Bacteria have three methods for horizontally transferring DNA: , and .

Plasmids can replicate independently of the . They are transferred to neighbouring cells by structures on the of bacteria.

Some bacteria can directly take up free DNA from their . This is known as natural transformation. Environmental DNA often comes from other bacteria splitting open and their contents.

are viruses that only infect bacteria. Phage can integrate their genetic material with the bacterial chromosome during their life cycle. Sometimes, when phage package their genetic material into coat proteins, they can accidentally package DNA as well. When these phage infect a new cell, DNA from the previous host is horizontally .