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. Prokaryotic cells exhibit a variety of 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

• 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, and chemicals.

Reproduction of Prokaryotic Cells

All prokaryotes reproduce asexually. The three main methods are:

• Binary fission: The most common method, involving the replication of DNA, elongation of the cell, and division into two 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 a bud on the parent cell, eventually detaching as a separate organism.

Other Reproductive Strategies

Some prokaryotes, such as Epulopiscium, reproduce by viviparity, where live offspring emerge from the body of the dead mother cell. This is 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 daughter cells remain attached. Common arrangements include:

• Diplococci: Pairs of cocci

• Streptococci: Chains of cocci

• Tetrads: Groups of four cocci

• Sarcinae: Cuboidal packets of eight or more cocci

• Staphylococci: Irregular clusters of cocci

Modern Prokaryotic Classification

Taxonomic Domains

Modern classification of prokaryotes is based primarily on genetic relatedness, especially rRNA sequences. 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, have unique membrane lipids with branched hydrocarbon chains, and use methionine as the start codon for protein synthesis. They reproduce by binary fission, budding, or fragmentation and display a variety of shapes. Archaea are not known to cause disease in humans, plants, or animals.

Extremophiles

Many archaea are extremophiles, requiring extreme conditions such as high temperature, acidity, or salinity to survive.

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

• Halophiles: Inhabit highly saline environments (over 9% NaCl), often producing red or orange pigments for protection against sunlight. Halobacterium salinarium is a well-studied example.

Methanogens

Methanogens are archaea that produce methane gas by converting carbon dioxide, hydrogen gas, and organic acids. They play a significant role in the carbon cycle and are a primary source of environmental methane, especially in sediments and the digestive tracts of animals.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

Deeply branching bacteria are considered similar to the earliest forms of life, often autotrophic and found in extreme environments. Phototrophic bacteria perform photosynthesis and are divided into groups based on pigments and electron donors:

• Cyanobacteria: Oxygenic photosynthesis, Gram-negative, some fix nitrogen, thought to have contributed to the oxygenation of Earth's atmosphere.

• Green sulfur bacteria: Anoxygenic photosynthesis, use H2S as electron donor.

• Green nonsulfur bacteria: Anoxygenic, use organic compounds as electron donors.

• Purple sulfur bacteria: Anoxygenic, deposit sulfur inside cells.

• Purple nonsulfur bacteria: Anoxygenic, use organic compounds as electron donors.

Low G + C Gram-Positive Bacteria

These bacteria have a low guanine-cytosine content in their DNA and include several medically and industrially important genera:

• Clostridia: Obligate anaerobes, endospore-formers, cause diseases such as tetanus and botulism.

• Mycoplasmas: Lack cell walls, smallest free-living cells, colonize mucous membranes.

• Bacillus: Endospore-formers, Bacillus thuringiensis produces insecticidal toxins, Bacillus anthracis causes anthrax.

• Listeria: Contaminates food, can reproduce at refrigeration temperatures, crosses placenta.

• Lactobacillus: Used in food production, rarely pathogenic.

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

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

High G + C Gram-Positive Bacteria (Actinobacteria)

These bacteria have a high guanine-cytosine content in their DNA and include:

• Corynebacterium: Pleomorphic, produces metachromatic granules, causes diphtheria.

• Mycobacterium: Waxy cell walls (mycolic acid), slow-growing, causes tuberculosis and leprosy.

• Actinomycetes: Form branching filaments, important for antibiotic production (e.g., Streptomyces).

Gram-Negative Proteobacteria

Overview and Classes

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

• Alphaproteobacteria

• Betaproteobacteria

• Gammaproteobacteria

• Deltaproteobacteria

• Epsilonproteobacteria

• Zetaproteobacteria

Alphaproteobacteria

• Nitrogen fixers: Azospirillum and Rhizobium associate with plant roots, important for agriculture.

• Nitrifying bacteria: Nitrobacter oxidizes nitrogenous compounds, crucial for nitrogen cycling.

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

• Other genera: Acetobacter, Gluconobacter, Caulobacter.

Betaproteobacteria

• Pathogens: Neisseria (various diseases), Bordetella (pertussis), Burkholderia (respiratory infections).

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

Gammaproteobacteria

• Purple sulfur bacteria: Anoxygenic phototrophs.

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

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

• Glycolytic facultative anaerobes: Includes Escherichia, Salmonella, Shigella, Klebsiella, Proteus, Yersinia, Vibrio, and Haemophilus.

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

Deltaproteobacteria

• Desulfovibrio: Sulfate-reducing bacteria

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

• Myxobacteria: Exhibit complex life cycles and social behavior

Epsilonproteobacteria

• Campylobacter: Causes gastrointestinal infections

• Helicobacter: Associated with peptic ulcers

Zetaproteobacteria

• Discovered through DNA sequencing; common in oceans

• 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 genera include Treponema (syphilis) and Borrelia (Lyme disease).

Bacteroids

• Bacteroides: Common in the digestive tracts of humans and animals; some species can cause infections.

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