Phage Therapy: The Future of Bacterial Infection Treatment?

Stylized visualization of a bacteriophage

(Adobe Stock 293883966 by Tatiana Shepeleva)

As doctors tried to find a way to treat bacterial infections more than a century ago, Félix d’Herelle proposed using bacteriophages against bacterial infections. Although this idea sparked interest, the discovery of the world’s first antibiotic, penicillin, in 1928 by Alexander Fleming pushed phage therapy to the sidelines.

Almost a century later, the threat of rising antibiotic resistance means the world desperately needs new therapies against bacterial infections. According to the CDC, treating 6 of the 18 most alarming antibiotic resistance threats contributes to more than $4.6 billion in annual health care costs.¹

Why Are Antibiotics Not Effective Anymore?

Antibiotics were once a powerful treatment option against bacterial infections. Today, they do not even work in certain cases for a simple reason—resistance against them, something Fleming warned about.²

Antibiotic tolerance occurs when bacteria susceptible to antibiotics survive an antibiotic regimen.² There are 3 main molecular mechanisms of resistance—inactivation of antibiotics, modification of the antibiotic target on the bacteria, and reduction of intracellular antibiotic concentrations. Bacteria that survive an antibiotic challenge constitute a significant threat to modern health care. It was noticed that antibiotic tolerance precedes antibiotic resistance and can push toward rapid antibiotic resistance.²

Patients’ and physicians’ misuse and overuse of antibiotics are major causes of antibiotic resistance. The lack of new antibiotics in the past 50 years has played a role in this silent pandemic. Pharmaceutical companies have moved from developing new antibiotics to producing more profitable medications for chronic conditions.

Health care-associated infections by Infection Control Today

What Is Phage Therapy?

Phage therapy was first proposed by d’Herelle1 after he successfully used phages to treat children suffering from bacterial dysentery.³ Despite not gaining popularity in the West, phage therapy was widely used in the Soviet Union, especially Georgia, due to d’Herelle’s collaboration with Georgian scientists. Russian studies of the time show that phage therapy was used in ophthalmology, pediatrics, surgery, dermatology, and other specialties.

Naturally occurring or environmental phages can be found virtually anywhere bacteria are found. This includes water bodies, soil, animals, and plants. Water and soil samples of interest are usually passed through 0.22-μm filters, and filtrates are added onto plates with lawns of bacteria to check for lysis.³ Although the idea of using phages is more than a century old, current research has been focusing on using phage lysins as a therapeutic option.⁴

Where Does Phage Therapy Stand Today?

Renewed interest in phage therapy due to rising anti­bacterial resistance has led to several clinical trials. Even though phage therapy was extensively used in the Soviet Union, Russian literature published on phage therapy cannot be taken as it is today. Poor study designs, unspecified phage titers and methods, lack of control groups, and other factors make these studies unusable.⁵

Phage Therapy in Tuberculosis

The treatment for one of the world’s top infectious killers has changed from 2 injections of streptomycin to a multi­drug regimen today. Most cases of tuberculosis today are drug-resistant, with some patients infected with totally drug-resistant tuberculosis.

A recent study published in Communications Biology found 2 bacteriophages (D29 and DS6A) capable of lysing Mycobacterium tuberculosis (Mtb) (Figure 6). However, researchers found that D29 could not kill Mtb in broth culture even at the highest multiplicity of infection (ratio of bacteriophage to bacteria), although it could do so in solid culture. This may be due to Mtb rapidly developing resistance to the phage and replacing D29-susceptible bacteria with resistant ones. DS6A, on the other hand, was found to kill Mtb even in primary human macrophages.⁶

Phage Therapy in Sepsis

Sepsis is a major health risk and can be fatal. Researchers looked at phage therapy against extensively drug-resistant Acinetobacter baumannii sepsis, for which very few drugs are available.

They used φkm18p, a phage isolated from hospital sewage, against KM18, a bacterial isolate of Acinetobacter baumannii that has shown resistance to all antibiotics except tigecycline and polymyxins. They found that the bacterial loads were significantly lower in mice receiving immediate and delayed phage therapy. However, they could also isolate phage-resistant bacteria from the mice.⁷

A single-arm Australian study (NCT03395769) injected 12 patients suffering from severe Staphylococcus aureus infections with 3 bacteriophages named AB-SA01 as an adjunctive therapy. None of the patients experienced adverse effects, and phage resistance was not seen.⁸

Phage Therapy in Intraocular Infections

Bacillus cereus causes one of the most severe forms of sight-threatening intraocular infections. This organism is inherently resistant to β-lactamase; other antibiotics are not enough to treat these infections.⁹

Figure. Bacteriophage Therapy in Treating Mycobacterium tuberculosis Infections6. Serial dilutions of different strains of bacteriophages were mixed with H₃₇Rv in ₇H₉ medium. Reproduced with permission from Yang et al⁶

A recent study explored the possibility of using bacterio­phages in B cereus infections. Researchers used PlyB, a phage lysin, in various mediums. They found that PlyB acted as a bac­tericidal agent against B cereus, killing the vegetative bacteria but not its spores. PlyB is a highly group-specific phage lysin that killed the bacteria in multiple growth mediums, including ex vivo rabbit vitreous humor. It also reduced bacterial counts in keratitis and endophthalmitis models, prevent­ing complications related to these infections. An in vivo mouse model also showed the efficacy of this phage lysin.9 Moreover, it was shown to be safe against human retinal cells and macrophages, making it a strong con­tender for future phage therapies.⁹

Phage therapy against Pseudomonas aeruginosa

Multidrug-resistant Pseudomonas aeruginosa is a dangerous bacteria with a high mortality rate. The main factor in infections by this organism is the biofilm it secretes.¹⁰

The first bacteriophages against P aeruginosa were discovered decades ago. Since then, multiple phages have been discovered that act against it. A bacteriophage cocktail, especially, has proved to be more effective against the biofilm. A cocktail may increase activity due to an increased number of hosts and may prevent resistance against bacteriophages.¹⁰

Phages MAG1 and MAG4 individually affected 50% of carbapenem-resistant bacterial isolates, but when used in a combination cocktail, the anti-biofilm activity rose to 72.9%. Another cocktail of Pa193, Pa204, Pa222, and Pa223 significantly reduced the rate of biofilm production by 89% in patients with chronic rhinosinusitis. Individually they had reduced the rate by 53-73%. Phage AZ1 inhibited and destroyed the planktonic form of P aeruginosa and its biofilm.¹⁰

Can we use phage therapy along with antibiotics?

Current studies have used a combination of antibiotics and phage therapy, and old Russian studies used phage therapies after unsuccessful antibiotic therapies.

Researchers to find a suitable phage-derived peptide against Mtb found that the peptides AK15 and its isomer AK15-6 showed synergistic effects along with Rifampicin.11 Phages were also used in combination with antibiotics against Staphylococcus aureus in the Australian study mentioned above.⁸

What advantage does phage therapy have over antibiotics?

As of now, the biggest advantage of phage therapy is its ability to be used as an alternative treatment option against antibiotic-resistant bacterial infections or infections in which the currently available antibiotics are of little to no use.

Regulatory approval of phage-encoded proteins would follow the same procedure as other regulatory proteins, making it a much shorter procedure than the one used for antibiotics.¹² Once the safety and regulatory concerns are cleared, phage preparations take relatively less time to prepare, giving doctors the option to order customized phage cocktails for their patients. With personalized medicine already gaining momentum in mainstream healthcare, customizing phage cocktails suited for every individual patient may become more common. This may also help in preventing misuse of phage therapy by the public, which has caused the rise of antibiotic resistance in the first place.

Bacteriophages have an advantage over antibiotics in infections caused by bacterial biofilm as they can penetrate the inner layer, unlike antibiotics, which only attack the surface. Cocktails, in particular, can increase antifilm activity and prevent resistance to phages. However, they do not seem to work as efficiently against high-density biofilms but can prevent further accumulation and diffusion of these films by reducing migratory bacteria. Large production of mature or alginate biofilms can inhibit the function of bacteriophages.¹⁰

What challenges are we likely to face while developing phage therapy?

While phage therapy looks like a promising alternative to antibiotics, it has its own limitations.

The development of phage-encoded proteins depends upon therapeutic safety, cell toxicity, and immunogenicity.¹² Bacterio­phages contain endotoxins with high immunogenicity, making safety a huge problem when it comes to commercial use of phages. Bacteriophage preparations must undergo extensive purification to make them safe for human consumption.¹²

Phage preparations, like other therapies, will need to undergo extensive regulation, starting from the selection of hosts to the removal of endotoxins.12 This may make it a lengthy process that pharmaceutical companies may not be willing to undertake.

Some kinds of bacteria, such as E coli, can evade phages and seek protection inside the cell. To try and combat this, we can investigate combining phages with lytic enzymes with cell-penetrating peptides.¹²

As already seen in the above experimental models, bacteria can develop resistance to phages. This means that we may face new challenges in developing phage therapies.

As the world grapples with the rising burden of antibiotic-resistant infections, researchers need to seriously look into other therapeutic options, especially phage therapy, considering that we have seen some level of effectiveness in old Russian trials and modern-day studies. Though there is a risk of bacteria becoming resistant to commercially available phage therapies, the wide variety of available phages and relatively shorter regulatory and approval time may mean that we can produce new therapies at a much faster rate.

Rising antibiotic resistance rates have led researchers to reconsider phage therapy. Although multiple clinical trials have been conducted, phage therapy is not widely available in most countries. Phage therapy, like other forms of treatment, has its challenges, one being resistance to phages. Regulation of phages, safety, and efficacy are other problems researchers may encounter while developing new treatment options against drug-resistant bacteria.

REFERENCES

1. CDC partners estimate healthcare cost of antimicrobial-resistant infections. CDC. Published April 22, 2024. https://www.cdc.gov/antimicrobial-resistance/stories/ partner-estimates.html

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3. Strathdee SA, Hatfull GF, Mutalik VK, Schooley RT. Phage therapy: from biological mechanisms to future directions. Cell. 2023;186(1):17-31. doi:10.1016/j.cell.2022.11.017

4. Van Belleghem JD, Merabishvili M, Vergauwen B, Lavigne R, Vaneechoutte M. A comparative study of different strategies for removal of endotoxins from bacteriophage preparations. J Microbiol Methods. 2017;132:153-159. doi:10.1016/j. mimet.2016.11.020

5. Chanishvili N. Chapter 1 - Phage therapy—history from Twort and d’Herelle through Soviet experience to current approaches. ScienceDirect. Published January 1, 2012.

6. Yang, F., Labani-Motlagh, A., Bohorquez, J.A. et al. Bacteriophage therapy for the treatment of Mycobacterium tuberculosis infections in humanized mice. Commun Biol 7, 294 (2024). https://doi.org/10.1038/s42003-024-06006-x

7. Wang JL, Kuo CF, Yeh CM, Chen JR, Cheng MF, Hung CH. Efficacy of φkm18p phage therapy in a murine model of extensively drug-resistant Acinetobacter bau­mannii infection. Infect Drug Resist. 2018;11:2301-2310. doi:10.2147/IDR.S179701

8. Petrovic Fabijan A, Lin RCY, Ho J, Maddocks S, Ben Zakour NL, Iredell JR; West­mead Bacteriophage Therapy Team. Safety of bacteriophage therapy in severe Staphylococcus aureus infection. Nat Microbiol. 2020;5(3):465-472. doi:10.1038/ s41564-019-0634-z

9. Mursalin MH, Astley R, Coburn PS, et al. Therapeutic potential of Bacillus phage lysin PlyB in ocular infections. mSphere. 2023;8(4):e0004423. doi:10.1128/ msphere.00044-23

10. Chegini Z, Khoshbayan A, Taati Moghadam M, Farahani I, Jazireian P, Shariati A. Bacteriophage therapy against Pseudomonas aeruginosa biofilms: a review. Ann Clin Microbiol Antimicrob. 2020;19(1):45. doi:10.1186/s12941-020-00389-5

11. Yang Y, Liu Z, He X, et al. A small mycobacteriophage-derived peptide and its improved isomer restrict mycobacterial infection via dual mycobactericidal-immunoregulatory activities. Journal Biol Chem. 2019;294(19):7615-7631. doi:10. 1074/jbc.RA118.006968

12. Aslam B, Arshad MI, Aslam MA, et al. Bacteriophage proteome: insights and potentials of an alternate to antibiotics. Infect Dis Ther. 2021;10(3):1171-1193. doi:10.1007/s40121-021-00446-2