Domain Bacteria
Gamma Proteobacteria

Escherichia coli

The Gamma order is the largest and most diverse group of the Proteobacteria.

They include the well-studied, scientifically important and usually harmless gut bacteria E. coli.

But they also include an exceedingly large number of dangerous pathogens that cause bubonic plague, cholera, salmonella, tularemia, and urinary tract and ear infections. Many cause secondary infections in ill or hospitalized patients. The Gamma Proteobacteria include:

  • Enterobacteriales, a very large group, found in water, soil or the intestines of humans and animals. The pathogenic genera include:
    • Escherichia coli (occasionally pathogenic)
    • Salmonella (food poisoning & typhoid fever)
    • Shigella (bacterial dysentery)
    • Yersinia (bubonic plague, which in the 14th century killed over one-fourth of the population from China to Europe.)
    • Serratia cause respiratory urinary & eye infections and when flushed out to sea kill coral reefs
    • Vibrionales cause cholera.
    • Legionellales cause Legionaires' disease.
    • Pasteurellales cause genital sores and ear infections.
    • Chromatiales, or purple sulfur bacteria, consume hydrogen sulfide.
    • Methylococcales consume methane.
    • Pseudomonadales spoil food, especially dairy products.
    • Thiotrichales, the biggest bacteria in size, cause tularemia.


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