Gut microbiota consists of over 100 trillion microorganisms including at least 1000 different species of bacteria and is crucially involved in physiological and pathophysiological processes occurring in the host. An imbalanced gastrointestinal ecosystem (dysbiosis) seems to be a contributor to the development and maintenance of several diseases, such as Alzheimer’s disease, depression, and type 2 diabetes mellitus. Interestingly, the three disorders are frequently associated as demonstrated by the high comorbidity rates. In this review, we introduce gut microbiota and its role in both normal and pathological processes; then, we discuss the importance of the gut-brain axis as well as the role of oxidative stress and inflammation as mediators of the pathological processes in which dysbiosis is involved. Specific sections pertain the role of the altered gut microbiota in the pathogenesis of Alzheimer’s disease, depression, and type 2 diabetes mellitus. The therapeutic implications of microbiota manipulation are briefly discussed. Finally, a conclusion comments on the possible role of dysbiosis as a common pathogenetic contributor (via oxidative stress and inflammation) shared by the three disorders.

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Approximately 30% of Parkinson's disease (PD) patients show impaired cognitive performance, which is suggestive of Mild Cognitive Impairment (MCI), representing a predictor of dementia, especially when present at diagnosis. The objective of the study was to evaluate the frequency and clinical predictors of MCI in a large hospital-based cohort of PD patients. We collected cross-sectional data from the Parkinson's disease cognitive impairment study (PACOS), a multicenter study involving two Movement Disorder centers, which are located in south Italy. The PD subjects were diagnosed according to the UK Brain Bank criteria and they underwent an extensive neuropsychological assessment. PD-MCI was diagnosed according to the Movement Disorder Society task force criteria for MCI. PD severity was evaluated in accordance with the Unified PD Rating Scale-Motor Examination (UPDRS-ME) and the Hoehn and Yahr scales. The study included 659 PD patients (57.5% men; mean age 67.0 ± 9.7 years), with a mean disease duration of 3.8 ± 4.6 years and a mean UPRDS-ME score of 25.8 ± 12.3. PD-MCI was diagnosed in 261 (39.6%) subjects and in 82 (31.7%) of 259 newly diagnosed patients (disease duration ≤ 1 year). An amnestic MCI multidomain phenotype was the most frequent MCI subtype (39.1% of the overall sample and 43.9% in newly diagnosed PD). A positive significant association between MCI, age and motor scores was found at multivariate logistic regression analysis, while a negative association was observed between educational level and MCI. In conclusion and in agreement with the literature data, the prevalence of MCI recorded in the PACOS sample was approximately 40 and 32% amongst newly diagnosed patients.

Oxidative stress (OS) has been demonstrated to be involved in the pathogenesis of the two major types of dementia: Alzheimer's disease (AD) and vascular dementia (VaD). Evidence of OS and OS-related damage in AD is largely reported in the literature. Moreover, OS is not only linked to VaD, but also to all its risk factors. Several researches have been conducted in order to investigate whether antioxidant therapy exerts a role in the prevention and treatment of AD and VaD. Another research field is that pertaining to the heat shock proteins (Hsps), that has provided promising findings. However, the role of OS antioxidant defence system and more generally stress responses is very complex. Hence, research on this topic should be improved in order to reach further knowledge and discover new therapeutic strategies to face a disorder with such a high burden which is dementia.

Background: Cognitive impairment in Parkinson’s disease (PD) includes a spectrum varying from Mild Cognitive Impairment (PD-MCI) to PD Dementia (PDD). The main aim of the present study is to evaluate the incidence of PD-MCI, its rate of progression to dementia, and to identify demographic and clinical characteristics which predict cognitive impairment in PD patients.Methods: PD patients from a large hospital-based cohort who underwent at least two comprehensive neuropsychological evaluations were retrospectively enrolled in the study. PD-MCI and PDD were diagnosed according to the Movement Disorder Society criteria. Incidence rates of PD-MCI and PDD were estimated. Clinical and demographic factors predicting PD-MCI and dementia were evaluated using Cox proportional hazard model.Results: Out of 139 enrolled PD patients, 84 were classified with normal cognition (PD-NC), while 55 (39.6%) fulfilled the diagnosis of PD-MCI at baseline. At follow-up (mean follow-up 23.5 ± 10.3 months) 28 (33.3%) of the 84 PD-NC at baseline developed MCI and 4 (4.8%) converted to PDD. The incidence rate of PD-MCI was 184.0/1000 pyar (95% CI 124.7–262.3). At multivariate analysis a negative association between education and MCI development at follow-up was observed (HR 0.37, 95% CI 0.15–0.89; p = 0.03). The incidence rate of dementia was 24.3/1000 pyar (95% CI 7.7–58.5). Out of 55 PD-MCI patients at baseline, 14 (25.4%) converted to PDD, giving an incidence rate of 123.5/1000 pyar (95% CI 70.3–202.2). A five time increased risk of PDD was found in PD patients with MCI at baseline (RR 5.09, 95% CI 1.60–21.4).Conclusion: Our study supports the relevant role of PD-MCI in predicting PDD and underlines the importance of education in reducing the risk of cognitive impairment.