Are ‘xerostomia’ and ‘hyposalivation’ the Same Thing? A Microbiological Narrative Review of Distinct Dry Mouth Aetiologies and Mechanisms

Objectives: The aim of this narrative review is to summarise the current evidence on alterations to the oral microbiome across different causes of dry mouth (Sjögren’s syndrome, rheumatoid arthritis, diabetes mellitus, tobacco smoking, head and neck radiotherapy, physiological ageing and neuroactive medications) and to evaluate the therapeutic potential of targeted probiotics. Methods: A structured literature search was conducted across PubMed, Scopus and Web of Science from inception to 2026. Out of the 384 records identified, 68 primary studies and reviews investigating the oral microbiological profiles of clinically defined xerostomia/hyposalivation cohorts were selected for qualitative synthesis. Methodological limitations were appraised using SANRA guidelines and domain criteria adapted from JBI checklists. Results and Conclusions: The evidence reveals a two-tiered dysbiotic pattern: a universal ‘stress signature’ characterised by acid-tolerant, cariogenic taxa (e.g., Streptococcus mutans and Lactobacillus spp.) and non-specific Candida albicans, driven by reduced salivary buffering. This is overlaid by distinct, aetiology-specific profiles. Specific pathomechanisms cause unique shifts in the microbial composition, such as the presence of citrullinating pathogens (e.g., Aggregatibacter actino-mycetemcomitans) in rheumatoid arthritis, the presence of opportunistic Staphylococcus aureus in physiological ageing and the presence of altered pioneer commensals (e.g., Streptococcus salivarius) in drug-induced receptor blockade. It is crucial to differentiate between generic hyposalivation responses and disease-specific microbial profiles in order to develop targeted diagnostic biomarkers and personalised probiotic therapies.