chapter 27

Bio1151 Chapter 27 9

Most prokaryotes are microscopic, and can be found almost anywhere.
- The glowing organ below the eye of this deep water flashlight fish harbors bioluminescent bacteria.
  The fish uses the light to attract prey and to signal potential mates.
The most common shapes are (cocci), (bacilli), and .
- Cocci (singular coccus) are spherical prokaryotes. They occur singly, in pairs, or in chains (such as these streptococci). Bacilli (singular, bacillus) are rod-shaped prokaryotes. They are usually solitary, sometimes the rods are arranged in chains. Spiral prokaryotes such as these spirochetes are corkscrew-shaped.
Many bacteria exhibit , the directional movement in response to a stimulus, by propelling themselves with .
- Prokaryotic flagellum. Unlike an eukaryote flagellum, the motor of the prokaryotic flagellum is a basal apparatus embedded in the plasma membrane. A curved hook turns to rotate the attached filament, which is composed of the protein flagellin. This allows the cell to respond to a stimulus in a directional movement called taxis. Diversity of prokaryotic motility:
The prokaryotic genome is usually a circular ring of DNA located in a region, and the membrane may be folded to perform specialized functions.
- A prokaryote such as this E. coli cell has a circular chromosome consisting of a single ring of DNA, located in a nucleoid region.
  Many cells also harbor smaller, non-genome rings of DNA called plasmids.
- Specialized membranes of prokaryotes. In-foldings of the plasma membrane, reminiscent of the cristae of mitochondria, function in cellular respiration in some aerobic prokaryotes. Photosynthetic prokaryotes called cyanobacteria have thylakoid membranes, much like those in chloroplasts.
The cell wall of many prokaryotes is covered by a protective , and many also form in harsh conditions.
- The polysaccharide or protein capsule surrounds the cell wall of this Streptococcus.
  The capsule enables the bacterium to attach to cells of the human respiratory tract.
  Other prokaryotes stick to the substrate by fimbriae.
- Some prokaryotes attach to surfaces or to other prokaryotes via protein appendages called fimbriae.
- An endospore.
  Bacillus anthracis, the bacterium that causes anthrax, produces persistent endospores.
  The thick, protective coat helps it survive in the soil for years.
Prokaryotes are capable of genetic via several processes.
- Genetic recombination in prokaryotes may occur by 3 means.
- transformation
- transduction
- conjugation
- Transformation.
  The "S" (smooth) strain of a Streptococcus are pathogenic since it has a protective capsule.
  The "R" (rough) strain lack a capsule and are non-pathogenic.
  However, when R bacteria are mixed with heat-killed S bacteria, some of the R cells became pathogenic.
  The R cells have been transformed by picking up DNA from dead S cells in the environment.
- Transduction is genetic recombination mediated by phages.
  A phage infects a "donor" bacterium.
  In its lytic cycle, the phage accidentally packages some "donor" DNA (e.g. A^+ allele) in its protein coat.
  When progeny phage then infect a "recipient" cell, DNA from the "recipient" may exchange with that of the "donor".
  The "recipient" becomes a recombinant (A^+ B^-).
- Conjugation occurs when DNA from one bacterium is transferred to another.
  A specialized fimbria called sex pilus allows 2 bacteria to draw together.
  A mating bridge then forms between the cells, transferring DNA and resulting in genetic recombination.
Response to stain classified bacterial species into two groups based on amount of in the cell .
- Bacteria are stained with a violet dye and iodine, rinsed in alcohol, and then stained with a red dye. Gram-positive bacteria have a cell wall with a large amount of peptidoglycan that traps the violet dye in the cytoplasm, masking the added red dye. Gram-negative bacteria have less peptidoglycan. The violet dye is easily rinsed from the cytoplasm, and the cell appears red after the red dye is added.
Molecular analysis has now revealed several among the gram-negative prokaryotes, while another group is now assigned its own domain of .
- Prokaryotes do not constitute a monophyletic clade.
  Archaea are more closely related to Eukarya than to Bacteria.
  Note that among Bacteria:
- the Gram-positive comprises a monophyletic clade.
- the rest are a paraphyletic group of Gram-negative organisms.
- A comparison of the three domains of life.
  Among the prokaryotes, Archaea and Bacteria share few traits, other than lacking organelles and having circular DNA.
- Many Archaea live in extreme conditions: thrive in hot environments, live in saline environments, and live in swamps.
  - Extreme thermophiles. Orange and yellow colonies of "heat-loving" Archaea in the hot water of a Nevada geyser.
  - Extreme halophiles.
    Colorful "salt-loving" Archaea thrive in these ponds near San Francisco.
    Used for commercial salt production, the ponds contain water that is five to six times as salty as seawater.
  - Owens Lake in California can reach a salt concentration of 32%, nine times that of seawater.
    Few organisms can live in such saline environments other than halophiles in domain Archaea.
    These prokaryotes contain red pigments called Bacteriorhodopsin.
  - Some Archaea are methanogens that produce methane as a metabolic waste product in anaerobic conditions.
    They occur naturally in the guts of cattle to help digest cellulose, and can also be used in sewage treatment plants.
- The gram- bacteria constitute their own clade.
  - Gram-Positive Bacteria Soil-dwelling Streptomyces are a source of many antibiotics, including streptomycin used to treat tuberculosis, which is caused by another gram-positive bacterium, Mycobacterium tuberculosis
    Mycoplasmas are tiny and have no cell walls. One species causes "walking pneumonia". Hundreds of mycoplasmas are seen here covering a human fibroblast cell.
- Gram- bacteria include cyanobacteria and proteobacteria.
- Cyanobacteria ("Blue-green algae") are photosynthetic and can form stromatolites.
- Oscillatoria
- Anabaena
- These two species of cyanobacteria called Oscillatoria form filamentous colonies. Cyanobacteria locomotion:
- Metabolic cooperation in a colonial prokaryote. In the filamentous cyanobacterium Anabaena, cells known as heterocytes fix nitrogen, while the other cells carry out photosynthesis, and these cells exchange metabolic products. +n+ Most N fixation by Rhizobium, next (Proteobacteria).
- Proteobacteria comprise a diverse group within the Gram-negative bacteria.
  A well-known member of the gamma subgroup is Escherichia coli, found in the human colon.
  Another important member is the nitrogen fixing Rhizobium.
- Rhizobium is another proteobacterium among the Gram-negative bacteria. They live inside root cells of legumes (plants of the pea/bean family), where the bacteria convert atmospheric nitrogen to organic compounds such as ammonia in a process called nitrogen fixation.
Some prokaryotes are human pathogens by releasing .
- Bacteria can be pathogenic by releasing toxins.
- Exotoxins are secreted proteins.
- Endotoxins are lipid components of bacterial membranes and released when they die. Lyme disease is caused by the spirochete Borrelia burgdorferi and transmitted by deer ticks.
Bacteria diversity: