## 2 Diversity of Gut Prokaryotes and the Principles of Microbiota Transfer
### Faecal Microbiota Transplantation as an Efficient Therapeutic Approach  
Faecal microbiota transplantation is not a strictly novel approach, as its early forms date back to the seventeenth century in Europe and the third century in Asia [51]. The procedure remains controversial despite its high efficacy in resolving gastrointestinal disorders, as aside from the natural scepticism expressed by patients towards the transfer of faecal matter between individuals, current data simply do not sufficiently explain all aspects of recipient response, and there are many well-founded doubts about biosafety, potential pathogen transmission, and long-term effects of altering the gut microbiota [52]. While the process--as all clinical interventions do--does carry some inherent risks, current research supports the notion that it is a safe form of treatment. A meta-analysis of studies performed in the first two decades of the XXI century found that adverse effects related to FMT occurred in 19 percent of patients, but these were mostly mild symptoms such as diarrhoea, cramping, or abdominal pain/discomfort. Serious adverse effects such as infection occurred in 1.4 percent of patients, and 0.12% of patients died after the procedure; however, all patients in these groups had been dealing with mucosal barrier injury, which presumably increased their susceptibility to pathogens [53]. Taking into consideration the high overall mortality rate of people suffering from CDI, sometimes estimated to be as high as 37% within one year after diagnosis [54], FMT does seem to be a preferable therapeutic alternative. Even though the incidence of serious adverse effects is indeed relatively small, additional steps aimed towards pathogen removal substantially contribute to the safety of the procedure [55]. Faecal microbiota transplantation has demonstrated high efficacy in treating recurrent _Clostridioides difficile_ infection, while its role in other conditions is still being explored and remains to be clearly established. These include _Clostridioides difficile_ infections (CDIs), a very common cause of contagious diarrhoea in hospital settings, and the possibility of effectively curing CDI and/or preventing its reoccurrence in patients (along with decreasing mortality rate in severe cases of CDI) has been proven by both numerous individual studies [3; 56] and meta-analyses [57; 58]. FMT can also be used to successfully treat irritable bowel syndrome (IBS) symptoms [59]. There are also some claims of FMT's usefulness in treating other conditions, such as epilepsy [60], depression [61] or autism [62]. While certainly promising, further studies are needed to assess the validity of these findings and elucidate specific mechanism of FMT's influence on human behaviour and mental well-being. FMT is also the subject of research oriented towards improving the health, well-being, growth rate and final product quality of various animals. There exists a procedure called transfaunation, which has been common in animal husbandry for at least a few hundred years, that is based on transferring cud or rumen fluid from healthy animals to individuals suffering from indigestion [63]. While these substances are not faeces per se, the principle of transferring beneficial biological factors remains essentially the same. Available studies on transfaunation are surprisingly scarce given the long history of the procedure, but the current literature confirms its efficiency in treating indigestion caused by various factors [64]. Another study showed that transfaunation can significantly increase growth rates in lambs [65]; interestingly enough, this effect was observed after supplementation not only with sheep rumen fluid but with cow rumen fluid, suggesting an intrinsic potential of interspecific alimentary tract microbiota transfers that may also be applicable in the context of FMT.