A New 'Phage' in Combatting AMR
With the World Health Organisation and the National Healthcare System highlighting antibiotic resistance as a critical threat to health, research into other methods to tackle bacterial infections must be utilised to improve rates of bacterial infections. The World Health Organisation’s European Region wants to ensure the safety of people and animals from antibiotic resistance by 2030. 53 Member States have endorsed the plan to encourage research and innovation to prioritise and implement interventions to combat antibiotic resistance.
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One potential solution is the use of bacteriophages, which are viruses that can target bacteria. Phages are made up of protein capsids with a DNA/RNA core and are the most common biological entity on Earth.
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Whilst both target and destroy bacteria, their methods are distinctly different. Antibiotics work by interfering with the growth or metabolism of the target bacterium. For example, they could interfere with the synthesis of bacterial cell walls or bacterial enzyme action.
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The most renowned antibiotic, penicillin, targets bacteria by affecting its cell walls. Bacterial cell walls are composed of peptidoglycan molecules, which are held together by cross-links. When a new bacterial cell grows, it secretes autolysins (enzymes that create small holes in the bacterial cell wall). These holes allow the bacterial cell wall to stretch as the cell grows, and new peptidoglycan molecules are able to join up via the cross-links. Penicillin stops these cross-links from forming by inhibiting the enzymes that catalyse the formation of the crosslinks. The autolysins continue to make holes in the cell wall, weakening the wall. As bacteria live in watery environments and take up water by osmosis, their weakened cell walls eventually burst as they can no longer withstand the pressure exerted on them from within the cell.
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However, phages target bacteria differently. Depending on their cycles, phages can either be virulent or temperate. The lytic cycle involves the destruction of the bacterial cell compared to the lysogenic cycle which ensures the bacterial cell’s immunity against other identical phages.
Lytic phases are relevant to combatting bacterial diseases in humans. With lytic phages, bacterial cells are broken open (lysed) and destroyed. As soon as the cell is destroyed, the phage can find new hosts to infect.
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Firstly, the phage injects its genetic material into the bacterium, which will be replicated via bacterial enzymes. The bacteria’s enzymes and ribosomes replicate the phage’s genetic material and form virions, which are phage particles. Then, the phage produces proteins which are directly involved in the lysis of the bacterial cell. The first protein is the transmembrane protein holin, and the second is a peptidoglycan cell wall hydrolase called lysin. These two proteins work together to trigger the lysis of the bacterial cell. Each phage has several unique lysin and holin enzymes, exemplifying the specificity of phage therapy.
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Holins accumulate in the bacterial cell during the phage infection cycle. At a certain point, holins create holes in the membrane, allowing lysins to reach the bacterial cell wall. Then, lysins access the peptidoglycan and degrade the cell wall by cutting specific bonds in hydrolysis, causing the bacterial cell to burst. This then releases more phages, which can consequently attach themselves to new bacteria and start the cycle over again. This will continue until the pathogen is eliminated. Then the phages, which cannot survive without a host, will then be degraded.
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A positive thing about phage therapy is that bacteria are less likely to gain phage resistance. For example, the lysin PlySs2 was highly effective against a range of pathogenic Streptococcus and Staphylococcus species, including MRSA (which is resistant to a wide range of bacteria). Antibiotics target specific enzymes or biological processes in bacteria, meaning bacteria can evolve against antibiotics by developing mutations. In comparison, phages specifically target the bacterial cell wall, which is integral to the structure of the bacteria. The bacteria will find it hard to block the phage mechanisms without compromising their own structure which could potentially lead to cell death.
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Additionally, the specificity of phage lysins towards bacterial species and strains ensures that the phages only target harmful bacteria. As phages only target harmful bacteria, it means beneficial bacteria, in the gut microbiome, for example, are left unharmed.
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Whereas with antibiotics, their broad application means they target both harmful and beneficial bacteria, disrupting the biome, which leads to side effects like dysbiosis (microbial imbalance) and antibiotic-associated diarrhoea. Antibiotics targeting the wrong bacteria in the gut microbiome can also lead to a higher risk of asthma, obesity, and diabetes. Whilst there is little research into the effects of phage therapy in the microbiome, it has been reported phage therapy results in fewer disturbances in the gut microbiome compared to antibiotics.
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Another positive aspect of phage therapy is its ability to penetrate through the biofilm. It has been proven antibiotics are not very effective in treating biofilm-based bacterial infections. However, phages have enzymes on the exterior of their capsid which can degrade extracellular substances and disperse the biofilms, allowing the phage to access bacteria embedded within. After the phage carries out the lytic cycle, new progeny phases (new phage viruses) are released. As this process of lysis continues, phages gradually break down the biofilm by destroying the biofilm-embedded bacteria layer by layer. The application of phages to colonies of the pathogenic bacterium P. aeruginosa was effective in inhibiting new biofilms forming and was able to degrade pre-existing ones. The application of phages to in vitro colonies of the pathogenic bacterium P. aeruginosa has been shown to both inhibit the formation of new biofilms and degrade pre-existing ones
With antibiotics, high doses are usually required to penetrate dense biofilms to inhibit bacterial growth, yet complete eradication is rare and typically regrowth of colonies begins after the end of antibiotic treatments. Administering high doses of antibiotics can lead to tissue toxicity, which can cause severe, long-lasting side effects due to impaired tissues and organ dysfunction.
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In medicine specifically, phage therapy could help tackle the problem of persistent infections caused by medical devices where biofilm formation is common. Phage treatments have been found to eliminate biofilms formed by the pathogenic bacteria L. monocytogenes, P. aeruginosa, and Staphylococcus epidermidis on the surface of medical devices, which could potentially be utilised to control infection in medical settings.
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Whilst using phage lysins may be a viable solution, combining phage lysins and antibiotics may be a more effective approach to combatting bacterial infections compared to using antibiotics alone. Although research into phage therapy is at initial stages, it could be a potential solution to addressing the rising threat of antibiotic resistance.
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