Question: What is the most common bacteria ?

Samuel Ellis answered on 9 Mar 2015:

Bacteria are often adapted to survive in different environments, so some are more common in certain places than others.

For example, in our gut we have lots of ‘friendly’ bacteria which help us break down food and fight off bad bacteria. One of the major types in the gut are called Bacteroides. Although usually helpful, like a lot of bacteria they can sometimes make you sick if they get somewhere they shouldn’t be, such as after an injury or surgery.

As for the most common bacteria on the planet? Some scientists think that might be the Pelagibacter family. These are really tiny bacteria (around 0.5 microns long, which is 0.00005 cm) which live in the oceans, and may make up almost half of all cells in the ocean during the summer.

There are an awful lot of bacteria out there though, many of which we haven’t found yet or know nothing about! We estimate that there are around 5 x10^30 bacteria on Earth, which is 5 followed by 30 zeros!