Horizontal gene transfer is the lateral movement of genetic material between unicellular and/or multicellular organisms, which is distinct from the vertical transmission of DNA from parent to offspring. There are three main types of horizontal gene transfer:

• Transformation: Uptake and incorporation of exogenous DNA into bacterium from its surroundings through the cell membrane.
• Conjugation: Transfer of genetic material between two bacterial cells in direct cell-to-cell contact.
• Transduction: Injection of foreign DNA into the host bacterium by a bacteriophage virus.