_applied sciences_  
_Article_  
Clinical Effects of Locally Delivered _Lactobacillus reuteri_ as  
Adjunctive Therapy in Patients with Periodontitis:  
A Split-Mouth Study  
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phenomenon included in the group of gingivitis. If left untreated, inflammatory and bacterial products can affect the supporting periodontal tissues, such as periodontal ligaments, cementum, or alveolar bone, with gingivitis evolving into periodontitis, a disease that can have as a negative endpoint teeth loss [5].  
Etiological periodontal therapy addresses precisely these factors that lead to the onset and evolution of destructive periodontal phenomena. The gold standard in periodontal therapy remains scaling and root planing (SRP), a non-surgical debridement therapy that aims to disorganize the bacterial biofilm and create biologically acceptable surfaces that allow the formation of a new periodontal attachment and prevent bacterial adhesion [6]. Over time, various forms of adjunctive therapy have been associated with SRP in order to improve the results and response of the human body to standard therapy. These forms of treatment include topically or systemically administered antibiotic/antiseptic therapy, or photoactivation therapy, among many other methods [7; 8].  
Particular attention was paid to the use of probiotics as an additional means in periodontal etiological therapy. Probiotics are viable microorganisms that, when administered in adequate amounts, provide a health benefit to the human body. The use of probiotics has been shown to be effective in controlling intestinal diseases and appears to act by resisting colonization and/or modulating the immune system [9]. Regarding the use of probiotics for the additional therapy of various diseases of the oral cavity, the strains of _Lactobacillus_, _Streptococcus_, and _Bifidobacterium_ are the most frequently investigated [10].  
A number of mechanisms of action have been proposed for probiotics, phenomena which include: (a) Direct interactions within the oral bacterial plaque, disruption of plaque biofilm formation by competition for binding sites on host tissues and for nutrients; probiotic species are also thought to produce a number of antimicrobial compounds (organic acids, hydrogen peroxide, peptides, bacteriocins, and anti-adhesion molecules) [11]; (b) Modulation of both innate and adaptive immune function, with an alteration of cytokine production and subsequent effects on general immunity [11]. Moreover, some probiotic species may improve mucin production and barrier function, regulate host defense peptides, as well as promote angiogenesis and wound healing [12].  
The most common bacterial strains used as probiotics include the _Lactobacillus_ species, and the genus reuteri appears to generate the most significant results and is the strongest of all [13]. _L. reuteri_ ATCC 55730 was originally isolated from breast milk and may be present in humans on the lining of the gastric body and antrum, duodenum, and ileum [14]. _L. reuteri_ ATCC 55730 was found to exhibit a potentially transferable resistance trait for tetracycline and limomycin. Therefore, it was replaced with _L. reuteri_ DSM 17938, a strain without undesired resistance [15].  
Given the presented benefits of probiotics, starting from the hypothesis that their local administration in periodontal pockets would generate beneficial effects, we conducted an interventional study aimed at evaluating periodontal clinical parameters before and after supplementing SRP with _L. reuteri_ DSM 17938 applied in periodontal pockets in patients with stages 3-4 periodontitis.