Bacteriophages are viruses that infect bacteria, and in the great war between humans and pathogenic bacteria they can act as allies for both sides. Phages that destroy their host bacteria can be used as antimicrobial therapy, complementing or replacing antibiotics. On the other hand as phages are essentially little capsules that carry DNA from one bacteria to another, they can spread the genes that make bacteria resistant to antibiotics.
The image below shows a stylised drawing of a bacteriophage. The DNA is the swirl contained in the icosahedral head at the top. Bacteriophages work in different ways; some enter their bacterial host and incorporate their genome into the bacterial DNA, happy to settle down and replicate with the host. Others multiply inside the bacteria to create new phage genomes, which then burst out of the host and spread. Other phages incorporate both strategies at different stages in their lifecycle – replicating with the bacteria when times are good, and spreading and destroying the host at any sign of stress.
The viruses that spread antibiotic resistance
Source: Lab Rat, Scientific American