Abstract Bacteriophage (phage) are viruses that specifically infect bacteria. Phage come in a large variety of sizes and shapes. They are globally classified in terms of morphology and genome type. A phage particle is composed of a single type of nucleic acid (either DNA or RNA) and a protein capsid that protects the genetic material. The vast majority of phage also possess a tail (made of proteins) that enables the specific recognition of a receptor at the surface of the host bacterium. Phage are now recognized as the most abundant biological entities on our planet and they play major roles in the ecological balance of the microbial life.