During the COVID-19 pandemic, the use of face masks was widely recommended. This study sought to characterize the oral microbiome present on the inner surface of masks, evaluate the abundance of specific bacterial species, and assess whether xerostomia influenced microbial composition. The study involved 55 generally healthy adults (45 females and 10 males; mean age 38.18 ± 12.49 years). For each participant, unstimulated (UFR) and stimulated (SFR) saliva flow rates were measured, and saliva samples were collected. Fourteen major oral bacterial species—including Porphyromonas gingivalis (P. gingivalis), Lactobacillus casei (L. casei), Tannerella forsythia (T. forsythia), and Treponema denticola (T. denticola)—were quantified on both the inner surface of masks and in saliva using real-time PCR. The findings indicated that total bacterial DNA was significantly greater in both UFR and SFR than on the mask surface (p < 0.001). Among bacteria on the mask, P. gingivalis was the predominant Gram-negative species, while L. casei dominated among Gram-positive species. Microbial profiles on the mask differed from those in saliva samples. Shannon’s diversity index was markedly higher in UFR and SFR (2.64 ± 0.78 and 2.66 ± 0.76, respectively) compared with the mask (1.26 ± 1.51, p < 0.001), and a strong positive correlation existed between UFR and SFR diversity (r = 0.828, p < 0.001), whereas no significant relationship was observed with mask diversity. The abundance of Red Complex bacteria (P. gingivalis, T. forsythia, and T. denticola) was greater in UFR than on masks. Notably, xerostomia did not significantly impact bacterial counts, total DNA, or diversity measures (p > 0.05).In summary, oral bacteria were transferred to and persisted on the inner surfaces of masks, but xerostomia had no discernible effect on microbial composition. Although masks harbored a unique oral microbiome, both bacterial load and diversity were lower than in unstimulated or stimulated saliva.
