A recent HBO series named The Last of Us received widespread acclaim among viewers. The show background describes the post-apocalyptic world after a fungus pathogen Cordyceps infects humans and controls their minds to assist the fungus’s reproduction. The same kind of fungus exists in the real world. Thankfully, it does not infect humans. Instead, Cordyceps use spores that attach and penetrate insect exoskeletons to infect ants. Once inside, the fungus begins to grow and expand across the ant’s inner soft tissues while avoiding vital organs to keep the ant alive as long as possible. As the fungus matures, it begins to release chemicals to change the behavior of the host ant. Instead of returning to the ant nest like a regular ant, the fungus chemically manipulates the ant to climb to a high location, often a plant stem or tree trunk, where wind flow is ideal for fungus spores to spread. By this time, the fungus controls the entire ant body, including the vital organs. Finally, the fruiting body of the fungus bursts from the ant’s head or body, releasing spores into the air to infect more ants below and killing the original host ant in the process.
In a disturbingly similar way, the same relationship existed for 3.8 billion years between phages and bacteria. Phages are viruses that infect bacteria. According to recent studies in this decade, phages existed before bacteria, descending from the lineage of primordial RNA. A phage consists of a geometrical twenty-sided “head” that encloses the genetic material. A coiled “sheath” extends from the capsid head and ends on a base plate. Six sharp, appendaged tails are attached to the base plate forming a shape resembling a spider or an octopus. When a phage encounters a target bacterium, it attaches itself to the bacteria via its tails. An injection needle protrudes from the sheath, acting like a syringe to force the phage genetic material through the bacterial cell membrane. Once inside, the phage genetic material pastes itself into the bacteria’s DNA. As bacteria transcribes and translates its compromised DNA, it produces a cocktail of pro-phage proteins that gradually take over the bacterial cellular machinery, directing the bacterial cell to produce more and more phages. Ultimately, the bacterial cell becomes filled with new phages. At this point, the pro-phage segments of genetic material direct the cell to release lysozymes that break down the cell wall, bursting the cell body and releasing the phages to infect more bacterial cells nearby.
Phage was first discovered in 1917 by the French-Canadian microbiologist Félix d’Hérelle by examining the filtrates of excrements of dysentery patients. He noticed some entity in the filtrates was killing the dysentery bacteria, and he hypothesized that something in this filtrate was eating the bacteria. He named the hypothetical agent “bacteriophage,” meaning bacteria eater. D’Hérelle produced further works on how the agents can be isolated to use as effective bacteria killers. This discovery was revolutionary during the era when small cuts or unsanitary food may lead to death due to bacterial infection. However, d’Hérelle’s works were soon forgotten when a traitor arose from the bacterial world. Antibiotics like penicillin were discovered by doctors. These agents were bacteria that specialize in killing other bacteria. Due to its efficiency, antibiotics became the main method of antibacterial operations in the last century. The proliferation of antibiotics led to the emergence of antibiotic-resistant bacteria, commonly known as superbugs. These bacteria have developed the ability to withstand the effects of the human arsenal of antibiotics, making infections caused by them difficult, and sometimes impossible, to treat. Superbugs were once predicted to be one of the most plausible potential agents that would end humanity. In the face of this threat, the wider medical world was reminded of a bacteria killer that had been fighting bacteria for 3.8 billion years, a war that lasted longer than human existence. Phages are not the ultimate cure, bacteria will also develop phage resistance. However, when a culture of bacteria evolves to resist phages, it becomes susceptible to antibiotics. Therefore, researchers propose a staged approach of antibiotic and phage cocktails to counter superbugs. This method has proved to be highly effective against a myriad of new bacteria strains. In this century, phages, with their ability to target and kill specific bacteria, offer a promising complement to traditional antibiotics to help researchers, and doctors combat the growing threat of bacteria.
Sources:
Azeredo, Joana, and Ian W. Sutherland. “The use of phages for the removal of infectious biofilms.” Current pharmaceutical biotechnology 9.4 (2008): 261-266.
Clokie, Martha RJ, et al. “Phages in nature.” Bacteriophage 1.1 (2011): 31-45.
Zinder, Norton D. “RNA phages.” Annual Reviews in Microbiology 19.1 (1965): 455-473.
Photo credits:
Koto_Feja/iStock/Getty Images; Jay Watson (Lu); John Soares (Cepko)
Cool. We learned about phages a few months ago in bio.
Note: when the ant climbs up to a high spot, it’s called summiting. Also, other insects and even spiders can get affected by cordyceps.
Astute observation. The reason I gravitated towards describing only its effects on ants is due to the relative lack of good studies on Cordycep’s effects on other anthropod. A friend I know who is familiar with the field also mentioned that there are ongoing debates on whether strains that affect other insects and arachnids are Cordyceps.