Impact of rapid maxillary expansion on nasal microbiota in mouth-breathing children: a prospective cohort study

Maxillary transverse deficiency (MTD) is a common malocclusion in children and is often associated with mouth breathing. Rapid maxillary expansion (RME) is a conventional treatment for MTD; however, its association with nasal microbiota composition in mouth-breathing children remains unclear. This study aimed to evaluate nasal microbial community composition before and after RME. This was a prospective cohort study. Thirty-two children aged 5–14 years were enrolled, including 16 with maxillary transverse deficiency (8 mouth-breathing and 8 nasal-breathing) and 16 healthy controls. Nasal swabs were collected before and one month after RME in the MTD groups and once in the control group. Microbial DNA was sequenced using the Illumina MiSeq platform. Analyses included alpha diversity, taxonomic composition, and differential abundance across groups. No significant differences in alpha diversity were observed between mouth-breathing children and controls, or before and after RME. Firmicutes, Proteobacteria, and Actinobacteria were dominant across groups, and compositional differences were observed after RME. In the mouth-breathing subgroup, the relative abundance of Streptococcus significantly decreased after RME (P < 0.05), while the relative abundance of Actinobacteria increased. Overall, post-RME samples in the mouth-breathing subgroup showed compositional shifts, including a trend toward a profile more similar to that observed in healthy controls. RME was associated with short-term changes in nasal microbiota composition while overall alpha diversity remained relatively stable. In mouth-breathing children, post-RME compositional shifts included reduced Streptococcus relative abundance. These findings should be interpreted as exploratory associations, and further studies with longer follow-up, standardized breathing-pattern reassessment, and clinical/inflammatory outcome measures are needed to clarify their clinical significance. Chinese Clinical Trial Registry (ChiCTR), ChiCTR2500112875, registered on November 20, 2025. Retrospectively registered. Available at: https://www.chictr.org.cn/showproj.html?proj=290888.