The element mercury (Hg) is a nonessential metal, recognized as a global pollutant, and exists naturally in the environment. Hg enters the environment due to the natural decomposition of minerals in soils and rocks via exposure to water and wind. The release of Hg from natural resources has been relatively uniform over the years . Hg is not naturally present in food products, while it may be distributed in food chains by smaller organisms used by humans, such as fish. Shellfish and fish are the major sources of the mentioned element . Hg is not commonly found in plants; however, when mercury-containing sprays are used in agriculture, it can enter the human body through crops and vegetables . This toxic element can be present in metal form, as an organic mercury compound, or as a mercury salt. Studies have shown that exposure to high levels of Hg can damage the heart, kidneys, lungs, brain, and immune system [4, 5]. FAO and WHO specify the exposure level of mercury as a Provisional Tolerable Weekly Intake (PTWI) of 1.6 µg/Kg of bodyweight . Many studies have revealed that the mentioned metal generates oxygen radicals and induces oxidative stress [7, 8]. Oxidative stress is a phenomenon that occurs as a consequence of an imbalance between the production of free radicals and antioxidant defences. Hg affects the antioxidant defence system through depletion of glutathione, affecting several enzymes like superoxide dismutase (SOD), glutathione peroxidase (GPx), and increasing DNA, protein, and lipid oxidation [9, 10]. Chelation therapy is used to remove heavy metal ions that are toxic to the human body. According to the side effects of chemical chelators (like vomiting, appetite loss, nausea, anorexia, and diarrhea) , research efforts have been directed towards bioremediation of heavy metal pollution.
The term “symbiotic” represents a dietary supplement combining both prebiotics and probiotics, which work synergistically to increase the colonization and survival of intestinal microbiota as well as probiotic microorganisms in the gastrointestinal (GI) tract .
Prebiotics are food components that pass undigested through the GI tract and have health benefits for the host by stimulating the activity and growth of advantageous microbiota, especially probiotics. Inulin is a group of fructooligosaccharides which is extracted from the chicory root and are recommended as a prebiotic compound .
Probiotics are living microorganisms that sufficient amounts consumption of them causes health benefits for the consumer . A number of lactic acid bacteria (LAB) like Bifidobacterium, Lactobacillus plantarum, and Lactobacillus acidophilus are recognized as the most common and important probiotics. Because of the nature and membrane compositions of lactobacilli, they are known as natural adsorbents for metals. Recently, much of the concern has been focused on the sensitivity of these probiotic species to the normal physiological conditions, including bile salts and acidity of the stomach . Therefore, a new probiotic has been introduced that is able to withstand the extreme conditions of the GI tract. It has been indicated that Bacillus coagulans is resistant, can survive in harsh conditions through spore production, and is also recognized as safe by the Food and Drug Administration (FDA) [16, 17].
Several studies have indicated the heavy metal biosorption ability and antioxidant impact of probiotic bacteria. It is stated that L. plantarum ID9263 and L. rhamnosus LC105 exhibited the potential to bind copper, cadmium, and lead in polluted water [18, 19]. Majlesi et al.  introduced B. coagulans and L. plantarum as suitable candidate species for the reduction of mercury toxicity in rats. Furthermore, Al-Wabel et al.  demonstrated the protective effects of feeding synbiotic fermented milk against the toxicity of lead acetate in rats. They demonstrated that this synbiotic diet could effectively protect the liver against oxidative damage by enhancing the activity of glutathione-S-transferase. In our previous study, we indicated the effectiveness of a synbiotic diet in preventing acute cadmium toxicity in a murine model . Hence, the objective of the current study was to compare the impacts of two probiotics, B. coagulans and L. plantarum, along with a prebiotic, inulin, to alleviate the toxic impacts of mercury in rats.