Bacteriophage are viruses that infect only bacteria. A typical virus contains some form of genetic material protected by a capsid made of proteins. The genetic material can vary widely from being RNA to DNA to single stranded to double stranded; It can even vary on the polarity of the genetic material: positive or negative. The capsid has three basic shapes: icosahedral, helical, or complex. A complex virus typically has an icosahedral or helical shape, but it also has extra protein components such as tail fibers. A virus can either be classified as naked or enveloped. Enveloped viruses are surrounded by a phospholipid bilayer while naked viruses are not (1).
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Viruses are considered obligate intracellular parasites. This means they require a host cell to replicate. They have a two general life cycles: lytic or lysogenic. The lytic cycle occurs when viruses immediately begin to replicate while the lysogenic cycle occurs when a virus waits for a stimulus to replicate—usually a host stressor. The first step to both life cycles requires locating and attaching to a suitable host. Attachment requires specific interactions between viral attachment proteins and host cell receptors. Once a virus particle has attached to the host cell, penetration of the virion can occur through capsid rearrangement, cell-mediated endocytosis, fusion, or translocation. Capsid rearrangement occurs when attachment results in a shift in capsid proteins which results in the release of the genetic material into the host cell. Cell-mediated endocytosis occurs when attachment results in the virion being endocytosed into the host cell. Fusion occurs in enveloped viruses when attachment results in integration of the virion’s phospholipid bilayer with the host cell’s cell membrane. Translocation is not understood but results in the penetration of the virion into the host cell. Once penetration has occurred, the virus must replicate its genetic material and produce crucial proteins such as capsid proteins as seen in the lytic cycle, or the viral genome can integrate into the host genome as seen by the lysogenic cycle. Once the host is stressed the virus will leave the lysogenic cycle and continue a lytic cycle. Production of virus varies widely based on complexity and type of genetic material of the virus. After assembly of new virus particles, the virus must be released from the host cell. This can either occur by bursting the host cell as seen by naked viruses or budding as seen by enveloped viruses as they steal host cell membrane. Once the virions have been released, they seek new host to infect to repeat the cycle (1).
Viruses which exhibit a lytic life cycle can prove to be efficient in controlling microbial loads. Their high specificity for suitable host cells makes them ideal candidates for targeting specific microbes. Lysogenic viruses are not desired since they can integrate their genome into the host’s genome. This could lead to unpredictable and dangerous gene transfers into the host cell. Worst case scenario creating a new pathogenic strain of a foodborne pathogen (2). The use of these bacteriophage to treat foodborne pathogens has been termed phage biocontrol. Phage biocontrol allows for the targeting of specific problematic foodborne pathogens. Foodborne pathogens remain a constant problem around the world. In 2010 approximately 600 million individuals were infected worldwide by a foodborne pathogen. Among the 6 million affected, approximately 420,000 cases resulted in death. Up to 40% of these cases occur in children below the age of 5 (3). With so many individuals falling ill to foodborne illnesses, preventative measures should be a continuing worldwide effort. Phage biocontrol has proven effective against many problematic bacteria including Listeria monocytogenes, Salmonella spp., Escherichia coli, Shigella spp., and Campylobacter jejuni (2).
L. monocytogenes is a problematic foodborne bacterium particularly due to its ability to replicate in temperatures between 2-8°C (2). In 2010 L. monocytogenes caused approximately 14,000 infections with 3,000 of them resulting in death (3). Symptoms can vary from an upper respiratory infection to a gastrointestinal infection. L. monocytogenes is a dangerous pathogen due to its ability to cross the placental barrier resulting in the infection of the fetus—ultimately a stillbirth (2). Since L. monocytogenes can successfully replicate in cold temperatures, it poses a threat to ready-to-eat (RTE) foods stored in cool temperatures, however, phage biocontrol has proven to be an effective treatment against L. monocytogenes. L. monocytogenes is susceptible to not only phage cocktails but also monphage treatments (2). Although phage biocontrol was able to reduce the L. monocytogenes, it was not able to eliminate L. monocytogenes from contaminated products (4). Phage biocontrol did prove more effective when paired with other antimicrobial compounds such as sodium diacetate and potassium lactate (4). Even though monophage treatment was successful, it is ideal to use a cocktail to prevent the rise of phage-resistant strains. Currently there are three FDA approved phage biocontrol products that target L. monocytogenes: PhageGuard, Listex™, and ListShield™ (2).
Non-typhoid Salmonella spp. serotypes are enteric bacterium known for causing gastroenteritis infections. They can lead to life-threatening symptoms typically due to dehydration (2). In 2010 Salmonella spp. caused 78 million foodborne illnesses with 60,000 cases resulting in death (3). Similar to L. monocytogenes, Salmonella spp. poses a safety hazard for RTE foods (2). Primarily due to contaminated equipment and unsanitary work conditions. Phage biocontrol is not only effective on food products. It is also effective at reducing microbial loads on equipment such as glass surfaces and stainless-steel surfaces (5). Phage cocktails can also be applied directly to contaminated meat products such as chicken breasts. The combination of phage biocontrol with a modified atmospheric environment prove to further decrease the microbial load (6). Non-typhoid Salmonella spp. serotypes not only affect humans but pets as well. Close association with pets can results in illness in pet owners therefore treatment of contaminated pet food can prove beneficial (7). Currently there are two FDA approved phage biocontrol products that target Salmonella spp.: PhageGuard S™ and SalmoFresh®. A third one named SalmoPro® is awaiting approval (2).
E. coli is an enteric bacterium that is part of the normal flora of the human digestive tract. Pathogenic strains of E. coli can disrupt normal flora processes. One such strain is E. coli O157:H7 which can produce a Shiga toxin capable of causing gastrointestinal infections (2). In 2010 E. coli strains capable of producing Shiga toxin such asO157:H7 caused 1 million cases of infection and at least 100 deaths with most deaths occurring in children and elderly patients (3). Phage biocontrol has been proven effective in E. coli in fresh produces such as groceries as well as pasteurized and unpasteurized milk. As long as temperatures below 4°C are maintained, microbial load will not increase after treatment, but if temperatures of the treated product rise then the E. coli load will increase over time (2). Rise in load may be due to a rise in phage-resistant E. coli strains, however, increasing the number of phages in a cocktail should reduce the likelihood of the development of resistant strains of E. coli (8). Phage specificity is so high that I can target different strains of E. coli—particularly pathogenic ones. Currently there are three FDA approved phage biocontrol products that target E. coli O157:H7: Ecolicide®, EcoShield™, and Finalyse®. Secure Shield E1 is a broader spectrum phage biocontrol for E. coli awaiting approval (2).
Shigella spp. are another enteric bacterium capable of causing gastrointestinal infections. In 2010 Shigella resulted in 50 million infections worldwide with 15,000 cases resulting in death. Most of these infections occurring in third world children (3). A wide use Shigella phage cocktail is capable of treating RTE foods such as produce, dairy products, deli products. It is also able to treat raw meat products such as chicken breasts (9). Currently there is only one FDA approved phage biocontrol for Shigella spp. called ShigaShield™ (2).
Campylobacter spp. such as C. jejuni is another foodborne pathogen capable of causing gastroenteritis. In 2010 Campylobacter resulted in 95 million cases of infection with 21,000 cases resulting in death. Campylobacter is part of the normal flora of avian and livestock intestinal tracts as well as the livers of chickens. Poor handling of raw meat products or consumption of undercooked meat products are typical modes of transmission to humans (3). Phages isolated directly from chicken fecal matter and livers have proven useful in reducing the microbial loads of Campylobacter spp., however, phage cocktails targeting different receptors are required for effective treatment (10). Currently there are no FDA approved phage biocontrol approved by the FDA, however, natural isolates from normal flora have been proven reliable (2). This means there could potentially be a viable phage cocktail availed in the future.
ListShield™ was the first phage biocontrol approved as a food additive by the FDA in 2006. Shortly after, Listex™ was Generally Recognized as Safe (GRAS) product by the FDA. Many of the other phage biocontrol treatments have sought to recognized as GRAS as a standard for phage treatment against foodborne pathogens. Phage biocontrol treatments are obtained naturally from the environment as they are already a part of the foods we consume (2). The USDA has even approved the use of specific cocktails which are considered safe during the meat production industry to reduce the microbial load of pathogenic microbes (2).
The use of phage in the treatment of food is desirable for many reasons. Their specificity to target microbes allows dangerous pathogenic to be selected against. This leaves normal flora of food products to remain in tact when compared to using harsh non-selective chemicals. There may potential nutritional significance in protecting the normal flora of food products. Most phage biocontrol treatments were isolated from naturally occurring phages. This means that their impact to being introduced to the environment will be minimal since they already part of the ecosystem (2). These phage biocontrol treatments also tend to be water-based solution meaning that they have virtually no impact on the taste or smell of treated foods (11). Many of these phage biocontrol treatments have also been labeled as Kosher and Halal as well as organic (2). This allows many different types of consumers to potentially enjoy phage biocontrol treated products. Products can be treated pre-harvest or post-harvest (12). This allows a wide variety of at-risk foods to be treated from RTE foods such as produce to raw meat products. A single use of phage biocontrol treatment can reduce microbial load 10-fold to 1000-fold (2). Phage biocontrol can prove to be cheaper then other conventional methods. Conventional methods such as HPP treatment or irradiation can cost as much as 10-30 cents per pound of food whereas application of phage biocontrol treatments can cost as little as 1-4 cents per pound of food (13).
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With any treatment procedure there are certain complications. Phage must be stored in refrigerated areas between 2-8°C to preserve viability (2). There have also been instances where the use of phage with other antimicrobial treatments have resulted in both positive or negative results. Harsh chemicals not only reduce microbial loads, but they also reduce the viability of phage (2). Understanding proper procedures for the use of phage still need to be understood. A sole phage biocontrol treatment has failed to completely eliminate targeted pathogenic cells. This means that phage biocontrol cannot replace other treatments but should instead be used to further reduce microbial loads. Treatments have also produced little production of new phage (2). This means that the very few phages locate a suitable host after initial treatment. The fear of phage resistant microbes could also be a potential threat.
Understanding the life cycle of phage allows companies to produce phage biocontrol treatments against foodborne pathogens. Foodborne illnesses are still a major problem world wide resulting in 600 million cases of illness in the year 2010 with 420,000 of those cases resulting in death (3). The use of phage biocontrol can reduce the targeted pathogens 10-fold to 1000-fold (2). Even a 10-fold reduction in L. monocytogenes can result in a substantial drop in mortality rates by 50% and a 100-fold reduction could result in a 74% drop in mortality (14). Although there are some complications with using bacteriophage to treat food products, understanding proper procedures could potentially save multiple lives.
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- Chibeu A, Agius L, Gao A, Sabour PM, Kropinski AM, Balamurugan S. 2013. Efficacy of bacteriophage LISTEX™P100 combined with chemical antimicrobials in reducing Listeria monocytogenes in cooked turkey and roast beef. US National Library of Medicine. National Institutes of Health. [Internet]. [cited 12 Nov 2018]. 167(2):208-214. Available from. https://www.ncbi.nlm.nih.gov/pubmed/24125778
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