## 5. Probiotic Strains Used in Yogurt and Related Probiotic Milk Beverages and Their Health Benefits  
One question a product developer working with yogurt needs to ask is whether to use a single strain probiotic or to use multiple strains for forming potential symbiotic relationships (similar to the relationship between _S. thermophilus_ and _L. bulgarcius_) or for potential health benefits. Peng et al. [234] manufactured yogurt with incorporated _L. casei_ Zhang, _B. lactis_ V9, or their combination. The use of this combination stimulated the growth of _B. lactis_ V9 compared to use of _B. lactis_ V9 by itself, likely because of valine, leucine, and isoleucine biosynthesis. However, the use of this combination did not stimulate the growth of _L. casei_ Zhang compared to the use of _L. casei_ Zhang by itself. Furthermore, the use of this combination stimulated short-chain fatty acid production. In a similar type of experiment, Fan et al. [50] manufactured yogurt with incorporated _Lacticaseibacillus casei_ CGMCC1.5956, _Levilactobacillus brevis_ CGMCC1.5954, or their combination. They found improved probiotic growth, increased hardness and adhesiveness, less syneresis, and enhanced antioxidant capacity in the yogurt prepared with both probiotics. In another study, Fan et al. [235] found that use of binary probiotics (_Lacticaseibacillus casei_ CGMCC1.5956 and _Lactiplantiobacillus plantarum_ subsp. _plantarum_ CGMCC1.5953) enhanced hardness, viscosity, and gumminess compared to use of these probiotics by themselves. McFarland [236] reviewed whether single strains or multiple strains are more effective in preventing and treating diseases. Although there were cases in which multiple strains were more effective than single strains in eradicating diseases, multi-strain mixtures were not usually more effective than single strain probiotics. However, Washburn et al. [237] reported that microbial gastrointestinal diversity was not significantly influenced in their study when healthy adults consumed _Bifidobacterium infants_ as a single probiotic species. It has been debated as to whether or not the yogurt starter cultures, _S. thermophilus_ and _L. bulgarcius_, should be considered as probiotic [238]. Obviously, if the yogurt is heat treated to kill the cultures after fermentation, then it is not probiotic yogurt. One would not consider yogurt starter cultures as probiotic if they were not acid and bile tolerant or if they did not survive within the intestinal tract [238]. In an early study, Cheplin and Rettger [239] were not able to implant _Bacillus bulgarcius_ (now _L. bulgarcius_) into the human gastrointestinal tract. However, Mater et al. [240] and Elli et al. [241] found that _L. bulgarcius_ and _S. thermophilus_ can survive within the human gastrointestinal tract. Martinovic et al. [242] reviewed whether or not _S. thermophilus_ survives within the gastrointestinal tract and concluded that most studies did not perform taxonomic studies with sufficient accuracy for distinguishing _S. thermophilus_ from _S. salivarius_ to determine if _S. thermophilus_ can be recovered. Uriot et al. [243] supported the idea that certain strains of _S. thermophilus_ be considered as probiotic, and Guarner et al. [244] likewise concluded that both _S. thermophilus_ and _L. bulgarcius_ should be considered as probiotic. Popovic et al. [143] showed that _S. thermophilus_ BGKMJ1-36 and _L. bulgarcius_ BGVLJI-21 can function as yogurt starter cultures and possess probiotic properties by modulating gut autophagy and improving the gut epithelial barrier. Recently, Taj et al. [245] found that certain exopolysaccharide producing strains (RIRT2, RIH4, and RIV) of _S. thermophilus_ fulfill the basic criteria to be considered as probiotics. In this paper, the emphasis is on yogurt that has probiotic cultures in addition to _S. thermophilus_ and _L. bulgarcius_. _Lactokacillus acidophilus_ NCFM (North Carolina Food Microbiology) (ATCC 700396) has been available since 1972 and is GRAS (GRN Number 357).