## 2 Materials and Methods  
The study was performed on a group of 40 patients. The criteria for inclusion in the study were patients with periodontitis stages 3-4, with at least 20 teeth present on the arches and the study excluded smoking patients, patients with systemic diseases that could interfere with periodontal status, patients who have followed by antibiotic/antiseptic therapy or periodontal therapy in the last 3 months prior to the start of the study, and pregnant or lactating patients.  
The study methodology was explained to each patient and signed informed consent was obtained. The methodology of the study complied with the rules set out in the Helsinki Declaration and approved by the \"[PERSON]\" UMPh Ethics Committee/30 July 2020.  
Each patient underwent a rigorous clinical examination to determine the following periodontal parameters: probing depth (PD), clinical periodontal attachment loss (CAL),and bleeding on probing index (BOP). The measurements were performed by two experienced and calibrated examiners, with an agreement of 99.85%, using the periodontal probe (Williams Color-Coded Probe PQW, Hu Friedy Mfg. Co., LLC, Chicago, IL, USA).  
In order to eliminate the inter-individual variability, a split-mouth design was used for the therapeutic approach. Full-mouth ultrasonic scaling (Woodpecker UDS-A-LED, Guilin Woodpecker Medical Instruments Co., Ltd., Guangxi, China) and root planing (Gracey curettes, Hu Friedy Mfg. Co., LLC, Chicago, IL, USA) were performed in one session for each subject.  
In addition, local application of _L. reuteri_ to periodontal pockets of two quadrants per patient was performed. The _L. reuteri_ treatment was randomly assigned to either the right or the left halves of the dentition. The applied solution comprised 10\({}^{8}\) living CFU _L. reuteri_ strain DSM 17938 per 0.2 mL (Protectis(r), BioGaia, Stockholm, Sweden), with a mean volume of 0.96 \(\pm\) 0.3 mL per subject. For the application, each site was isolated with cotton rolls, gently dried and _L. reuteri_ solution was instilled in the pocket with a blunt syringe. The area was kept isolated for 10 min. The procedure was performed in the same session with SRP and repeated at 7, 14, 21, and 28 days, respectively. Forty-four patients were initially enrolled in the study but 4 of them (9.09%) could not follow all the treatment sessions and were, therefore, excluded. The clinical measurements were reprised at 3 months after the treatment completion (4 months from baseline).  
Oral hygiene instructions were given in terms of toothbrushing and dental flossing and the patients were required to avoid the use of antiseptics, antibiotics, anti-inflammatory drugs or any other probiotics.  
All the data were registered for each subject and statistically analyzed (Microsoft Excel 2021 and Wizard 2 for Mac, [PERSON](r)). A power analysis was performed for CAL as a parameter, estimating an 8% reduction, with a 90% power and alpha 0.05, generating a result of 32 subjects dimension group; we also estimated a 10% rate of abandon. A Shapiro-Wilk test was conducted as normality test. Normally distributed values were compared with the paired _t_-test and for non-normally distributed values we used the Mann-Whitney U test.