Does Diabetes Influence the Microbiological Response to Non-Surgical Periodontal Therapy? A Systematic Review

Background/Objectives: The aim of this systematic review was to synthesise evidence on how non-surgical periodontal therapy (NSPT) alters the oral microbiota and periodontal parameters in patients with diabetes and periodontitis compared with non-diabetic patients with periodontitis. Methods: Electronic searches of MEDLINE/PubMed, Scopus, Embase, Ovid Emcare, JBI EBP, Web of Science, Dentistry and Oral Sciences Source, and grey literature sources were conducted from inception to March 2025. Eligible studies were clinical studies that included participants with diabetes and periodontitis, used NSPT as the primary intervention, and reported pre- and post-treatment oral microbiological and periodontal parameters as outcomes. The risk of bias was assessed using the Risk of Bias 2 (RoB 2) tool for randomised trials, and ROBINS-I for non-randomised studies and certainty of evidence was evaluated with GRADE. The review protocol was registered with PROSPERO (CRD420251005668). Results: Fourteen studies involving 374 participants with diabetes and 276 non-diabetic participants with periodontitis were eligible for inclusion. Thirteen studies used targeted methods restricted to pre-specified taxa, and one used 16S rRNA gene sequencing. In the targeted studies, NSPT was associated with reductions in periodontal pathogens, particularly Porphyromonas gingivalis, Tannerella forsythia, and Aggregatibacter actinomycetemcomitans, which were found in both diabetic and non-diabetic groups, with small or no differences observed between the groups. There were improvements in probing depth, clinical attachment, bleeding on probing and plaque scores after NSPT in both groups, with no consistent between-group differences. In most studies, RoB 2 was rated "some concerns", and GRADE ranged from very low to moderate for microbiological and clinical outcomes. Conclusion: NSPT reduces selected periodontal pathogens and improves periodontal parameters in adults with periodontitis, with and without diabetes. Community-level microbiome evidence is limited to one small study, and the available data do not show whether diabetes modifies the microbiological response to NSPT.