By Nishka Mahajan
The technique of bacteriophage or phage therapy, involving the usage of viruses as a remedy for bacterial infections, has existed for about a century now. It was developed based on how phages work: they recognise, bind to, and replicate within bacterial host cells, finally causing cell lysis. However, phage therapy remained a popular practice only in select parts of the world after the invention and extensive use of antibiotics. Antibiotics fundamentally induce selective toxicity on engaging with distinct bacterial cellular targets. The genes for these cellular targets are subject to change due to mutation and gene transfer. These mutations promoting resistance can amplify within populations via natural selection. Although antibiotics are still successful means of treating major bacterial infections, significant exceptions where this therapy can no longer be considered reliable exist. Methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE) and multi-drug-resistant Mycobacterium tuberculosis (MDR-TB) are all examples of resistant bacteria that have generated a search for antibiotic-free alternatives, revisiting and renewing interest in phage therapy.1
To evaluate this concept, it is imperative to understand the basics of phage biology. Phages are simple (composed of DNA/RNA encased in a protein capsid), naturally occurring bacterial parasites that are incapable of reproducing on their own, ultimately making them rely on their respective bacterial host for survival. On infecting a suitable host cell, phages typically bind to specific targets (proteins or sugar moieties) present on the bacterial cell surface. Following binding, the phage then injects its genetic material, transferring it into the host cell. Now, with respect to the virus, the phage opts between one of two lifecycles. In the case of virulent phages, the viral genetic material is immediately replicated within the host cell – resulting in rapid destruction of the host cell via the lytic pathway. In contrast, temperate phages initiate the lysogenic pathway, integrating the viral genetic material into the bacterial genome (leading to the formation of ‘prophage’). The former remains largely repressed; continually replicates itself while it’s incorporated within the host cell. When environmental factors are suitable, lytic proteins are activated and eventually lyse the host cell, releasing produced phages to infect other cells.1-3
Scientific researchers have studied the lifecycle and genomic structure of several temperate phages.4 Their analysis has shown that many encode bacterial virulence factors, or possess the ability to mobilize said factors to previously low-pathogenic strains or species. Although there are speculations about the intimate linkage between temperate phages and bacterial virulence, this property makes them unsuitable for human therapy. Since virulent phages skip the prophage state, virulence factor genes are rarely included or transferred across bacterial cells. Immediately after infecting a host cell, they subvert its metabolism – by degrading the cell’s genome and other macromolecules – to build their own virions. Some phages use host cell pathways to encode their own metabolic machinery, whereas the rest simply hijack pre-existing pathways for viral multiplication.5
A phage’s host range (respective bacterial species/strains that a phage clone effectively infects) is usually narrow, often limited to a single bacterial species or even only certain strains within that species. Whether a phage effectively infects a host cell depends on all the said stages of infection being carried out successfully. Blockage of any of these steps by bacterial cell mechanisms leads to an unsuccessful infection of the bacterial strain.6, 7 Additionally, even if a phage goes on to infect a bacterial species/strain, there are chances of that strain rapidly developing resistance to the respective phage (for instance, the bacteria could modify or downregulate the phage receptor). However, phages themselves are prone to mutation and evolve in correspondence to bacterial adaptation.
The use of bacteriophages for therapeutic purposes necessitates utilizing phages capable of infecting the bacterial strain responsible for clinical pathology. Phage replication is essential to achieve a considerable decrease in bacterial populations, allowing the infection to be either eradicated or controlled by the immune system. Administering phages in large numbers, such that each bacterial cell is lysed by an administered phage, reduces the need for phages to replicate. At the same time, one of the numerous benefits of phage therapy includes the requirement of a very small inoculum to function – depending on its unique replication ability to facilitate a reduction in bacterial numbers, eliminating the infection.1
Considering the problems associated with the phage’s host range, evolution of bacterial resistance along with the experimental and observation-based nature of clinical practice, phage cocktails (composed of different phage clones) can ideally be an option for therapeutic use. Many modern scientific trials involving phage therapy have conducted research associating single bacterial and phage strains. However, when it comes to clinical practice, it can be far more challenging to determine the precise bacterial aetiology of a disease and how responsive it is to a given phage. Here, phage cocktails make it easier by serving as a remedial treatment for multiple bacterial strains; whilst reducing the likelihood of a strain developing resistance to the cocktail if many phages successfully infect the same bacterial isolate.8 This treatment style is similar to existing, currently in practice, methods of using multiple antiretroviral agents that operate via distinct mechanisms for treating HIV (HAART). Another such example involves the usage of multiple antibiotics for treating tuberculosis.1
Furthermore, the action pathway utilized by phages is very different from that of antibiotics. This implies that even though a bacterial strain is resistant to antibiotics, it could be susceptible to infection by phages. While bacterial strains adapt to environmental changes, phages too possess the ability to consequently evolve. Antibiotics, on the other hand, are chemically defined microbial products and are immune to mutation. The issue the scientific world is forced to face is that the pipeline of novel antibiotics is close to drying up, and several new antibiotics developed seemed to have already induced resistance in their target bacteria. These new antibiotics, therefore, play a limited role, reducing their commercial value and slowing down the pipeline all the more.1 Lastly, antibiotics are administered for a temporary period – this significantly decreases the ideal return on research investment. Phages, owing to their abundance and adaptability, may be able to potentially bypass these issues while remaining effective as long as new phage strains are discovered with increasing bacterial resistance.
References:
- Burrowes B, Harper D, Anderson J, McConville M, Enright M. Bacteriophage therapy: potential uses in the control of antibiotic-resistant pathogens. Expert Review of Anti-infective Therapy. 2011, 9:9, 775-785. http://dx.doi.org/10.1586/eri.11.90
- Lin DM, Koskella B, Lin HC. Phage therapy: An alternative to antibiotics in the age of multi-drug resistance. World J Gastrointest Pharmacol Ther. 2017, 8(3):162-173. doi: 10.4292/wjgpt.v8.i3.162
- Luong T, Salabarria AC, Roach DR. Phage Therapy in the Resistance Era: Where Do We Stand and Where Are We Going? Clinical Therapeutics. 2020, 42(9), 1659-1680. https://doi.org/10.1016/j.clinthera.2020.07.014
- Ptashne, M. A Genetic Switch: Phage Lambda Revisited. 3rd ed., Cold Spring Harbor Laboratory Press, 2004. https://library-search.imperial.ac.uk/permalink/44IMP_INST/mek6kh/alma995511554401591
- Boyd EF, Brüssow H. Common themes among bacteriophage-encoded virulence factors and diversity among the bacteriophages involved. Trends Microbiol. 2002, 10(11), 521-529. doi:10.1016/s0966-842x(02)02459-9
- Hyman P, Abedon ST. Bacteriophage host range and bacterial resistance. Adv Appl Microbiol. 2010, 70, 217-248. doi:10.1016/S0065-2164(10)70007-1
- Labrie S, Samson J, Moineau S. Bacteriophage resistance mechanisms. Nat Rev Microbiol. 2010, 8, 317–327. https://doi.org/10.1038/nrmicro2315
- Goodridge LD. Designing phage therapeutics. Curr Pharm Biotechnol. 2010, 11(1), 15-27. doi:10.2174/138920110790725348
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