Bacteria .

Bacteria cells are single-celled organisms – they can live by themselves. This image is of a Campylobacter bacteria – a common cause of diarrhoea and dysentery.

Bacteria are situated on extreme ends when it comes to numbers. They are really tiny – measured in micrometres (between 10-6 m and 10-7 m), but are really abundant – there are approximately 5 nonillion (5x1030) bacteria on Earth, with around 40 million bacteria in a single gram of soil!

Image: Public domain – U.S. Government