Bacteria

A domain consisting of prokaryotic organisms. Bacterial characteristics include the ability to adhere to and invade host cells, being toxic and being able to avoid the hosts immune system. Bacteria that cause disease are pathogens.

Bacteria were one of the first life forms on earth, and even though they are microscopically small, of only a few micrometres in length, they are very good at adapting to their environment around them, leading them to inhabit most habitats on Earth. There are many different names for bacteria depending on where they inhabit, such as Physcrophile, Mesophile, Thermophile, Hyperthermophile. 

Bacteria can also be classified on whether they are gram positive or gram negative, which is dictated by their cell wall composition. This can be tested by performing a gram strain procedure.