Chapter 16: Microbial Life: Prokaryotes

16.1 Prokaryotes are diverse and widespread

  • The fossil record shows that prokaryotes were abundant billions of years ago. Today prokaryotes are found wherever there is life.
  • They are extremely abundant – a handful of fertile soil will house more prokaryotes than the total number of people who have ever lived!
  • Most prokaryotes have a diameter of 1-5µm (a typical eukaryote is 10-100µm). Despite their size, they continue to have a profound impact on the world.
  • The ones we typically hear about are the ones that cause disease, pathogens. However, there are many more beneficial bacteria than those that are harmful.

16.2External features contribute to the success of prokaryotes

Prokaryotes have a cellular arrangement that is different from eukaryotes. Some of their diversity is evident in external features. These features are useful in identifying the prokaryote, but also in keeping it alive.

  • Cells shape: One of the first steps to identifying a prokaryote is to determine its shape. Three shapes are common to prokaryotes:

Coccus (cocci) – are spherical in shape

Bacillus (bacilli) – are rod-shaped

Curved or spiral

  • If the bacteria are shaped like a comma, they are called vibrios
  • If they are helical, they are called spirilla
  • If they are longer and more flexible, they are called spirochetes
  • External Structures – one of the most important features of prokaryote is the cell wall. The wall maintains cell shape, provides protection, and prevents bursting in a hypotonic environment.

In hypertonic environments, the cell will lose water and shrink away from their wall. Severe water loss prevents reproduction, which is why salt can be used to cure foods for storage.

Gram staining is an invaluable tool for the identification of bacteria. Cell walls fall into two general types – both of which are identifiable by this technique. Many times, gram staining is used to indicate the type of antibiotic to prescribe.

  • Gram positive bacteria – have simpler walls with a relatively thick layer of peptidoglycan, a polymer of sugars cross-linked by short polypeptides. This will take up a violet color when stained.
  • Gram negative bacteria – have a thinner layer of peptidoglycan and are more complex due to lipids bonded to carbohydrates. They will take up a pink color when stained. Generally, these are more threatening bacteria due to toxicity of the lipids in the wall. They are also more protected against the body’s natural defenses.

The cell wall is often covered by a sticky layer of proteins called the capsule. It enables prokaryotes to adhere to a surface. They also provide some protection against host defenses.

  • Projections – Many bacteria are equipped with a flagella to help them move. In response to chemical or physical signals they can move towards or away from a stimulus.

Bacterial flagella are different than eukaryotic flagella in structure. The bacterial flagella will have a system of rotating rings that attach them to the cell membrane and wall, giving them a propeller type motion. Flagella may be scattered all over the cell’s surface or concentrated at one or both ends of the cell. The prokaryotic flagellum is a protein structure lacking microtubules.

Some prokaryotes have appendages called fimbriae to attach to their substrate or to one another. These can allow pathogenic bacteria to latch on to host cells, as seen with Neisseria gonorrheae.

16.3 Populations of prokaryotes can adapt rapidly to changes in the environment

  • Prokaryotes are extremely successful, in part, because of their ability to reproduce quickly. In favorable conditions, they divide rapidly by binary fission. Many prokaryotes being able to produce a new generation within 1-3 hours, some in only 20 minutes.
  • Each time DNA is replicated prior to binary fission, spontaneous mutations can occur. Their rapid reproduction generates a great deal f genetic variety in a population. If an individual possesses a beneficial mutation, it can quickly take advantage of it, as seen in antibiotic resistance.
  • The genome of a prokaryote only has about one-thousandth of that in a eukaryote and forms a circular chromosome. Smaller DNA rings, plasmids, carry genes that can harbor resistance, metabolism, or other contingency factors.
  • Endospore – a specialized inner cell that is produced when conditions become less than ideal. The outer layer of the cell will disintegrate, leaving the inner cell. The endospore has a thick protective coat and can dehydrate to become dormant. When conditions improve, it rehydrates and resumes growth.

16.4 Prokaryotes have unparalleled nutritional diversity

In classification, mode of nutrition is often used to describe how an organism obtains two main resources: carbon and energy. Prokaryotes have more diverse methods of obtaining these things than eukaryotes do.

  • Types of Nutrition –

Many are autotrophs, “self-feeders.” They get their energy from sunlight or other inorganic chemicals.

  • Photoautotrophs – use sunlight for energy and carbon dioxide for carbon. (cyanobacteria)
  • Chemoautotrophs – obtain energy from inorganic sources instead of sunlight.

Heterotrophs, “other feeders,” obtain carbon from organic sources

  • Photoheterotrophs – obtain energy from sunlight but carbon from organic compounds
  • Chemoheterotrophs – can use almost any organic molecule for a food source.

16.5 Biofilms are complex associations of microbes

  • Some prokaryotes cooperate and form colonies. Connections between cells allow them to exchange chemicals that they need for survival. In some cases, the cooperation results in a surface coating called a biofilm.
  • Biofilms consist of one or several species of prokaryotes, but may include species of protists or fungi as well. They can form on almost any support material, living or not.
  • Formation starts when prokaryotes secrete signaling molecules to attract nearby cells into a cluster. Once the cluster is large enough, it secretes a gooey coating that enables them to stick together. As it grows, it generates chemical signals for communication. The biofilm will function as members of a community, with division of labor, defense, and other activities.
  • Biofilms are common in bacteria that cause human disease. This includes the plaque that forms on teeth, the ones that can form in the lungs of cystic fibrosis patients, and those that can form on medical equipment in the body. Antibiotics may have little effect on a biofilm because they can penetrate deep in to the community.

16.6 Prokaryotes help clean up the environment

Pathogenic prokaryotes are a small group of prokaryotes. Most species are actually helpful. All life depends on cycling elements between the living and nonliving portions of the environment. Soil has nutrients added by bacteria that recycle nitrogen; decomposition takes organic wastes and recycles them. Without decomposers, essential nutrients could not be returned to the soil.

Bioremediation is the use of organisms to remove pollutants from soil, air, and water.

For example, the use of decomposers to treat sewage.

16.7 Bacteria and archaea are the two main branches of prokaryotic evolution

  • There are two different kinds of prokaryotes. By comparing gene sequences, it was concluded that some bacteria are more closely related to eukaryotes, and therefore, should have their own classification.

Thus, bacteria are now divided into two domains: Archaea and Bacteria.

The belief is that at one point, bacteria and archaea diverged, and that archaea and eukaryotes may have come from one common ancestor.

  • Differences between bacteria and archaea show up in the cell wall and membranes. Almost all prokaryotes have a cell wall outside of the membrane that maintains the shape and provides protection. Yet bacterial walls contain a unique material called peptidoglycan.

16.8 Archae thrive in extreme environments – and in other habitats

  • Archae are abundant in many habitats, including areas where few other organisms could survive.

Extreme halophiles – thrive in salty places.

Extreme thermophiles – thrive in very hot water.

Methanogens – thrive in anaerobic environments and give off methane as a waste product.

16.9 Bacteria include a diverse assemblage of prokaryotes

  • Domain Bacteria is currently divided into 9 groups, five of which are a subgroups of gram negative bacteria called proteobacteria. It is divided into several categories:
  • Alpha – include Rhizobium species which live on roots and fix nitrogen or Agrobacterium which produce tumors on plants
  • Gamma – include sulfur bacteria that oxidize H2S. May include species that inhabit the intestines (Salmonella), those that cause cholera (Vibrio cholera), and E. coli.
  • Delta – include slime-secreting myxobacteria that form colonies
  • A second subgroup is the chlamydias. One type, Chlamydia trachomatis, is the most common cause of blindness in the world. Another type is the most common STD in the US.
  • A third subgroup is the spirochetes. Spirochetes are helical bacteria and many are pathogens.
  • Treponema pallidum causes syphilis
  • Borrelia burgdorferi causes Lyme disease
  • Gram-positive bacteria rival the proteobacteria in diversity.
  • Actinomycetes – form colonies of branched chains and are common in soil

Steptomyces are used to produce antibiotics

  • Also includes Bacillus anthracis, Staphylococcus, Streptococcus
  • Cyanobacteria – only plant-like, oxygen-generating photosynthesizers.

16.10 Some bacteria cause disease

  • We are constantly bombarded with bacteria, but are generally unaware of it because the body’s natural defenses take care of it. Sometimes are defenses cannot take care of it, and we become ill. Pathogenic bacteria cause about half of all human disease. Most cause illness by producing a poison such as an exotoxin (secreted by bacterial cells) or an endotoxin (found naturally in the cell wall).
  • Exotoxins include organisms such as botulism
  • Endotoxins include organisms such as Salmonella

16.11 Koch’s postulates are used to prove that a bacterium causes a disease

To test the hypothesis that a certain bacterium causes a disease, four conditions must be met. Robert Koch formulated this process in the 19th century. They are termed Koch’s Postulates.

  1. Find the candidate bacterium in every case of the disease
  2. Isolate the bacterium from a person who has the disease and grow it in a pure culture
  3. Show that the cultured bacterium causes the disease when put in a healthy subject (usually an animal)
  4. Isolate the bacterium from the experimentally infected subject

Using these techniques, Australian microbiologist Barry Marshall was able to prove that Helicobacter pylori was the cause of gastric ulcers.

16.12 Bacteria can be used as biological weapons

Any time a bacteria is used as a weapon, it is considered a biological weapon.

B. anthracis was mailed as a powder to members of the US Senate and news media.

Yersinis pestis (plague bacteria) – bodies of plague sufferers were hurled by the army at their enemies.