**1. Introduction**

The microbiota exerts a marked influence on the host during homeostasis linked with metabolic diseases in humans, but demonstration of causality remains a challenge [1].

Humans as hosts have co-evolved with microorganisms over millions of years, and each body habitat has a unique set of microorganisms shaping its microbiota [2].

These bacteria live on the skin, in the corners of the eyes, in the oral cavity, under fingernails, and most importantly, in the guts. Several perinatal determinants, such as caesarean section delivery, type of feeding, the use of antibiotics, gestational age or environment can affect the pattern of bacterial colonization, resulting in gut dysbiosis. The establishment and development of the gut microbiota over the lifecycle moved from the previous accepted dogma that the mammalian healthy placenta and foetus were germ-free and considered to be sterile, and that these conditions were critical to the developing newborn's immune system, to the actual knowledge that in utero humans are now known to harbour unique prenatal microbiomes [3,4].

Amniotic fluid may contain microorganisms, increasing the complexity of fetal microbiota, and having implications for the long-term health and susceptibility to disease, as placental microbiota could trigger immune responses in the fetus. Early gut microbiota settlement influences the maturation of the infant's immune system [5] and subsequent health, although the evidence in support of the "in utero colonization hypothesis" is considered extremely weak by some authors [6].

Health authorities are now becoming fully aware that one cannot be considered to be in good health without a well-balanced microbiota composition in the gut, our "forgotten organ" [7], and of the fundamental role of a diverse and healthy gut microbiota on the subsequent maintenance of future health and well-being of the host [8,9]. Indeed, although it is broadly mentioned that there are 10 times more cells from microorganisms in our bodies than there are human cells [10], this claim has been challenged, and others have estimated that the number of bacteria is similar to that of human cells [11].

Many species of bacteria, specifically those found in the invisible universe of the human microbiota, e.g., composed of nonpathogenic commensal microbiota from the *Firmicutes*, *Bacteroidetes*, *Actinobacteria*, *Proteobacteria* and *Verrucomicrobia* phyla [12], are unsusceptible to petri dish cultivation. They can be successfully cultivated in association with other microbes, meaning in communities of different bacteria species. But without being able to isolate them, research is difficult [13]. Commensal microbiota gradually deteriorates in sick patients. Therefore, research is being conducted to generate new technologies to study the rest of the human microbiome using advances in DNA-sequencing technologies and associated computational methods [14]. Metagenomic sequencing of total fecal DNA samples offers complementary support to classical microbiology, and enables researchers to access previously-inaccessible genomic information from gut bacteria [15,16].

In recent years, a number of functional species and strains have been identified in human metabolic diseases [17]. Gut bacteria can produce various bioactive metabolites which can be detrimental to the host's health, such as those with cytotoxicity, genotoxicity, or immunotoxicity [18], shifting the paradigm of understanding the root cause of the onset and progression of several human metabolic diseases [19].

Gut microbiota modulates the expression of many genes in the human intestinal tract [20], including genes involved in immunity, nutrient absorption, energy metabolism, and intestinal barrier function. It is important to understand genomic diversity of specific members of the gut microbiota if precise nutrition-based approaches are to be realized [21].

In the oral use of live bacteria, there is more research concerning isolated probiotic commercial supplements than there is work concerning health benefits of common fermented foods, since major industries usually do not fund this type of research [22]. Many studies suggest that probiotics may help with diarrhea or symptoms of irritable bowel syndrome, but strong evidence to support their use for most health conditions is lacking in people with sepsis, and probiotics are no panacea [23,24].

Probiotics should not be universally given as a 'one-size-fits-all'; most trials were based on stool samples, which may not really reflect the bacteria living in the gut, as shedding takes place continuously [25]. Besides, taking probiotics after treatment with broad spectrum antibiotics may actually delay the return of normal gut microbiome, a new potential adverse side effect [26].

The One Health concept, introduced at the beginning of the 2000s [27], is a worldwide strategy for promoting multidisciplinary partnerships and information in all facets of health care sciences, perceiving the interrelationship between humans, animals, plants, and their common environment [28]. By working with physicians, veterinarians, osteopathic physicians, dentists, pharmacists, nurses, ecologists, wildlife professionals, and other scientific-health and environmentally-related specialists, it will be possible to monitor and control public health threats and learn how diseases spread among people, animals, and the environment [29].

The point of this mini review is to highlight if the requirement for multiconvergence of the research fields of One Health (Human-Animal-Environment), the relationship between microbiota-nutrition and fermented foods, and to underline the idea that future gut-brain research is feasible and achievable (Figure 1).

**Figure 1.** A general picture of the One Health (Human-Animal-Environment) concept as a trans-disciplinary effort. Contribution of the three branches of public health. Microbiota and human metabolic diseases, animal health, and environmental epidemiology.

## **2. Microbiota and General Health**

Having an active and natural variety of microorganisms in the gut may improve general health [30]. The good, healthy bacteria make food more digestible through their enzymes, increased vitamin synthesis, and the preservation of nutrients, and also help to reduce sweet cravings, maintain the immune system, and benefit overall gut wellness [31].

The microbiome, consisting of microorganisms and their collective genomes, modulates the host metabolic phenotype, and influences the host immune system. It is now well established that gut bacteria are closely tied to immune health [32]. The gut microbiota regulates L-tryptophan metabolism and identifies the underlying molecular mechanisms of these interactions [33].

imbalanced, it is hard for the body's immune system to function properly [34]. There are also a A large majority of the immune system resides in the tonsils and gut, so when gut health is imbalanced, it is hard for the body's immune system to function properly [34]. There are also a number of common factors in modern life that can throw human gut bacteria off, such as processed foods and antibiotics. The use of antibiotics does have several short and long-term implications in the ecology of the normal microbiota and gut motility [35].

Research on the health benefits of probiotics is still emerging, mainly from the food and beverage industries and their commercial interests. In contrast, strong, independent scientific evidence to support specific uses of probiotics for most health conditions is still lacking [36].

The administration of probiotics/prebiotics has been shown to alter the composition and functionality of the gut microbiota [37]. Recent evidence indicates that the effects of probiotics are likely to be different from one person to the next [38].

*Foods* **2018**, *7*, 195

In addition, probiotics might be ineffective, and possibly counterproductive, in restoring the baseline gut microbiome after it has been altered by antibiotic treatment. Indeed, probiotics may not be quite as good as was commonly thought, and they could even be harmful if taken after antibiotics [39].

Serious disorders such as obesity, anorexia, irritable bowel syndrome, autism, and posttraumatic stress disorder—which have been thought to be solely psychological—share a common symptom: a hypersensitivity to gut stimuli [40–42]. — — –

The role of environmental factors in the development of autism is a crucial and an important area of research concerning how the environment influences and interacts with genetic susceptibility. Factors such as parental age at conception, maternal nutrition, infection during pregnancy, and premature birth are risk factors [43]. Autism (ASD, autism spectrum disorder), a developmental disorder characterized by disturbance in language, perception, and socialization, with no exact known cause, is usually linked with bioenergetic metabolism deficiency [44] and neuro-inflammatory conditions [45], and immune system dysregulation and dreadful gut concerns may improve with better diet and fermented foods (e.g., fermented raw coconut milk) [46,47].

Specific benefits from the direct dietary modulation of the human gut microbiota has been described [48]. Despite the wide array of beneficial mechanisms deployed by probiotic bacteria and fermented foods and beverages, relatively few effects have been supported by clinical data [49].

The interactions (Figure 2) between ingested fermented food and intestinal microbiota, and their correlations to metabolomics profiles and health, represent an important perspective, and independent research on health benefits is still emerging [50,51]. Microbiota is specific to each individual, despite the existence of several bacterial species shared by the majority of adults. A diverse and propitious microbial ecosystem (e.g., *Bacteroides fragilis*, *Bifidobacterium* spp. and *Faecalibacterium* spp.) favors homeostasis, particularly at the level of the disease–immune dialogue [52,53]. –

**Figure 2.** Interactions between dynamics of microbiota in humans, animals, and the associated environment with disease occurrence, salubrity, and well-being.
