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 fraction are 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: Rigid, spiral-shaped cells.

• Spirochete: Flexible, spiral-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, chemicals, and radiation.

Reproduction of Prokaryotic Cells

All prokaryotes reproduce asexually. The main methods include:

• Binary Fission: The most common method, where the cell duplicates its DNA and divides 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, separating the cells.

• Budding: A new cell develops from a bud due to the outgrowth of the parent cell.

Some actinomycetes produce spores for reproduction, which are distinct from endospores.

Viviparity in Prokaryotes

Epulopiscium and its relatives exhibit viviparity, a unique reproductive method where live offspring emerge from the body of the dead mother cell. This is the first known case of viviparous behavior in prokaryotes, with internal offspring formation resembling early endospore development.

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:

• Cocci: Diplococci, streptococci, tetrads, sarcinae, staphylococci.

• Bacilli: Single bacilli, diplobacilli, streptobacilli, palisade, V-shape.

Modern Prokaryotic Classification

Three Domains of Life

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 prokaryotes that lack true peptidoglycan in their cell walls, have unique membrane lipids with branched hydrocarbon chains, and use methionine as the start amino acid for protein synthesis. They reproduce by binary fission, budding, or fragmentation and display a variety of shapes. Archaea are not known to cause disease.

Extremophiles

Many archaea are extremophiles, requiring extreme conditions such as high temperature, acidity, or salinity. Major groups include:

• Thermophiles: Thrive at temperatures above 45°C; hyperthermophiles require temperatures over 80°C. Examples: Thermococcus, Pyrodictium.

• Halophiles: Inhabit highly saline environments, requiring over 9% NaCl. Many produce red or orange pigments for protection from sunlight. Example: Halobacterium salinarium.

Methanogens

Methanogens are archaea that produce 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, including methane trapped in ocean sediments.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

Deeply branching bacteria are considered among the earliest forms of life, adapted to extreme environments similar to early Earth. They are autotrophic and can survive in hot, acidic, and UV-intense habitats. Examples include Aquifex and Deinococcus.

Phototrophic bacteria use light as an energy source and are divided into groups based on pigments and electron donors:

• Blue-green bacteria (Cyanobacteria)

• Green sulfur bacteria

• Green non-sulfur bacteria

• Purple sulfur bacteria

• Purple non-sulfur bacteria

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

Low G+C Gram-Positive Bacteria

These bacteria have a low proportion of guanine and cytosine in their DNA. Major groups include:

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

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

• Bacillus: Endospore-formers, common in soil. Bacillus thuringiensis produces an insecticidal toxin; Bacillus anthracis causes anthrax.

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

• Lactobacillus: Used in food production, rarely pathogenic.

• Streptococcus, Enterococcus, Staphylococcus: Important human pathogens, some are multi-drug resistant.

High G+C Gram-Positive Bacteria

These bacteria have a high proportion of guanine and cytosine in their DNA. Major groups include:

• Corynebacterium: Pleomorphic aerobes, produce metachromatic granules.

• Mycobacterium: Slow-growing, acid-fast rods due to mycolic acid in cell walls.

• Actinomycetes: Form branching filaments, important in soil ecology and antibiotic production.

Gram-Negative Proteobacteria

Overview

Proteobacteria are the largest and most diverse group of Gram-negative bacteria, divided 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 nitrogen compounds, important in agriculture.

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

• Other genera: Acetobacter, Gluconobacter, Caulobacter.

Betaproteobacteria

• Pathogens: Neisseria (gonorrhea, meningitis), Bordetella pertussis (whooping cough), Burkholderia (opportunistic infections, melioidosis).

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

Gammaproteobacteria

• Purple sulfur bacteria: Phototrophic, deposit sulfur internally.

• Intracellular pathogens: Legionella pneumophila (Legionnaire's disease), Coxiella burnetii (Q fever).

• Glycolytic facultative anaerobes: Includes Escherichia coli, Salmonella, Shigella, Proteus, Klebsiella, Yersinia pestis, Vibrio cholerae, Haemophilus influenzae.

• Pseudomonads: Aerobic, flagellated rods, important pathogens and spoilage organisms.

Deltaproteobacteria

• Desulfovibrio: Reduces sulfates, produces hydrogen sulfide.

• Bdellovibrio: Predatory on other Gram-negative bacteria.

• Myxobacteria: Soil bacteria with complex life cycles, aid in decomposition.

Epsilonproteobacteria

• Campylobacter: Causes gastroenteritis and blood poisoning.

• Helicobacter: Causes gastric ulcers and increases risk of stomach cancer.

Zetaproteobacteria

Discovered through DNA sequencing, these bacteria are common in oceans. Mariprofundus ferrooxydans is the only formally named species.

Other Gram-Negative Bacteria

Chlamydias

Obligate intracellular parasites with both DNA and RNA, cytoplasmic membranes, and ribosomes. They lack peptidoglycan in their cell walls. Chlamydia trachomatis is the most common sexually transmitted bacterium in the U.S. and can cause urethritis, pelvic inflammatory disease, and trachoma (leading to blindness).

Spirochetes

Spiral-shaped bacteria that move via axial filaments, producing a corkscrew motion. Some are normal flora, while others are pathogenic:

• Treponema pallidum: Causes syphilis.

• Borrelia burgdorferi: Causes Lyme disease, transmitted by ticks.

Bacteroids

• Bacteroides: Common in the digestive tract, aid in digestion, but some species are pathogenic.

• Cytophaga: Aquatic, degrade polysaccharides, important in sewage degradation.

Additional info: Where tables were referenced but not fully visible, key characteristics and groupings were inferred from standard microbiology knowledge and the context provided.