The human gut microbiota directly affect human health. Alterations in the composition of commensal bacteria can lead to chronic inflammation also predisposing individuals to fungal infections. Commensal fungi are important in human health and changes in commensal fungal populations have been shown to deeply affect pathologies not directly related to fungi, such as Inflammatory Bowel Diseases. Rett syndrome (RTT) is a neurodegenerative disorder caused by a mutation in MECP2. High proportion of RTT subjects suffer of gastrointestinal disorders, implying a link between RTT and the gut microbiota. We studied the bacterial and fungal gut microbiota of patients affected by RTT by a multilevel approach to understand whether changes in gut microbiota of RTT patients could be associated with GI abnormalities, inflammatory status and cytokine dysregulation previously observed. Our findings revealed that RTT patients harbor a distinct and less diverse bacterial and fungal microbiota compared to healthy subjects in which Bifidobacterium and Candida are the most abundant genera. The immunological responses exerted by RTT yeast isolates in our in vitro cell model suggest that fungi could contribute to the gastrointestinal abnormalities and intestinal inflammation observed in our RTT study cohort.