## 6 Mechanisms Used by Microorganisms to Impact the Bioavailability of Micronutrients  
The role of microbiota in the modulation of the nutritional status of the host is of great importance. This is best demonstrated by the studies on germ-free animals displaying low levels of some micronutrients that are enhanced after introducing the microbiota [138; 139]. Understanding mechanisms by which the microorganisms may impact the bioavailability of nutrients is crucial for realizing the potential of microbiota in modulating the absorption and use of these nutrients in the human body. The microbiota influences intestinal function and absorption processes through a variety of actions. Beneficial microorganisms produce various metabolites, including short-chain fatty acids (SCFAs) that were found to accelerate the proliferation of intestinal epithelial cells and thus increase the absorptive surface of the intestine, as well as to improve the intestinal blood supply [140]. Nevertheless, the best-known mechanism through which probiotics may impact the regulation of the intestinal barrier is the antagonism to pathogens that damage these functions [141]. The inflammatory status caused by the imbalance of microbiota in obesity may also indirectly impact the bioavailability of micronutrients. For example, levels of hepcidin that are known to depend on this status have an impact on the absorption of Fe [40]. Other than regulating intestinal absorption, gut microorganisms could be considered a bioreactor in the human intestinal tract, transforming various compounds into beneficial or harmful metabolites, thus having a crucial role in maintaining the availability of micronutrients for use and storage in the body [142]. Some of the water-soluble vitamins may be synthesized de novo by the microbiota. For example, vitamin B12 is produced by _Propionibacterium freudureichii_, _Salmonella enterica_, and _Listeria innocua_; some lactobacilli, such as _L. reuteri_; and others [143], while bifidobacteria, especially _B. bifidum_ and _B. longum_, are renowned for the synthesis of vitamin B9 in high concentrations [144]. Other B-group vitamins produced by beneficial members of intestinal microbiota that may impact the nutritional status of the host include thiamin (B1) [145] and riboflavin (B2) [146]. Gut microbiota was shown to influence the bioavailability of fat-soluble vitamins in several studies. Some microorganisms were capable of performing ex novo synthesis of carotenoids through the mevalonate pathway, as well as the production of proteins involved in vitamin A transport into the enterocytes [147]. Similarly, the intestinal uptake of vitamin D was improved by microbial stimulation of the vitamin D receptor expression [148], as well as through modulation of the activity of fibroblast growth factor 23 produced mainly by osteocytes and osteoblasts [149]. Furthermore, gut microbiota was shown capable of initiating the cytochrome P450-dependent metabolism of vitamin E in the intestinal lumen [150], while few commensal bacteria (_Bacteroides_, _Enterobacter_, _Veillonella_, and _Eubacterium lentum_) could synthesize de novo several forms of vitamin K (menaquinones) [151]. Of importance are also metabolites of microorganisms that may either aid the absorption of micronutrients or decrease it. For example, members of gut microbiota, including pathogens such as _Salmonella_, produce lipopolysaccharidesaccharides, which contribute to the inflammatory response of the intestine and also decrease the uptake of vitamin C by downregulating the intestinal transporters of this vitamin [152]. Furthermore, microbiota, especially in dietary Fe deficiency, was shown to produce Fe-binding metabolites that helped the microorganisms utilize this micronutrient but limited its bioavailability to the host [153]. On the other hand, microbial metabolites may capture minerals and promote their intestinal uptake through active transport.