**5. Lactic acid bacteria as source of vitamins**

Human life cannot exist without vitamins, because they are involved in essential functions e.g. cell metabolism and antioxidant activities. Humans cannot synthesize most of these vitamins, although it is well known that some intestinal bacteria like LAB can produce some vitamins (folate, vitamin B12 or cobalamin, vitamin K2 or menaguino, riboflavin and thiamine) [128,129]. The gut microbiota has been recognized as a source of some watersoluble vitamins, while such vitamins have also been reported as results of the LAB fermentation in yogurt, cheeses and other fermented foods.

### **5.1. Folate**

Folate is the term used to describe the folic acid derivatives, such as the folyl glutamates which are naturally present in foods and folic acid that is the chemically synthesized form of folate, commonly used for food fortification and nutritional supplements. Folate belongs to the B-group of vitamins and participates in many metabolic pathways like the biosynthesis of DNA and RNA and the inter-conversions of amino acids. Moreover, folate possesses antioxidant capacity that protects the genome by preventing free radical hack of DNA [130].

Dietary folate is essential for humans, since it cannot be synthesized by mammalian cells. Folate can be found in legumes, leafy greens, some fruits and vegetables, in liver and fermented dairy products [131], especially in yogurts, where it may be increased depending on the starter cultures used and the storage condition, to values above 200 μg / lt [132]. Epidemiological studies indicated that folate deficiency is associated with a variety of disorders like Alzheinmer's disease, coronary heart diseases, osteoporosis and increased risk of breast and colorectal cancer [130,133].

LAB having the ability to produce folate belong to the *Lactobacillus spp (L. lactis, L. plantarum, L. bulgaricus), Streptococcus spp.* and *Enterococcus spp*. Nevertheless, some lactobacilli strains (*L. gasseri, L. salivarius, L. acidophilus* and *L. johnsonii*) used as both starter cultures and probiotics, cannot synthesize folate due to their lacking in some genes involved in folate biosynthesis [130]. Furthermore, it has been reported that some starter cultures and probiotic lactobacillus strains in non-dairy foods utilize more folate than they produce [128,130]. For this reason nowadays the food industry focuses on the strategy to select and use folate producing probiotic strains, to produce fermented products with elevated amounts of "natural" folate concentrations, without increasing production cost, although increasing health benefits [130,133].
