Pros and cons of using phages and enzybiotics

Truly alarming, is how to best describe the most recent data about resistance to antibiotics for pathogenic bacteria. After all, they also affect the human health situation. It is estimated that per year, it is the cause of 25,000 human casualties in the European Union (EU) alone; worldwide, 700,000 lives are lost. In 2050, this tendency would lead to a situation that infection-related deaths will be more common than deaths as a result of cancer. Apart from human losses, antibiotic resistance also represents an economic burden. Health spending in the EU because of resistance-related problems are estimated to cost €1.5 billion. In that context, the European authorities have started up various programmes in an attempt to slow down the spread of harmful bacteria.

One Health action plan

The EU programme ‘One Health action plan against antimicrobial resistance’ has been created to coordinate and implement laws aimed at preventing and controlling the problem in all countries. In addition, the plan was designed to invigorate research and development into novel alternative therapeutic methods.

With regard to animal health, the problem has been particularly serious as a large number of zoonotic bacteria are causing infections in humans as well. On top of that, it is important to note that the indiscriminate usage of antibiotics in certain parts of the animal production industry has led to the development of resistant bacteria that can even be found in hospitals. Any solution for this global problem, affecting various market segments (agribusiness, health care, environment), should be considered from various angles. That is why there is a plea for a reduction of the usage of antibiotics and disinfectants, which have to be substituted by other types of antimicrobial approaches that will ideally lead to a low frequency of resistant bacteria. In addition, it should be key to find alternatives that one can use to fight multi-resistant bacteria.

Bacteriophages and enzybiotics

Bacteriophages or simply phages (see Figure 1) are viruses that exclusively infect bacteria. Within these bacteria, they replicate and produce new virus particles which are set free as soon as the bacteria either disintegrates or explodes. This capacity of being able to destroy bacteria makes phages into powerful antimicrobials and exactly that characteristic could be exploited while treating various infectious diseases, i.e. by phage treatment. This type of treatment was discovered by Felix d’Herelle in the beginning of the 20^th century and it was used in Eastern Europe for many years until the usage of antibiotics became commonplace. Nevertheless, in Poland and the former Soviet Union, the usage of phages for antimicrobial purposes has continued. These days, more western nations are also looking more into the use of phages in an attempt to fight infections.

Figure 1 – Illustration of the antimicrobial activity of bacteriophages.

Enzybiotics are proteins (enzymes) with bactericidal activities. In the context of phage therapy, enzybiotics are proteins coded by phages. Enzybiotics are capable of destroying certain bacteria, when they are being added externally. The majority of the enzybiotics are endolysins, and are derived from phages. The specific function of these endolysins is to destroy the bacteria that is host to the phages, so newly created virus particles can be set free, once the multiplication cycle has been finished, see Figure 2.

Figure 2 – Illustration of antimicrobial activity of enzybiotics.

Pros and cons

When suggesting a novel substance as an antimicrobial, an evaluation requires a comparison with existing methods regarding its pros and cons.

1. Bacteriophages are micro-organisms that are extraordinarily abundantly available, as they can be found in their normal form in the environment. For instance, they have been isolated in soil samples at a density of 108 particles/g. This means that humans, animals and plants are in contact with phages for most of our lives without them causing apparently adverse reactions.
2. The antimicrobial action of bacteriophages and enzybiotics is very specific. When used in a therapy, they only target that specific pathogenic bacteria and the rest will remain untouched.
3. Whether applied to antibiotic resistant bacteria or to susceptible bacteria, the elimination efficacy of bacteriophages and enzybiotics is identical.
4. Bacteriophages multiply when infecting pathogenic bacteria, consequently increasing their antimicrobial properties.
5. Enzybiotics act on essential targets for the bacteria’s viability and for that reason, it is difficult for bacteria to modify them. This appears to be the reason why until now there has not been any description of bacteria resistant to enzybiotics.

Antimicrobials derived from bacteriophages, however, do have a couple of disadvantages.

1. Acceptation by part of society may be an issue, as bacteriophages are associated with dangerous viruses. These viruses, however, are totally specific to these bacteria and for that reason, they are harmless to humans, animals, plants or the environment.
2. The usage of bacteriophages requires a detailed study into its biology as well as its genetic characteristics. The goal of this research should be to assure that in no case are the phages carriers or capable of transmitting virulence factors to other bacteria.
3. The specificity of the bacteriophages for their target bacteria is very high, which means that, to make sure that all strains will be eliminated, it is necessary to use a mixture of various phages with a different host range.
4. The appearance of resistance against phage-derived antimicrobials might be a theme of concern. In the case of bacteriophages, bacteria can become resistant to these. Using a mixture of different phages minimises the probability of acquiring resistance to those. It is good to note that there have not been any descriptions of bacteria being resistant to enzybiotics, although numerous studies of repeated exposure to subinhibitory quantities of them have been made.

Current status of phage therapy

The number of studies about phage therapy is growing, indicating that more data will become available about phage and enzybiotic efficacy and safety, especially with regard to the elimination of pathogenic bacteria in human health, animal health or the environment.

Noteworthy is the significance of research by some, mostly US-based biotechnology-based companies, as they received permission for various products. These products can be used in different segments of the agribusiness production chain, from the treatment or prevention of disease in farm animals, to the disinfection of farm houses. The following applications are available in relation to agriculture.

• AgriPhage (Intralytix, USA) is a commercially available product which consists of a mixture of phages which are active against pathogenic bacteria for plants. Its goal is to fight certain diseases in plant crops (think of tomatoes and peppers).
• In the field of food, Listex P100 (Micreos, the Netherlands) is a product that combines 6 phages to get rid of Listeria monocytogenes in certain nutrients and in industrial facilities.
• With regard to animal health, Intralytix has developed phage preparations for the prevention and treatment of animal diseases, which includes farm animals as well as pets. They are commercially selling products against Salmonella (PLSV-1) and Clostridium perfringens (INT-401) to treat chickens.
• Other products, like ListShield, EcoShield, SalmoFresh and ShigaShield, active against L. monocytogenes and E. coli O157:H7, Salmonella and Shigella respectively, are being used as a prevention measure in animals to avoid that these bacteria will enter the food chain.
• In this context, it is good to add that products have also been developed for the disinfection of the skin of live animals prior to slaughter. These include Ecolicide PX (Intralytix) and BacWash (OmniLytics, USA).
• Last but not least, there are also products developed to avoid infection by pathogenic bacteria in pet food, like Ecolicide, SalmoLyse and LystPhage (Intralytix).

Perspectives for the future

The list could of course be expanded with applications for humans. It is clear, however, that these products just constitute a precedent for the use of new phage-based compounds as antimicrobials, which application in different sectors. For a fact, one of the most important problems before commercialisation of this type of products is the lack of clear regulations for them in every country. For instance, the United States Department of Agriculture (USDA) and the US Food and Drug Administration (FDA) have approved various products with a use in the agribusiness. Nevertheless, the European Food Safety Authority (EFSA) has completed an evaluation of certain food preservatives, but has not found concluding data for its use in the European Union. With respect to products for animal health, in the EU, none of these products has received approval at this moment. Every time again, more research points to the big potential in this sector. For that reason, it is essential to continue contributing necessary data, so authorities can understand the real value of their later use.

This article earlier appeared in the Spanish magazine Albéitar. It was reprinted with the permission and cooperation of the authors and original publishers. References are available on request.

Authors: Pilar García Suárez And Ana Rodríguez González, Dairy Research Institute Of Asturias (Ipla), Spain