These are true bacteria. They are found absolutely everywhere. Although some are pathogens, many are essential to life and are also important in the recycling of nutrients.

Classification of bacteria based on shape

Shapes of Bacteria

In 1872, Ferdinand Cohn, considered to be the father of modern bacteriology, classified bacteria into 4 types based on their shapes as follows: sphaerobacteria (round), microbacteria (short rods or cylinders), desmobacteria  (longer rods or threads), spirobacteria (screw or spiral). Today these shapes are called cocci (spherical), bacilli (rod), vibrios (comma), spirilla (spiral),  and spirochaetes (corkscrew). There are also actinomycetes (branching filamentous) and mycoplasma (without a stable shape)

As well as shape, modern methods of identification and classification include:

• Nature of cell wall – differentiated by colour on Gram staining into Gram positive (red) and Gram negative (purple)
• Mode of nutrition –
  • heterotrophs (get their energy from organic carbon) 
  • autotrophs (produce their own)
    • photoautotrophs (via photosynthesis)
    • chemoautotrophs (chemosynthesis )  
• Temperature requirement –
  • psychrophiles – can grow at 0 °C or below but prefer 15 °C, and cannot grow above 20 °C
  • facultative psychrophiles  – can grow at 0 °C but prefer 20-30 °C
  • mesophiles – grow best between 25-40 °C but optimum is 37 °C (ie body temperature) and include most of the human pathogens (eg  Escherichia coli, Listeria, Pesudomonas, Staphylococcus, Streptococcus, and Clostridium species).  The human intestinal flora, or gut microbiome, contains beneficial mesophilic bacteria (eg. dietary Lactobacillus acidophilus).
  • thermophiles – can grow above 40 °C
  • hyperthermophiles –  prefer temperatures above 80 °C
• Oxygen requirement –
  • aerobes (need oxygen) Some types can cause problems for the human environment, such as corrosion, fouling, problems with water clarity, and bad smells. 
  • anaerobes (cannot grow in the presence of oxygen)  eg in the GI tract They can also cause gas, gangrene, tetanus, botulism, and most dental infections. 
  • facultative anaerobes (can live with oxygen but prefer no oxygen). They are mostly found in soil, water, vegetation and some normal flora of humans and animals. Examples include Salmonella.
• pH of growth medium –
  • acidophiles  – grow best at an acidic pH
  • alkaliphiles  – prefer alkaline pH
  • neutrophies  – grow best at neutral pH (6.5-7.5) (incudes most bacteria)

Other identifiers are reproductive methods, number of flagella, salt concentrations, genetic (methods of gene transfer, recombination), molecular (proteins and nucleic acids) , serological (antigens), and biochemical (test reactions).

Below are some of the major bacterial groups (phyla):

• Actinobacteria – An important group of filamentous bacteria, widely distributed in both terrestrial and aquatic ecosystems. In soil they play an essential role in recycling of nutrients, growing extensive mycelia like fungi. Some are nitrogen-fixing living in a symbiotic relationship with plants. A number of antibiotics, and other important compounds, have been discovered from marine actinobacteria, especially from the genus Streptomyces (eg actinomycin). Some are pathogens causing diseases like diphtheria, tuberculosis, and leprosy. (Gram+)
• Chlamydiae – Parasitic bacteria reproducing within host’s cells and include Chlamydia trachomatis (causes chlamydia STD) and Chlamydophila pneumoniae (cause pneumonia). (Gram-)
• Cyanobacteria – Although commonly called ‘blue-green algae’, due to their chlorophyll giving them their characteristic colour and allowing them to get their energy from photosynthesis, these are bacteria. They are thought to have played an important role in increasing the oxygen levels of our planet. They are found in a variety of environments, including thermal springs (eg the Grand Prismatic Spring, Yellowstone) and can form large visible colonies. Many are nitrogen fixing and a few are symbiotic (eg with fungi to form lichen). Several inland water species are used as food sources, animal feeds, fertilizers, and health products. (Gram-)
• Firmicutes – Second largest bacterial phylum and include Clostridia, Bacilli, Erysipelotrichia (gut), Thermolithobacteria (thermophils) and mycoplasmas (bacteria without a cell wall). One of the major gut bacteria with a high ratio possibly associated with obesity. (Most Gram+, a few Gram-)
• Proteobacteria – Largest, and most diverse, bacterial group, which include wide variety of pathogens (eg like Escherichia coli, Salmonella, Heliobacter pylori and Legionellales) as well as nitrogen fixing and oxidative bacteria. (Gram-)
• Spirochaetae – These are mostly long, corkscrew-shaped, free-living, anaerobic bacteria with diverse characteristics occupying diverse ecological niches. Many are serious human pathogens like Borrelia burgdorferi (cause Lyme disease) and Treponema pallidum (cause syphilis) to name a couple. (Gram-)

The the largest bacteria are Thiomargarita namibiensis and Epulopiscium fishelsoni. Being 600 micrometers in length and 75 micrometers in diameter they are easily visible to eyes.

Extremophiles

Extremophiles can withstand conditions considered too extreme for most life forms.

• thermophiles (temperature in the range 75 to 80°C),
• hyperthermophiles (temperatures up to 113°C)
• halophiles, found only in a salty environment
• acidophiles, some of which live in environments as acidic as pH 0
• alkaliphiles, living in alkiline environments up to pH 10.5
• psychrophiles, found in cold temperatures, for example, in glaciers

The hot springs of Yellowstone National Park have many extremophiles.

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The rainbow colours of the Grand Prismatic Spring is caused by thermophilic bacteria. Water from hot springs flows unobstructed, creating a nonstop cycle of hot water rising, cooling and falling. This constant cycle creates rings of distinct temperatures around the center: very, very hot water bubbles up from the middle and gradually cools as it spreads out across the spring’s 

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Table of thermophilic bacteria found in Yellowstone National Park¹

Name	Temperature	Description	Location
Cyanobacteria Calothrix	30–45°C 86–113°F	Colour: dark brown mats Metabolism: photosynthesis by day; fermentation by night	Mammoth Hot Springs Upper, Midway, and Lower Geyser Basins
Phormidium	35–57°C 95–135°F	Color: orange mats Metabolism: photosynthesis	Mammoth Hot Springs Upper, Midway, and Lower Geyser Basins
Oscillatoria	36–45°C 96–113°F	Color: orange mats Metabolism: photosynthesis; oscillating moves it closer to light sources.	Mammoth Hot Springs Chocolate Pots
Synechococcus	52–74°C 126–165°F	Color: green mats Metabolism: photosynthesis by day; fermentation by night	Mammoth Hot Springs Upper, Midway, and Lower Geyser Basins
Green Sulfur Chlorobium	32–52°C 90–126°F	Color: dense, dark green mats Metabolism: anaerobic photosynthesis— produces sulfate and sulfur, not oxygen.	Mammoth Hot springs Calcite Springs
Green non-sulfur Chloroflexus	35–85°C 95–185°F	Color: green mats Metabolism: anaerobic photosynthesis	Mammoth Hot Springs Upper, Midway, and Lower Geyser Basins
Aquifex Hydrogenobaculum	55–72°C 131–162°F	Color: yellow and white streamers Metabolism: uses hydrogen, hydrogen sulfide and carbon dioxide as energy sources; can use arsenic in place of hydrogen sulfide.	Norris Geyser Basin Amphitheater Springs
Deinococcus-Thermus Thermus	40–79°C 104–174°F	Color: bright red or orange streamers Contains carotenoid pigments that act as sunscreen.	Lower Geyser Basin

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References

1. Thermophilic Communities – Yellowstone National Park Services