Bacteriophage
Phage genomes may code for as few as four genes, and as many as hundreds of genes. Phages have a life cycle that begins when the phage attaches to the bacterium. Then they inject their genome into the bacterium. The genome uses the parts of the bacterium to replicate inside it. When there are many phages inside the bacterium, they put enzymes in the bacterium that weaken the outer cell wall so they can burst through it to infect new bacterias.