Characterizing and Classifying Prokaryotes

General Characteristics of Prokaryotic Organisms

  • Morphology of Prokaryotic Cells
    • Most diverse group of cellular microbes
    • Thrive in various habitats
    • Only a few are capable of colonizing humans and causing disease
    • Exhibit a variety of shapes:
    • Coccus (spherical)
    • Bacillus (rod-shaped)
    • Spirillum (spiral)
    • Vibrio (comma-shaped)
    • Spirochete (flexible spiral)
    • Coccobacillus (short rod), etc.
  • Reproduction of Prokaryotic Cells
    • All reproduce asexually
    • Three main methods:
    • Binary Fission (most common)
      • DNA replication and elongation of cell
    • Snapping Division
    • Budding
      • DNA replication and cell elongation to form a bud
  • Arrangements of Prokaryotic Cells
    • Arrangements depend on the planes of division during binary fission and separation of daughter cells
    • Example formations include:
    • Diplococci (pairs)
    • Streptococci (chains)
    • Tetrads (groups of four)
    • Staphylococci (clusters)

Modern Prokaryotic Classification

  • Classification is based on the genetic relatedness of rRNA sequences
  • Three main domains:
    • Archaea
    • Bacteria
    • Eukarya

Survey of Archaea

  • Common Features

    • Lack true peptidoglycan and have branched hydrocarbon chains in cell membranes
    • Two main phyla:
    • Crenarchaeota
    • Euryarchaeota
    • Some reproduce by binary fission, budding, or fragmentation
    • Extremophiles require extreme conditions (temperature, pH, salinity) to survive:
    • Examples include thermophiles and halophiles

  • Methanogens

    • Largest group of archaea
    • Produce methane by converting carbon compounds
    • Found in colons of animals and sediment

Survey of Bacteria

Deeply Branching and Phototrophic Bacteria

  • Deeply Branching Bacteria

    • Similar to earliest bacteria, inhabit primordial habitats
    • Example: Deinococcus

  • Phototrophic Bacteria

    • Autotrophic; divided into groups based on photosynthetic pigments:
    • Cyanobacteria, Green sulfur bacteria, Purple nonsulfur bacteria, etc.
Low G + C Gram-Positive Bacteria
  • Clostridia
    • Obligate anaerobes, many form endospores
  • Mycoplasmas
    • Lack cell walls, smallest free-living cells, colonize mucous membranes
High G + C Gram-Positive Bacteria
  • Mycobacterium
    • Aerobic, slow growth due to mycolic acid in cell wall
  • Actinomycetes
    • Form branching filaments resembling fungi, important for antibiotic production
Gram-Negative Proteobacteria

  • The largest and most diverse group of bacteria, categorized into six classes:

    • Alphaproteobacteria
    • Betaproteobacteria
    • Gammaproteobacteria
    • Deltaproteobacteria
    • Epsilonproteobacteria
    • Zetaproteobacteria

  • Pathogenic genera include Rickettsia (causes typhus) and Neisseria (causes gonorrhea)

  • Gammaproteobacteria includes important pathogens like Pseudomonas and Legionella

Other Gram-Negative Bacteria
  • Chlamydias
    • Intracellular parasites, often sexually transmitted
  • Spirochetes
    • Corkscrew motion, includes Treponema (syphilis) and Borrelia (Lyme disease)