Useful Reading
Campbell, Biology 6th Ed - Chapter 27, pgs 526-544
Campbell, Biology 7th Ed - Chapter 27, pgs 534-548
Traditional classification of the diversity of life organizes organisms into five kingdoms (Monera, Protista, Plantae, Fungi and Animalia). Monera is the only "kingdom" containing prokaryotic organisms. Current classification uses 3 domains of life. All prokaryotes (Kingdom Monera) are now divided into 2 domains – the Archaebacteria and the Eubacteria. Common “bacteria” are in domain Eubacteria. All eukaryotes (remaining four Kingdoms) are grouped into the third domain – Eukarya.
Prokaryotic cells are characterized by having no membrane bound organelles. This means they have no nucleus or any other specialized sections of the cell. The primary genetic material is on a single, circular molecule of DNA. In addition, many small rings of DNA, called plasmids, can be found inside a bacterial cell. Due to the lack of membrane-enclosed organelles, these cells are limited by the rate of diffusion across the cell to obtain nutrients and other necessities. As a result, these organisms are single cells and very small.
Click for more on structure of bacteria.
Prokaryotic cells can be compared to more advanced cells, the eukaryotic cells, which have membrane bound organelles. These organelles are specialized areas within the cell that are partitioned via sections of cell membrane. The specialization allows for movement other than simple diffusion across the cell, further allowing for an increase in cell size, and ultimately to the specialization of groups of cells (tissues), giving rise to multicellular creatures.
Although bacteria are unicellular, some species regularly aggregate into groups or colonies. In addition, they contain diverse cell shapes, chemical properties, metabolic pathways, and ecological associations. Many bacteria are important or necessary for the functioning of multicellular organisms and ecosystems.
Click for more on physiological and ecological diversity of bacteria.
Images courtesy of Molecular Expressions
Click here for a brief time line of the history of earth.
Classification
Bacteria are generally described and classified by colony morphology and the shape of the cell. Colonies of bacteria can have varying shape, margins, and surface characteristics, and are commonly round with smooth margins and surfaces. Bacterial cells come in three common shapes (shown below) – the cell shape is the first way of classifying bacteria.
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Bacilli (rod shaped) |
Cocci (ball shaped) |
Spirilli (corkscrew shaped) |
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Gram staining is another method of classifying bacteria, based on the structure of the cell wall. Bacterial cell walls are composed of peptidoglycan molecules arranged in a grid-like structure around the cell. A gram positive cell will appear purple because the cell wall is composed only of these peptidoglycans. A gram negative cell will appear pink because in addition to the peptidoglycans, there is a lipid bilayer coating the grid, preventing the gram stain from sticking to the cell. These differences in cell wall define major taxonomic groups.
Ecology and Control
Because of the diverse ecology and chemistry of bacteria, species differ in what type of habitat they can or will be found in. Thus, a bacterial habitat like a glass of milk will change in its species composition over time as the milk spoils (community succession). Similarly, different bacterial (and fungal) communities will occupy habitats such as chicken, soil, and human hands.
Antibiotics are substances that prevent the proliferation of bacteria. They tend to be effective against particular bacteria depending on cell wall structure. We observed plates of bacteria treated with various antibiotics to see if the bacteria were sensitive to the antibiotic. Sensitivity of bacteria to an antibiotic can be assessed by measuring the zone of inhibition around the antibiotic disk. Antiseptics and disinfectants are often also used to control the growth of bacteria by reducing the number of bacteria present.
Symbiosis
is defined as two or more organisms living together. It is common that one of
the organisms lives in or on the other. Mutualism is where both organisms
benefit from this association. Commensalism is when one organism benefits and
the other is neither helped nor harmed. Parasitism is where one benefits by
harming the other.
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Symbiotic Type |
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Mutualism |
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Commensalism |
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Parasitism |
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One example of symbiosis is that between legume plants and bacteria of the genus Rhizobium. Rhizobium are nitrogen fixing bacteria. They are important in terms of ecology and agriculture by providing plants with organic nitrogen (usable nitrogen) from the atmospheric nitrogen the bacterium took in. The bacteria induce the formation of and live in nodules on the roots of the legumes.
Cyanobacteria are photosynthetic prokaryotes. We looked at Oscillatoria and Anabaena.
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Anabaena |
http://www.bacteriamuseum.org/
Review Questions
-Define prokaryote, specifically how it differs from a eukaryote.
-Define symbiosis.
-Describe the symbiotic relationship that Rhizobium is usually found in: with what type of organism does it interact, what if the nature of the interaction, and what are the ecological implications?
-How can knowledge of the Rhizobium symbiosis inform agricultural efforts?
-What are the three cell shapes of bacteria?
-What feature of bacteria makes them “gram-positive” or “gram-negative”?
-Is gram-stain response a good trait to use for classification of bacteria?
-Define antibiotic. Where do antibiotics come from?
-Would you expect the bacterial community in air around a farm to differ from the community in city air?
-What would happen to the community composition and bacterial density on an agar plate If a single antibiotic were repeatedly applied?
