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 above 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.
As many antibiotics are naturally found in the environment, antibiotic resistant genes are also present and can be carried around within phages. Furthermore, natural bacterial habitats in the soil can be exposed to medical antibiotics through agricultural run-off and animal feeding operations. These antibiotics will be diluted as they move through the natural environment making it far more likely that the bacteria they encounter will start selecting for, and spreading around, resistance genes. Phages containing antibiotic resistance genes have been found in samples from urban sewage and river water as well as drainage from hospitals. Phages get everywhere.
Further exploring antibiotic resistance in the natural world revealed some interesting patterns. Comparing samples from oceans, soils, freshwater, human faeces and waste-water treatment plants showed a range of phage genomes found in all environments. Resistance to B-lactam antibiotics (including penicillin derivatives) was highest in soil bacteria and also found fairly frequently in waste-water treatment plants. In contrast resistance to the antibiotic tetracycline was found highly in human faeces with very little in any other environment.
Antibiotic resistance is most usually studied in clinical settings but it is important to appreciate that it is found across environmental habitats. Many antibiotics already exist in the environment as they are used by bacteria to defend themselves, and others leach into the natural world via farms and hospitals. Understanding the sources of antibiotic resistance, and how it spreads through bacterial populations, is critical for developing effective strategies to combat resistance and minimise its affect on public health.
Reference: Balcazar JL (2014) Bacteriophages as Vehicles for Antibiotic Resistance Genes in the Environment. PLoS Pathog 10(7): e1004219. doi:10.1371/journal.ppat.1004219