

The arrangement of prokaryotic cells is determined by the planes in which cells divide and whether daughter cells remain attached after division. Common arrangements include:

• Cocci: Diplococci (pairs), Streptococci (chains), Tetrads (groups of four), Sarcinae (cuboidal packets), Staphylococci (clusters)

• Bacilli: Single bacillus, Diplobacilli (pairs), Streptobacilli (chains), Palisade and V-shape arrangements

Endospores

Formation and Significance

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

Modern Prokaryotic Classification

Taxonomic Domains

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

• Archaea

• Bacteria

• Eukarya

Survey of Archaea

General Features

• Lack true peptidoglycan in their cell walls

• Cell membrane lipids have branched hydrocarbon chains

• The AUG codon codes for methionine (like eukaryotes)

• Three main phyla: Crenarchaeota, Euryarchaeota, Korarchaeota

• Reproduce by binary fission, budding, or fragmentation

• Most are cocci, bacilli, or spiral forms; pleomorphic forms exist

• Not known to cause disease in humans

Extremophiles

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

• Thermophiles: Require temperatures above 45°C; hyperthermophiles require temperatures over 80°C. Examples include Geogemma and Pyrodictium.

• Halophiles: Inhabit highly saline environments, requiring more than 9% NaCl. Many produce red or orange pigments for protection against visible and UV light. The most studied is Halobacterium salinarium.

Methanogens

Methanogens are the largest group of archaea, producing methane gas from carbon dioxide, hydrogen gas, and organic acids. They play a crucial role in converting organic wastes in aquatic sediments to methane and are a primary source of environmental methane.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

• Deeply Branching Bacteria: Thought to resemble the earliest forms of bacteria; autotrophic and inhabit environments similar to early Earth. Examples include Aquifex and Deinococcus.

• Phototrophic Bacteria: Contain photosynthetic lamellae and are divided into five groups based on pigments and electron sources: cyanobacteria (blue-green), green sulfur bacteria, green nonsulfur bacteria, purple sulfur bacteria, and purple nonsulfur bacteria.

Low G+C Gram-Positive Bacteria

• Clostridia: Rod-shaped, obligate anaerobes important in medicine and industry.

• Mycoplasmas: Facultative or obligate anaerobes lacking cell walls; smallest free-living cells.

• Bacillus: Common in soil; some species produce insecticidal toxins (e.g., Bacillus thuringiensis).

• Listeria: Contaminates milk and meat products.

• Lactobacillus: Grows in the body but rarely causes disease.

• Streptococcus and Enterococcus: Cause numerous diseases.

• Staphylococcus: Common inhabitants of humans.

High G+C Gram-Positive Bacteria

• Corynebacterium: Pleomorphic aerobes and facultative anaerobes; produce metachromatic granules.

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

• Actinomycetes: Form branching filaments resembling fungi; important genera include Actinomyces, Nocardia, and Streptomyces.

Gram-Negative Proteobacteria

Proteobacteria are the largest and most diverse group of bacteria, with many having prosthecae (extensions for attachment and nutrient absorption).

• Alphaproteobacteria: Includes nitrogen fixers (Azospirillum, Rhizobium), nitrifying bacteria (Nitrobacter), purple nonsulfur phototrophs, and pathogens (Rickettsia, Brucella).

• Betaproteobacteria: Includes pathogens (Neisseria, Bordetella, Burkholderia) and nonpathogens (Thiobacillus, Zoogloea, Sphaerotilus).

• Gammaproteobacteria: Includes purple sulfur bacteria, intracellular pathogens (Legionella, Coxiella), methane oxidizers, glycolytic facultative anaerobes (family Enterobacteriaceae), and pseudomonads (Pseudomonas, Azotobacter, Azomonas).

• Deltaproteobacteria: Includes Desulfovibrio, Bdellovibrio, and myxobacteria.

• Epsilonproteobacteria: Includes Campylobacter and Helicobacter.

Other Gram-Negative Bacteria

• Chlamydias: Genus Chlamydia

• Spirochetes: Genera Treponema and Borrelia

• Bacteroids: Genera Bacteroides and Cytophaga

Summary Table: Major Groups of Prokaryotes

Group	Key Features	Examples
Archaea	No peptidoglycan, extremophiles, methanogens	Halobacterium, Geogemma
Low G+C Gram-Positive	Low GC content, endospore formation, pathogens	Bacillus, Clostridium, Streptococcus
High G+C Gram-Positive	High GC content, filamentous forms	Mycobacterium, Streptomyces
Proteobacteria	Gram-negative, diverse metabolism	Rhizobium, Pseudomonas, Neisseria
Other Gram-Negative	Unique cell structures, pathogens	Chlamydia, Borrelia

Additional info: This summary table is inferred to provide a concise comparison of the major prokaryotic groups discussed in the notes.