![Campylobacter, Bacillus subtilis and Helicobacter bacteria.](http://static.sciencelearn.org.nz/images/images/000/000/168/full/Bacteria-cells20150907-25061-18skvka.jpg?1674163834)
Bacteria cells are single-celled organisms – they can live by themselves.
From left to right: Campylobacter bacteria; the Bacillus subtilis bacterium found in soil and the Helicobacter bacteria that is known to cause stomach ulcers.
Helicobacter bacteria courtesy of David Gregory & Debbie Marshall, Wellcome Images