Chapter 25: 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.

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.

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.

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.

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.

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.