Eubacteria is a huge, diverse kingdom of prokaryotic microbes that includes chlamydia, E. coli and cyanobacteria. Eubacterial organisms are used to make cheese and wine. They can cause serious infections in humans and sometimes, as in the case of human gut flora, they change quickly from helpful to harmful.
Eubacteria are all prokaryotes. This means that members of this kingdom, which is as large and diverse as the plant and animal kingdoms, lack a cell nucleus and generate energy without the symbiotic relationships that define eukaryotes.
Cyanobacteria, a type of eubacteria, are among the earliest life forms to have appeared in the fossil record. Colonial stromatophores were building large mounds, called stromatolites, on Earth 3 1/2 billion years ago. These blue-green algae, as they are often called, were the first major producers of oxygen on Earth, and they remain central to maintaining the equilibrium of Earth's atmosphere. Some cyanobacteria fix nitrogen directly from the air, enriching soil and permitting plants to thrive.
Harmful eubacteria exist as parasites. Chlamydia and the spirochetes responsible for syphilis are two examples of destructive eubacteria. E. coli is arguably another harmful bacillus. This species normally resides in the gut, where it aids digestion, but it can become dangerous in certain cases. Vancomyicin-resistant E. coli is a common infection found in hospital wards.