**1. Introduction**

The term microbiota defines the whole of microorganisms, not only bacteria but also fungi, protozoa, and viruses present in our organism. In particular, the human gastrointestinal tract is colonized by about 1013–1014 microorganisms, of which 15,000 different bacterial strains are located mainly in the colon in a symbiotic relationship with the host [1]. In normal conditions, the microbiota is characterized by the predominance of obligate anaerobic members of *Firmicutes* and *Bacteroidetes phyla*, which guarantee intestinal and general health, while the loss of homeostasis, known as dysbiosis, is linked to the proliferation of some bacterial populations such as the Enterobacteriaceae or the absence of important commensal bacteria helps to create a more favorable environment for the growth of pathogens, predisposing the organism to pathological conditions [2,3] (Figure 1). According to what was recently reported in a study by the Human Microbiome Project and the European consortium Meta HIT, the human intestinal bacterial flora, despite being composed of a very high number of different species, can be divided into three most represented genus: *Bacteroides* and *Prevotella*, belonging to the phylum *Bacteroidetes,* and *Ruminococcus* belonging to the phylum *Firmicutes* [4]. In the time from birth to adulthood, the microbiota undergoes numerous changes; in fact, the neonatal microbiota is precociously formed by *Escherichia coli* of the birth canal followed by *Bacteroides, Bifidobacteria*, and *Clostridium* in the first week of life and reaches stabilization already appearing similar to that of adults only around 2–3 years old [5]. The intestinal microbiota has many important functions for maintaining the health of the host, such as the formation and maintenance of the intestinal barrier, through the production of short-chain fatty acids (SCFAs) resulting from the fermentation of undigested nutrients, immunostimulation, and immunotolerance, synthesis

**Citation:** Sirufo, M.M.; De Pietro, F.; Catalogna, A.; Ginaldi, L.; De Martinis, M. The Microbiota-Bone-Allergy Interplay. *Int. J. Environ. Res. Public Health* **2022**, *19*, 282. https://doi.org/10.3390/ijerph19010282

Academic Editors: Diana María Cardona Mena and Pablo Roman

Received: 3 December 2021 Accepted: 24 December 2021 Published: 28 December 2021

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of substances, metabolic-trophic function, metabolism of drugs and toxins [6]. The balance between the intestinal microbiota and the host is maintained through various mechanisms, including the secretion of gastric acid, mucus, bile salts, and mucous Ig, mucosal pH, intestinal motility, local and systemic immunity, and interactions between different microbial species. An alteration in the microbiota-host relationship could potentially cause the onset of gastrointestinal or extra-intestinal diseases, defined as "intestinal microbiota related diseases". Among the most known, we remember allergic diseases, inflammatory bowel diseases, obesity, metabolic syndrome, type 1 and 2 diabetes, cardiovascular disease and even osteoporosis (OP) [1].

**Figure 1.** The human gastrointestinal tract is colonized by about 1013–1014 microorganisms, of which 15,000 different bacterial strains are located mainly in the colon in a symbiotic relationship with the host. In normal conditions, the microbiota is characterized by the predominance of obligate anaerobic members of Firmicutes and Bacteriodetes phyla, which guarantee intestinal and general health, while the loss of homeostasis, dysbiosis, linked to the proliferation of some bacterial populations such as the Enterobacteriaceae or the absence of important commensal bacteria helps to create a more favorable environment for the growth of pathogens, predisposing the organism to pathological conditions.
