Type of cellular organisms lacking nuclei.

The bacteria are one domain of prokaryotic organisms, i.e., domain Bacteria, though the term more loosely has been used to describe prokaryotic organisms in general.

All prokaryotic pathogens are members of domain Bacteria and bacteria in combination with archaea otherwise are the most numerous cellular organisms on , with an estimated more than 10³⁰ cells in total.

The human body has approximately 100 trillion bacteria associated with it, outnumbering human body cells by about ten to one. See the concepts of normal flora, normal microbiota, and microbiome, including .

Bacteria play key and crucial ecological roles as decomposers, nutrient "fixers", and nutrient mineralizers. Bacteria play essential roles in biotechnology and a number of familiar are products of bacterial fermentation such as .

The word prokaryote is often used as a synonym for bacteria. There is a movement afoot, however, to consign the latter term to the history books. The use of "bacteria" in this context technically is a little bit inaccurate. That is, strictly, one can speak of members of domain Bacteria and domain Archaea, where both are collections of prokaryotic organisms and, used loosely, the word "bacteria" can be used to describe either. In technical writing, however, the use of "bacteria" as a synonym for prokaryote should be avoided, hence the utility of the term prokaryote!

Video (Upbeat introduction to bacteria; watch out, though, with the definition of endotoxin)