## 3 Intestinal Bacteriophages and Faecal Virome Transplants
### Advantages and Potential Benefits of Facal Virome Transplants  
In the light of various adaptations and optimisations of FMT techniques briefly described above, it can be assumed that broadening their usage in the future will lead to safer and more effective procedures. However, considering faecal virome transplantation as an alternative to FMT would mean that the issue of introducing potentially pathogenic bacteria into the gut of the patient is essentially removed from the equation, which makes FVT a promising candidate for a novel therapeutic approach. In fact, while FVT can be still considered to be in its infancy, there are studies that were already able to highlight its efficiency in altering gut microbiota and affecting the metabolism of animals. A FVT using material from mice with high gut abundance of _Akkermansia muciniophilia_, a known probiotic species, was able to significantly increase the prevalence of this bacteria in mice naturally deficient in it; the same study also identified significantly increased fertility of the mice as a surprising and yet unexplained side effect of the FVT procedure [94]. Another study discovered that FVT based on material from obese mice on a high-fat diet can, aside from noticeably influencing the gut microbiota of the recipient mice, significantly affect their growth rates and induce an obese phenotype, even though the recipients were continuously fed a regular diet; a reverse dynamic was also observed, where mice fed a high-fat diet gained weight more slowly after a gavage of FVT from mice on a regular diet [95]. In a study conducted on broilers with LPS-induced intestinal injury, FVT proved to have significant effect on immune response within the gastrointestinal tract, effectively decreasing expression of IL-8 and IFN-\(\gamma\) on par with an FMT procedure [4]. These findings have tremendous implications, as they show that FVT can be an effective tool for altering gut microbiota and influencing animal phenotypes, making it possible to confer positive traits not only between closely related individuals but--as is already the case with FMT--between species. However, FVT can also be used to restabilise GIT microbiotas without needing to source material from other individuals. This was shown by a study on autochthonous viral transplants in which antibiotic-treated mice were administered with phage-containing filtrate from faeces that had been collected before the start of the treatment. The GIT microbial composition of FVT-recipient mice was highly similar to their original microbiome, while the microbiomeof mice that had not received the filtrate of their own faeces after the administration of antibiotics was noticeably more different [96]. There are also interesting reports of phage resistance being correlated with increased susceptibility to antibiotics in and/or decreased ability to colonise the gut in certain bacteria [73], possibly implying that bacteriophage presence may lead to passive selection of strains with decreased potential for pathogenicity. Even though it may be tempting to debate the superiority of one therapeutic approach over another, it seems more rational to instead think of FMT and FVT as similarly valid counterparts that can both be further developed in different directions. However, utilising FVT as a substitute for FMT might have certain advantages, such as the previously mentioned elimination of potentially pathogenic bacteria and toxic substances from the transferred material--provided, of course, that suitable purification and quality control steps are undertaken. In this regard, it resembles the \"washed\" microbiota transplant procedure [70]. Moreover, identifying bacteriophage species that are crucial for maintaining gut stability and devising efficient, sustainable ways of mass-producing specific bacteriophage strains could prove to be economical enough for such preparations to be used not only in therapeutic approaches but in feed supplementation and therefore enhance the well-being of animals and the quality of animal-derived products.