## 2 Probiotics and Their Benefits
### Significant Effects of Probiotics on Oral Health Status  
Probiotics primarily contribute to gastrointestinal health, but their diversity has been increasing and now they greatly contribute to oral health care [66]. Oral disease treatment is one of the most expensive health care treatments; that is why new approaches are being used to treat these infections, such as the use of probiotics [67]; in other words, oral lactic acid bacteria have been characterized after isolation and purification to use for oral health, especially to cure periodontal diseases, caries, and halitosis (bad breath) [19].  
These ailments are some of the most widespread diseases throughout the world, but the treatment of these infections require potent antibiotics that cause severe side effects in the gastrointestinal tract. The use of probiotics as a relatively less harmful treatment has been advised to avoid such side effects [68].  
Additionally, the buccal cavity is an excellent bacteriological medium, where microorganisms can grow very easily if certain factors are out of balance due to poor oral health, poor diet, or immunodeficiency. These parameters alter the pH of the mouth and create a favorable environment for micro-biota growth. Micro-organisms are deposited on the oral cavity like biofilm and create a bacterial hold and aggressively colonize the buccal cavity [69].  
Probiotic strains of _Bifidobacterium_, _Lactobacillus_, and _Streptococcus_ are primarily used to prevent or cure oral infections. Plaque or dental biofilms in the buccal cavity leads to poor oral health, but lactic acid bacteria are probiotics that interact with that biofilm/plaque and destroy the causative agents with LAB's antimicrobial activity [70].  
Indirectly, probiotics enhance immunity and strengthen the immune system by the interaction of lactic acid bacteria with immuno-competent cells, which leads to modification in the production of cytokines and ensuing effects on the overall immune system [71]. Lactobacillus used as a probiotic in mild gingival inflammation causes a great decline of Interleukin-8 discharge in the gingival crevicular fluid [72]. Probiotics have been used in the treatment of periodontitis and gingvitis: they modify the buccal cavity homeostasis, with possible numerous health benefits such as combatting dysbiosis and moderating inflammatory pathways to minimize the inflammation of periodontitis [73; 74].  
Probiotics are also being used in the treatment of halitosis; for this purpose, bacteriocins producing _Streptococcus salivarius_ K12 strain were developed. This strain treats the mouth odor by actively helping de-colonize bacteria like _Solobacterium moorei_, _Pariomonas micra_, and _Eubacterium sulci_, which produce volatile sulfur compounds(VSCs) on the tongue [75; 76].  
A new study demonstrated the effects of widely commercially used chewing gum containing probiotic bacteria (xylitol), which greatly impacted plaque and gingival scores by reducing _Streptococcus mutants_ counts in plaque and saliva [77]. Probiotics played an essential role in the clinical manifestation of dental disorders and periodontal diseases and perhaps impacted halitosis as well. Probiotics provide long and short-term therapeutic effects. Advance studies about particular strains and oral micro-biome transplants could also increase probiotics' role or efficacy in oral health, and probiotics consumed as part of the diet may also enhance oral health and strong oral immunity or hygiene [78; 79].