Characteristics of Taxonomic Groups with Soil-inhabiting Forms
The most numerous and widespread organisms on earth. They are prokaryotic and
lack a nucleus or other membrane-bounded organelles. The cell wall, outside the
plasma membrane, is partially composed of peptidoglycan, a complex
structural molecule not found in eukaryotic cells.
The most ancient of all organisms. Three groups:
- Thermoacidophiles - live in acidic waters of hot springs.
- Methanogens - obligate anaerobes; oxidize CO2 during cellular
respiration to produce methane (CH4). Inhabit mammalian digestive
tracts and volcanic deep-sea vents.
- Halophiles - saline environments including the Dead Sea and salt lakes.
Often pink in large concentrations.
Separated by reaction to Gram Stain:
Most eubacteria (about 75%) are gram negative.
They include the gliding bacteria, the spirochetes, the vibrios and spiral
bacteria, rods, cocci, rickettsias, chlamydias and the photosynthetic
- Phototrophic Bacteria: fix carbon using sunlight as an energy source
and sulfide as an electron donor.
- Gliding Bacteria: motile but no flagellae.
- Budding Bacteria: reproduce by budding.
- Aerobic Rods and Cocci: include Agrobacterium and N-fixers such as
Rhizobium and Azotobacter.
- Facultatively Anaerobic Rods: produce acid under anaerobic conditions.
- Anaerobic Rods: inhabit body cavities of vertebrates.
- Cocci: Mainly parasites.
Include rods, cocci, and the
- Chemilithotrophic Bacteria: oxidize reduced inorganic substances to obtain
energy to fix carbon. Include nitrifying bacteria Nitrosamonas
- Methane-producing Bacteria: anaerobic, inhabit anaerobic mud; oxidize
hydrogen or formate for energy to reduce CO2 to methane.
- Cocci: clumped forms, mainly aerobic.
- Endospore-forming bacteria: often resistant to heat.
- Rod-shaped Bacteria: fermentation of animal and plant products.
- Coryneform Bacteria: morphologically similar but probably not related,
some attack cellulose.
A diverse group of bacteria with filamentous growth. Classified by
mycelium pattern and substrate. Streptomyces is a well-known
example. Include gram negative and gram positive forms.
Actinomycetes are abundant in soils that are rich in organic matter and are
the source of the characteristic smell of organic soils.
Smallest known cells with
minimum DNA needed to code for a functioning cell. They lack cell walls and most
exist as intracellular parasites of plants or animals.
Coiled actinomycete-like bacterial cells
Dindal, D. 1990. Soil Biology Guide. John Wiley and Sons, NY.
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