Bacteria is a large domain of prokaryotic microorganisms. They are only a few micrometers in length, come in a variety of shapes, and inhabit almost every ecosystem on Earth. They were among the first organisms and developed 3.8bya. While some are beneficial to animals (including humans), others can be harmful and cause diseases. Well known examples of bacteria include E. coli, Salmonella, and Staphylococcus.
They are distinguished from other domains by being unicellular, lacking a true nucleus, and the presence of peptidoglycan in their cell walls. Archaea are often mistaken as bacteria, but they lack peptidoglycan and host a distinct RNA structure. While archaea have 3 different types of RNA (making it more similar to eukaryotes), bacteria only hold one type.