Characterizing and Classifying Prokaryotes: Study Notes

Characterizing and Classifying Prokaryotes

General Features of Prokaryotes

Prokaryotes represent the most diverse group of organisms on Earth, occupying a vast range of habitats. While only a small fraction are capable of colonizing humans and causing disease, their ecological roles are essential.

• Habitats: Found in environments ranging from Antarctic glaciers and thermal hot springs to animal colons, distilled water, brine, disinfectant solutions, and even basalt rocks.

• Diversity: Only a few prokaryotes are pathogenic to humans.

Morphology of Prokaryotes

Prokaryotes exhibit a variety of shapes and arrangements, which are important for identification and classification.

• Cocci: Spherical cells; arrangements depend on the plane of division and separation of daughter cells.

• Bacilli: Rod-shaped cells; arrangements include diplobacilli, streptobacilli, and V-shaped bacilli.

• Other Forms: Spiral and unusual shapes also exist, especially among archaea.

Reproduction in Prokaryotes

All prokaryotes reproduce asexually, utilizing several methods to ensure survival and propagation.

• Binary Fission: The most common method; a cell divides into two genetically identical daughter cells.

• Snapping Division: A variation of binary fission where the cell wall ruptures at a specific point.

• Budding: A new organism develops from a protrusion of the parent cell.

• Reproductive Structure Formation: Some prokaryotes form specialized reproductive structures.

Arrangement of Prokaryotic Cells

The arrangement of cells after division is a key taxonomic feature.

• Cocci Arrangements:

  • Diplococci: Pairs of cocci.

  • Streptococci: Chains of cocci.

  • Tetracocci: Groups of four cocci.

  • Staphylococci: Irregular clusters of cocci.

• Bacilli Arrangements:

  • Diplobacilli: Pairs of bacilli.

  • Streptobacilli: Chains of bacilli.

  • V-Shaped Bacilli: Bacilli arranged in a V shape.

Endospores

Endospores are highly resistant, dormant structures formed by certain Gram-positive bacteria as a defense against unfavorable conditions.

• Producers: Genera Bacillus and Clostridium.

• Formation: Each vegetative cell forms one endospore; each endospore can germinate into one vegetative cell.

• Resistance: Endospores are extremely resistant to drying, heat, radiation, and chemicals.

• Longevity: Can remain viable for tens to thousands of years.

• Significance: Major concern in food processing, healthcare, and biosecurity.

Example: Bacillus anthracis endospores can survive in soil for decades, posing a risk for anthrax outbreaks.

Taxonomic Classification of Prokaryotes

Modern classification is based primarily on genetic relatedness, especially rRNA sequences, dividing life into three domains.

• Domains: Archaea, Bacteria, Eukarya

• Basis: rRNA sequence analysis

Domain Archaea

General Features of Archaea

Archaea are prokaryotes with unique molecular and structural characteristics, distinct from bacteria.

• Cell Wall: Lack peptidoglycan.

• Membrane Lipids: Contain branched hydrocarbon chains.

• Genetics: Lack tRNA specific for thymine; AUG codon codes for methionine (not N-formylmethionine as in bacteria).

• Reproduction: Binary fission, budding, or fragmentation.

• Shapes: Cocci, bacilli, spiral, and unusual forms.

• Pathogenicity: Not known to cause disease in humans or animals.

Classification of Archaea

• Extremophiles: Require extreme conditions of temperature, pH, or salinity.

  • Thermophiles: Thrive above 45ºC; hyperthermophiles require >80ºC. Examples: Thermococcus, Pyrodictium, Geogemma.

  • Halophiles: Require >9% NaCl; contain red/orange pigments for UV protection. Example: Halobacterium salinarium.

• Methanogens: Convert CO2, H2, and organic acids to methane; largest group of archaea; important in carbon cycling.

Domain Bacteria

Deeply Branching Bacteria

These bacteria are considered to be among the earliest forms of life, with rRNA sequences and growth characteristics similar to ancient bacteria.

• Habitat: Often found in extreme environments.

• Example: Aquifex is considered one of the earliest bacterial branches.

Phototrophic Bacteria

Phototrophic bacteria use light as an energy source and possess photosynthetic lamellae. They are autotrophic and play key roles in global nutrient cycles.

• Groups:

  • Blue-green bacteria (cyanobacteria)

  • Green sulfur bacteria

  • Green nonsulfur bacteria

  • Purple sulfur bacteria

  • Purple nonsulfur bacteria

Gram-Positive Bacteria

Gram-positive bacteria are classified based on their DNA G+C content into low and high G+C groups.

• Low G+C Gram-Positive Bacteria:

  • Clostridia

  • Mycoplasma

  • Bacillus

  • Listeria

  • Lactobacillus

  • Streptococcus and Enterococcus

  • Staphylococcus

• High G+C Gram-Positive Bacteria:

  • Corynebacterium

  • Mycobacterium

  • Actinomycetes (including Actinomyces, Nocardia, Streptomyces)

Gram-Negative Bacteria

Gram-negative bacteria are highly diverse, with the largest group being the Proteobacteria, which are divided into several classes.

Proteobacteria Classes

• Alphaproteobacteria:

  • Nitrogen Fixers: Azospirillum, Rhizobium

  • Nitrifying Bacteria: Nitrobacter

  • Purple Nonsulfur Phototrophs

  • Pathogens: Rickettsia, Brucella, Ehrlichia

  • Other: Acetobacter, Gluconobacter, Caulobacter

• Betaproteobacteria:

  • Pathogens: Neisseria, Bordetella, Burkholderia

  • Nonpathogens: Thiobacillus, Zoogloea, Sphaerotilus

• Gammaproteobacteria:

  • Purple Sulfur Bacteria

  • Intracellular Pathogens: Legionella, Coxiella

  • Methane Oxidizers

  • Glycolytic Facultative Anaerobes: Families Enterobacteriaceae, Vibrionaceae, Pasteurellaceae

  • Pseudomonads

• Deltaproteobacteria:

  • Desulfovibrio

  • Bdellovibrio

  • Myxobacteria

• Epsilonproteobacteria:

  • Campylobacter

  • Helicobacter

• Zetaproteobacteria:

  • Discovered in the mid-2000s

  • Mariprofundus ferrooxydans: oxidizes iron as an energy source

Other Gram-Negative Bacteria

• Chlamydias: Chlamydia

• Spirochetes: Treponema (syphilis), Borrelia (Lyme disease)

• Bacteroids: Bacteroides, Cytophaga

Summary Table: Major Groups of Prokaryotes

Group	Key Features	Examples
Archaea	Extremophiles, lack peptidoglycan, unique membrane lipids	Halobacterium, Thermococcus, Methanogens
Deeply Branching Bacteria	Ancient, extremophilic	Aquifex
Phototrophic Bacteria	Photosynthetic, autotrophic	Cyanobacteria, green/purple sulfur/nonsulfur bacteria
Low G+C Gram-Positive	Thick peptidoglycan, low G+C DNA	Bacillus, Clostridium, Staphylococcus
High G+C Gram-Positive	High G+C DNA, often filamentous	Mycobacterium, Streptomyces
Proteobacteria	Gram-negative, diverse metabolism	Escherichia, Neisseria, Legionella
Other Gram-Negative	Unique cell structures, often pathogenic	Chlamydia, Treponema, Bacteroides

Key Terms and Definitions

• Binary Fission: A method of asexual reproduction in which a cell divides into two genetically identical cells.

• Endospore: A highly resistant, dormant structure formed by certain bacteria for survival in adverse conditions.

• Extremophile: An organism that thrives in extreme environmental conditions.

• G+C Content: The percentage of guanine and cytosine bases in DNA; used in bacterial classification.

• Proteobacteria: A major phylum of Gram-negative bacteria, including many pathogens.

Additional info:

• Classification based on rRNA sequences has revolutionized our understanding of microbial phylogeny.

• Endospore formation is a key factor in the persistence of certain bacterial pathogens in the environment.