## 6 Clinical Considerations on Probiotics' Effectiveness  
In general, for a strain to serve as a probiotic, it should be capable of firmly attaching to the oral surfaces [94]. However, _Lactobacillus_, present weak adhesiveness on the tooth structure [95]. The latter raises a variety of speculations around their long-term restraint in retention sites. Data from research studying the effect of probiotics on their saliva concentration and their tooth structure content are limited. However, according to [PERSON] et al., during the second week after the discontinuation of probiotic treatment with a yoghurt supplemented with LGG, LGG's salivary counts show a decrease in subjects who were following the treatment [20]. Likewise, [PERSON] et al. investigated LGG, _L. acidophilus_ and _B. bifidum_'s ability of adhesiveness on the tooth structure in vitro and in vivo [96]. In vitro data suggested that LGG possesses a by far inferior ability of adhesion to the clear enamel (without salivary pellicle) and the pellicle-coated enamel compared to the corresponding ability _L. acidophilus_. This difference was attributed to the hydrophilic character of LGG. Salivary samples collected from individuals who were subjected to the daily intake of these bacteria through a bio-yoghurt were free of Lactobacillus. It was concluded that the ecological conditions in the oral cavity of test persons were unfavorable for Lactobacillus to grow, as temporary colonization could not be achieved even in individuals without any evincible amounts of Lactobacillus. Another study demonstrating the temporary colonization of the oral cavity by probiotic Lactobacillus is [PERSON] et al. [97]. The authors investigated if _S. thermophilus_ and _L. bulgarcus_-containing yoghurt presented any activity against the oral microbiome regarding whether these probiotics could colonize the human oral cavity. Some activity against oral Streptococcus was detected, but this has not resulted from the probiotics' colonization because it elapsed once the treatment was discontinued. [PERSON] et al., except for demonstrating LGG's inability to colonize young adults' oral cavity, signalized that permanent LGG colonization is possible, providing that early administration in childhood has taken place [98]. [PERSON] and [PERSON] [99] explained that the latter finding was correlated with the instability of the resident microbiota in children [100]. After that, many studies were conducted to examine the possible long-term effect of probiotics in the child population. These trials are synopsized in Table 3.  
One study mentioned above evaluated the possible correlation between probiotic use in combination with an agent controlling the oral microbiota [23]. [PERSON] et al. tested this eventuality by combining the salutary advantages of chlororexidine (CHX) in oral microbiome control using LGG. They concluded that CHX increases--at least 5 weeks after ceasing the regimen--the stability of LGG oral colonization. [PERSON] et al. evaluated the influence of _B. animalis lactis_ Bb-12 (Bb-12) early administration on _S. mutans_ and Bb-12 oral colonization [45]. Subjects 1-2 months old in the test group received tablets with the probiotic strain, whereas control groups consumed xylitol (X) and sorbitol (S) in the same manner. The whole regimen lasted until the infants became 2 years old. Qualitative PCR showed bare and no Bb-12 oral colonization at 8-month-old and 2-year-old children, respectively, significantly reduced _S. mutans_ levels, and had no effect on Lactobacillus. Such results were extracted in a study carried out one year later for the strain _L. paracasei_ F19 [34]. In 2014, [PERSON] and his team studied whether caries prevalence could be subverted through oral probiotic administration before establishing the oral microflora [28]. It is worth mentioning that although a significant reduction in caries incidence was reported, no caries-associated microbiological alterations were observed. Other strains of _L.