LAUSR

Purpose Oral mucositis (OM) remains a significant complication developed by many patients undergoing radiotherapy (RT) to the head and neck region. Emerging data suggest oral microbes may contribute to the onset and severity of this acute side effect. Methods In this study, saliva and oral swabs from head and neck cancer patients undergoing RT were collected. We employed molecular microbiological techniques to study the bacterial communities present in saliva, and both the bacterial and fungal communities present on the buccal mucosa and lateral tongue. Changes in microbiota composition with increasing radiation dose and the presence of mucositis were examined. Results The data suggest that the salivary microbiota remain stable during RT and are consistently dominated by Streptococcus , Prevotella , Fusobacterium and Granulicatella . Obligate and facultative anaerobic Gram-negative bacilli (GNB) Bacteroidales G2, Capnocytophaga , Eikenella , Mycoplasma and Sneathia , as well as anaerobic GNB in the periopathogenic genera Porphyromonas and Tannerella , were all positively correlated with ≥ grade 2 OM. Significant increases in the relative abundances of Bacteroidales G2, Fusobacterium and Sneathia were identified in buccal mucosa swabs at sites of ≥ grade 2 OM ( p  < 0.05). Furthermore, the abundance of several GNB ( Fusobacterium , Haemophilus , Tannerella , Porphyromonas and Eikenella ) on the buccal mucosa may influence patient susceptibility to developing OM. Candida was widely detected in buccal mucosa swabs, regardless of mucositis status. Conclusions Our findings support previously hypothesized associations between oral health and the pathogenesis of OM, highlighting the importance of oral health interventions for head and neck cancer patients.