Molecule of the Month: Therapeutic Phage

Bacteria are among the most successful forms of life on Earth, inhabiting nearly every ecological niche imaginable. They can be found in vast numbers in oceans, deserts, and even in clouds. Over 30 trillion bacteria call your body home. Despite this staggering abundance, bacteria are themselves outnumbered by something even smaller: bacterial viruses known as bacteriophage or phage. For every bacterial species, there are often numerous phage species adapted to infect it.

Phages infect bacteria by attaching to molecules on the bacterial surface and delivering their genetic material into the cell. Once inside, they hijack the bacterial machinery to produce new viral particles. Eventually, the infected cell bursts open, releasing numerous new phages capable of infecting neighboring bacteria. However, some phages are able to integrate quietly into the bacterial genome and remain dormant until activated by some external stimulus.

Phages are classified based on both their morphology and the type of genetic material that makes up their genome. The overwhelming majority, estimated to represent more than 95% of known phages, are tailed double-stranded DNA phages. These phages resemble microscopic lunar landers, with a large head containing genetic material attached to a tail used for recognizing and infecting bacterial cells. Tailed phages are divided into three major morphological groups. Siphophages have long, flexible, non-contractile tails. Myophages have contractile tails that function almost like molecular syringes, driving genetic material into the host bacterium. Podophages are distinguished by their short tails. Examples of each of these phages are illustrated in this article. E217 is a myophage that infects the bacterium Pseudomonas aeruginosa, and is shown on the right. The siphophage JBD30, which also infects Pseudomonas aeruginosa, and the podophage Andhra, which infects Staphylococcus epidermidis, are shown below.

The rise of multidrug-resistant bacteria has become one of the most serious public health threats of the twenty-first century. Antibiotics, once considered miracle drugs, are losing effectiveness as bacteria evolve resistance mechanisms faster than new drugs can be discovered. Globally, antibiotic-resistant infections contribute to millions of deaths each year.

As conventional antibiotics become less reliable, researchers have intensified efforts to develop alternative approaches to treating bacterial infections, including the use of bacteriophage to selectively kill pathogenic bacteria. Although phage therapy may seem like a new innovation, commercial phage treatments were widely available globally from the 1920s through the 1940s. Enthusiasm for the use of therapeutic phage waned, however, as results proved inconclusive and penicillin and other antibiotics were widely adopted.

Interest in phage therapy has resurged in recent years, however, particularly following the success of compassionate-use cases involving patients with life-threatening antibiotic-resistant infections. There are currently dozens of active or planned clinical trials investigating phage therapies worldwide.

Despite its promise, phage therapy faces several significant challenges that have slowed widespread clinical adoption. Unlike broad-spectrum antibiotics, which may kill many bacterial species simultaneously, most phages are highly specific. A single phage may infect only one bacterial species, or even only certain strains within that species. This specificity can be advantageous because it allows harmful bacteria to be targeted without disrupting beneficial microbes. However, it also means that an effective treatment must closely match the patient’s infection.

Phages identify their bacterial hosts through interactions between receptor-binding proteins on the phage tail and specific molecules on the bacterial surface. For some phages, scientists have identified the exact bacterial receptors involved. For most others, however, the target molecules remain unknown and difficult to predict. As a result, successful treatments often require carefully designed phage cocktails: mixtures of multiple phages selected specifically to attack the bacteria causing a patient’s infection. These cocktails reduce the likelihood that bacteria will rapidly evolve resistance.

Scientists are actively researching ways to make phage therapies more effective. This includes efforts to engineer phages to expand their host range and aid in bacterial defense system evasion. Other studies are exploring how phage therapy can complement traditional antibiotics in the treatment of bacterial infections.