The Structure of Bacteria and Archaea

• Prokaryotic cells are very small and do not have membrane-enclosed organelles such as nuclei and mitochondria.

• Prokaryotic cells have several common shapes: spherical (cocci), rod-shaped (bacilli), and spiral.

  • Spiral bacteria include the spirillum, which is a rigid helix, and spirochete, which is a flexible helix.

• Most bacteria have cell walls composed of peptidoglycan.

  • The walls of gram-positive bacteria are very thick and consist mainly of peptidoglycan.

  • The cell walls of gram-negative bacteria consist of a thin peptidoglycan layer and an outer membrane resembling the plasma membrane.

  • Some species of bacteria produce a capsule or slime layer that surrounds the cell wall.

• Some prokaryotes have hairlike appendages called fimbriae.

  • Pili also extend from the surface of some prokaryotes.

  • Both fimbriae and pili help cells adhere to one another or to certain other surfaces, including cells they infect.

  • Cannulae and hami are recently discovered hairlike appendages unique to archaea.

• Bacterial flagella are structurally different from eukaryotic flagella; each flagellum consists of a basal body, hook, and filament.

  • They produce a rotary motion.

Prokaryote Reproduction and Evolution

• The genetic material of a bacterium typically consists of a circular DNA molecule and one or more plasmids, smaller circular fragments of DNA.

• Prokaryotes reproduce asexually by binary fission (the cell divides, forming two cells), budding (a bud forms and sepa- rates from the mother cell), or fragmentation (walls form inside the cell, which then separates into several cells).

• In prokaryotes genetic material can be exchanged by trans- formation, transduction, or conjugation.

  • In transformation a prokaryotic cell takes in foreign DNA released by another cell.

  • Homologous segments of foreign and host DNA are exchanged.

  • In transduction a phage carries bacterial DNA from one bacterial cell into another.

  • In conjugation a donor cell transfers plasmid DNA to a recipient cell.

• Rapid reproduction ensures that mutations are rapidly passed to new generations.

  • Horizontal gene transfer—by transformation, transduction, or conjugation—contributes to rapid evolution in prokaryotes.

Nutritional and Metabolic Adaptations

• Most prokaryotes are heterotrophs that obtain carbon from other organisms; some are autotrophs that make their own organic molecules from simple raw materials.

• Chemotrophs obtain energy from chemical compounds; phototrophs capture energy from light.

• Autotrophs may be photoautotrophs, which obtain energy from sunlight, or chemoautotrophs, which obtain energy by oxidizing inorganic chemicals such as ammonia.

• Photoheterotrophs obtain carbon from other organisms but use chlorophyll and other photosynthetic pigment to trap energy from sunlight.

  • The majority of bacteria are chemoheterotrophs.

  • They are mainly free-living decomposers that obtain both carbon and energy from dead organic matter.

• Most bacteria are aerobic; that is, they require oxygen for cellular respiration.

  • Some prokaryotes are facultative anaerobes that metabolize anaerobically when necessary; others are obligate anaerobes that can carry on metabolism only anaerobically.

• Some bacteria and archaea carry on nitrogen fixation; that is, they reduce nitrogen in the atmosphere to ammonia.

  • Other prokaryotes convert ammonia to nitrite or nitrate in a process called nitrification.

The Phylogeny of the Two Prokaryote Domains

• Prokaryotes are assigned to domain Archaea and domain Bacteria.

• Unlike those of bacteria, the cell walls of archaea do not have peptidoglycan.

  • The translational mechanisms of eukaryotes more closely resemble those of archaea than those of bacteria.

• The Crenarchaeota include many extreme thermophiles, archaea that can inhabit very hot, sometimes acidic, environments and archaea that are marine dwellers.

  • Euryarchaeota include methanogens, extreme halophiles, and some extreme thermophiles.

  • Methanogens are obligate anaerobes that produce methane gas from simple carbon compounds.

  • Extreme halophiles inhabit saturated salt solutions.

  • Korarchaeota have been found in terrestrial hot springs.

  • Nanoarchaeota has only one member to date, Nano­ archaeum equitans, a very small, extreme thermophile discovered in a hydrothermal vent.

• Major groups of bacteria include proteobacteria, gram-positive bacteria, cyanobacteria, chlamydias, and spirochetes.

Impact on Ecology, Technology, and Commerce

• Many bacteria are symbiotic with other organisms.

• Mutualism is a symbiotic relationship in which both partners benefit.

• In commensalism one partner benefits, and the other is neither harmed nor helped.

• In parasitism the parasite benefits, and the host is harmed.

• Bacterial pathogens cause disease, but are usually not considered obligate parasites.

• Biofilms are dense communities of microorganisms, in which cells adhere to one another on a surface.

• Biofilms may include bacteria, archaea, protists, and fungi.

• Prokaryotes play essential ecological roles as decomposers and are important in recycling nitrogen and other nutrients.

• Some bacteria carry out photosynthesis.

• Some prokaryotes produce antibiotics.

• We have developed the technology for using certain bacteria to produce vaccines, insulin, and other important compounds.

• We use bacteria in the production of many foods, including cheese, yogurt, vinegar, and chocolate.

• We also use microbes in sewage treatment and in bioremediation.

Bacteria and Disease

• Louis Pasteur demonstrated that sterilization prevented bacterial growth.

• Koch’s postulates are a set of guidelines developed by robert Koch to demonstrate that a specific pathogen causes specific disease symptoms:

• (1) the pathogen must be present in every individual with the disease,

• (2) a sample of the microorganism taken from the diseased host can be grown in pure culture,

• (3) a sample of the pure culture causes the same disease when injected into a healthy host, and

• (4) the microorganism can be recovered from the experimentally infected host.

• Some pathogenic bacteria release strong poisons called exotoxins; others produce endotoxins, poisonous components of their cell walls that are released when bacteria die.

• Many bacteria have become resistant to antibiotics.

• Plasmids that have genes for antibiotic resistance are called R factors.