Domain: Bacteria
The domain bacteria is characterized by prokaryotic, single celled organisms. Because these cells are prokaryotic, they lack a true nucleus and instead have a nucleoid region which contains free floating DNA. These cells undergo rapid growth and cell division which can cause them to quickly adapt to their environment. Bacteria include many human pathogens including anthrax (Bacillus anthracis), syphilis (Treponema pallidum), Streptococcus, and botulism (Clostridium botulinum).

Kingdom: Eubacteria
The kingdom Eubacteria are known as the “true bacteria”. Many antibiotics such as streptomycin and tetracycline are derived from eubacteria. Salmonella (Salmonella enteritidis) is also classified with the Eubacteria. As the above image on the left shows, all other organisms share a common ancestor with the Eubacteria.
Phylum: Proteobacteria
These bacteria are gram-negative. This means that the outer membrane is composed primarily of lipopolysaccharides and underneath is a thin layer of peptidoglycan. Because of this, the gram-negative bacteria are resistant to many antibiotics including penicillin because they cannot get through the outer membrane. On a gram-stain, these bacteria will appear pink-red because the peptidoglycan does not absorb the crystal violet dye as it does in gram-positive bacteria. E. coli and Neisseria Meningitidis are other examples of proteobacteria.
Class: Gammaproteobacteria
This class of organisms have a wide range of aerobicity, trophism (chemoautotrophism and photoautotrophism), and temperature tolerance. These diverse habitats allow this class of organisms to exploit a large range of environments, including those without oxygen for cellular respiration. It also includes different morphologies of bacteria including rods, curved rods, cocci, spirilla, and filaments. Many pathogens are included in this class including the bacteria that caused the Bubonic plague, Yersinia pestis, Vibrio parahaemolyticus, as well as the bacteria responsible for Legionnaire’s Disease (Legionella pneumophila).
Order: Aeromonadales
This grouping consists of anaerobic, rod-shaped bacteria.
Family: Aeromonadaceae
This family of bacteria do not form endospores or microcysts and thrive primarily in aquatic inhabitants. Many species are pathogens to humans, fish, and frogs. This family was recently proposed in 1986 when it was found that it shared significant evolutionarily differences from the Vibrionaceae family and that the Aeromonadaceae are more closely related to the Enterobacteriaceae than Vibrionaceae. The phylogenetic tree on the right represents the evolutionary relationships based on DNA-rRNA competition data, further showing that the genus Aeromonas is more closely related to the Enterobacteriaceae family than the Vibrionaceae family. This image was redrawn from the article "Proposal to Recognize the Family Aeromonadaceae" by Colwell, R., et al. 1986.
Genus: Aeromonas                                                         
This genus is characterized by their aquatic environments. A polar flagellum allows these bacteria to be motile in these environments throughout the world including in ground and drinking water, water distribution systems, lakes, rivers, and storage reservoirs.

   Species: Aeromonas hydrophila
Aeromonas hydrophila. Leifson flagella stain (digitally colorized).This motile species can be free-living or parasitic. It can survive in a wide range of aquatic environments and it can also survive as a pathogen in an invertebrate or vertebrate host. This pathogen was found in the floodwater following hurricane Katrina and was also the most common cause of skin and soft tissue infections following the 2004 Thailand tsunami.

To learn more about its diverse habitats, click here.

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