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Introduction to Organisms

Author(s): Deanne Erdmann, MS

The Kingdom Monera - Eubacteria

Bacteria are the most numerous and ancient life forms found on Earth. They can live in places normally found inhospitable to other organisms (too cold, too dark, too hot, etc.). Bacteria are unicellular organisms that do not contain a nucleus or internal compartments, and their genome does not contain introns.Most species of bacteria can be assigned to two groups, based on the amount of peptidoglycan found in their cell walls. Bacteria with a thick layer of peptidoglycan in their cell walls are called "gram-positive" because they retain a blue color after staining (following a technique developed by Christian Gram.) Bacteria with a thin layer of peptidoglycan sandwiched between other layers stain orange-red following the same procedure and are called "gram-negative." The three most common shapes of bacteria are spherical (cocci), rod (bacilli), and helices (spirilla). 

The number of ways that bacteria can obtain nutrition and respire contributes to their ability to inhabit so many diverse places on Earth. To obtain energy and carbon, bacteria can be photoautotrophic—harness light energy to drive metabolic processes and use CO2 as a carbon source, while others are chemoautotrophic—oxidize inorganic substances for energy and use CO2 as a carbon source, photoheterotrophic—use light to generate energy but obtain carbon from other organic molecules, or chemoheterotrophic—consume organic molecules for both energy and carbon. The chemoheterotrophs include saprobes, decomposers that absorb their nutrients from the body fluids of living hosts. Bacteria also form many diverse symbiotic relationships with other organisms.

Bacteria exhibit wide variation in their use of oxygen and can be classified based on their dependence upon it. Obligate aerobes must have oxygen for cellular respiration; facultative anaerobes use oxygen if it is present, but also can grow by fermentation in an anaerobic environment. Obligate anaerobes can not tolerate oxygen at any level. 

Bacterial reproduction normally occurs asexually by binary fission. Bacteria do have the ability to transfer genes or segments of genes, and they do so using three mechanisms: conjugation, transformation and transduction.  Conjugation involves the direct transfer of genetic material between prokaryotes. In transformation, the cells absorb fragments of DNA from the surrounding environment (even from other species). Transduction occurs when bacterial viruses play a role in transferring genetic material between prokaryotes. 

These abilities, along with a rapid reproductive rate, leaves little surprise as to why bacteria are "masters" of change and adaptation.