**4.1 Metal chelating ability**

Some probiotics exert their antioxidant potential by preventing metal ions from oxidizing. They use chelators to trap metal ions. These chelators include bathophenanthroline disulfonic acid (BPS), desferrioxamine, and ethylene diamine tetraacetic acid (EDTA) [45].

Different strains of probiotic bacteria were studied for this antioxidant mechanism; for example, among the various strains capable of chelating iron (II) or copper (II), the strains of *Lactobacillus casei* KCTC 3260 and *Streptococcus thermophilus* 821 have a much higher ability to chelate iron (II) and copper (II) [46]. Also, the high potency of *Lactobacillus helveticus* CD6 intracellular cell-free extraction in chelating iron (II) ions can be mentioned [47].

These chelating agents are not fully understood in probiotic bacteria. However, studies have shown their role in inhibiting phosphate ester displacement enzymatic reactions, as well as the production of radicals (such as peroxyl radical and alkoxyl radical) due to the decomposition of hydroperoxide compounds [48].
