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 shapes, which are important for identification and classification.

• Coccus: Spherical-shaped cells.

• Bacillus: Rod-shaped cells.

• Spirillum: Rigid, 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 survive extreme conditions and later germinate into a new cell. Endospores are a major concern in food safety and healthcare due to their resilience.

• Defensive strategy: Endospores allow survival during unfavorable conditions.

• Significance: Their resistance makes them difficult to eliminate by standard sterilization methods.

Reproduction of Prokaryotic Cells

All prokaryotes reproduce asexually, primarily through binary fission, but also by snapping division and budding. Some groups, such as actinomycetes, produce spores for reproduction, while Epulopiscium exhibits viviparity, where live offspring emerge from the mother cell.

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

• Snapping division: A variation of binary fission where the cell wall ruptures unevenly.

• Budding: A new cell develops from a small outgrowth (bud) on the parent cell.

• Viviparity: Unique to Epulopiscium and relatives; offspring develop inside and are released upon the death of the mother cell.

Arrangements of Prokaryotic Cells

The arrangement of prokaryotic cells results from the planes in which cells divide and whether daughter cells remain attached. These arrangements are useful for identification.

• Diplococci: Pairs of cocci.

• Streptococci: Chains of cocci.

• Tetrads: Groups of four cocci in a square.

• Sarcinae: Cubical packets of eight or more cocci.

• Staphylococci: Irregular clusters of cocci.

• Bacilli arrangements: Single, pairs (diplobacilli), chains (streptobacilli), V-shapes, or palisades.

Modern Prokaryotic Taxonomy

Classification Based on rRNA Sequences

Modern taxonomy of prokaryotes relies heavily on genetic relatedness, particularly rRNA sequence analysis. Most prokaryotes have not been cultured and are known only from their genetic fingerprints. The three domains of life are Archaea, Bacteria, and Eukarya.

• rRNA similarity: Closer rRNA sequences indicate closer evolutionary relationships.

• Domains: Archaea, Bacteria, Eukarya.

Survey of Archaea

General Features

Archaea are prokaryotes distinct from bacteria. They lack true peptidoglycan in their cell walls, have unique membrane lipids, and use methionine as the start amino acid for protein synthesis. Archaea are divided into two main phyla: Crenarchaeota and Euryarchaeota. They reproduce by binary fission, budding, or fragmentation and are not known to cause disease.

Extremophiles

Many archaea are extremophiles, thriving in extreme environments such as high temperature, acidity, or salinity. Prominent groups include thermophiles and halophiles.

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

• Halophiles: Require high salt concentrations (>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 are important in the carbon cycle and are found in sediments and the digestive tracts of animals. Methanogens are a major source of environmental methane.

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

Deeply branching bacteria are considered to resemble the earliest forms of life. They are autotrophic and inhabit environments similar to early Earth. Phototrophic bacteria use light as an energy source and are divided into groups based on pigments and electron donors.

• Cyanobacteria (blue-green bacteria): Gram-negative phototrophs that contributed to the oxygenation of Earth's atmosphere. Some fix nitrogen in specialized cells called heterocysts.

• Other groups: Green sulfur bacteria, green nonsulfur bacteria, purple sulfur bacteria, purple nonsulfur bacteria.

Low G+C Gram-Positive Bacteria

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

• Clostridia: Obligate anaerobes, many form endospores, important in medicine and industry.

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

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

• Listeria: Can grow at refrigeration temperatures, contaminates food, can cross placenta.

• Lactobacillus: Used in food production, rarely pathogenic.

• Streptococcus, Enterococcus, Staphylococcus: Common 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. Notable genera include:

• Corynebacterium: Pleomorphic, produce metachromatic granules.

• Mycobacterium: Slow-growing, waxy cell wall (mycolic acid), causes tuberculosis and leprosy.

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

Gram-Negative Proteobacteria

Proteobacteria are the largest and most diverse group of bacteria, divided into six classes (Alpha-, Beta-, Gamma-, Delta-, Epsilon-, Zetaproteobacteria). They include many important environmental and pathogenic species.

• Alphaproteobacteria: Nitrogen fixers (Azospirillum, Rhizobium), nitrifying bacteria (Nitrobacter), pathogens (Rickettsia, Brucella), and others.

• Betaproteobacteria: Pathogens (Neisseria, Bordetella, Burkholderia), environmental bacteria (Thiobacillus, Zoogloea).

• Gammaproteobacteria: Includes purple sulfur bacteria, intracellular pathogens (Legionella, Coxiella), methane oxidizers, glycolytic facultative anaerobes (Yersinia, Vibrio, Haemophilus), and pseudomonads (Pseudomonas).

• Deltaproteobacteria: Includes Desulfovibrio, Bdellovibrio (predator of other bacteria), and myxobacteria (complex life cycle).

• Epsilonproteobacteria: Includes Campylobacter (gastroenteritis) and Helicobacter (gastric ulcers).

• Zetaproteobacteria: Recently discovered, common in oceans, few cultured species.

Other Gram-Negative Bacteria

Chlamydias

Chlamydias are obligate intracellular parasites of mammals, birds, and some invertebrates. They are among the smallest bacteria and are responsible for several human diseases, including the most common bacterial sexually transmitted infection in the United States.

Spirochetes

Spirochetes are motile bacteria with a corkscrew motion. They inhabit diverse environments and include important pathogens such as Borrelia burgdorferi (Lyme disease) and Treponema pallidum (syphilis).

Bacteroids

Bacteroids are common inhabitants of the digestive tracts of humans and animals. Some species can cause infections. Cytophaga is an aquatic, gliding bacterium important in the degradation of raw sewage.