Seasonal Variation of Airborne Bacteria and Potential Pathogens Adhere to Fine and Coarse Particles in a Coastal City

Airborne bacteria exhibit high taxonomic diversity and a well-documented capacity to influence human health through pathogenic infection, allergenic sensitization, and immunomodulatory or commensal exposure. However, characterization of the airborne bacterial community structure, and particularly the seasonal dynamics, size-resolved distribution, and functional potential of predicted potential pathogens in Qingdao, a coastal megacity on the Yellow Sea, remains limited. Results revealed pronounced seasonal variation in community structure: Staphylococcus, Lactobacillus, Escherichia-Shigella, and Klebsiella were consistently dominant across all samples. LEfSe analysis identified Staphylococcus as significantly enriched in autumn, supporting its utility as a robust seasonal biomarker (LDA > 3.5, p < 0.05). This genus is predominantly associated with human sources and comprises opportunistic pathogens, thereby posing a potential risk to human health. Significant compositional divergence between summer and autumn communities was observed as a higher beta deviation index, whereas there were no statistically significant differences between particle size groups. The Chao and Shannon indices were consistently higher in PM₁₀ than in PM_2.5 across all seasons; however, this size-associated difference reached statistical significance only for autumn PM₁₀ (p < 0.05), indicating a seasonally amplified effect of coarse particulate matter on microbial richness and evenness. These analyses provide new insight into the total airborne bacterial and potential pathogens in an urban coastal environment.