Selection-driven shifts in airborne bacterial communities and potentially pathogenic genera across environmental and PM_2.5 gradients in urban Beijing

Air pollution poses the greatest public health threat among environmental risks, and fine particulate matter with diameter ≤2.5 μm (PM_2.5) is one of the most harmful air pollutants. PM_2.5 is often associated with airborne microorganisms, making their investigation important for both microbial ecology and potential exposure assessment. In this study, airborne bacterial communities in PM_2.5 samples collected during autumn and winter in urban Beijing were analyzed using 16S rRNA gene sequencing, null-model community assembly analysis, and co-occurrence network analysis. Distinct differences in microbial communities were observed between autumn and winter sampling periods based on PCoA and PERMANOVA analyses (p < 0.05), whereas no consistent differences between daytime and nighttime samples were detected. Community assembly shifted from stronger stochasticity in autumn to greater homogeneous selection in winter, with homogeneous selection contributing 47.22% and 53.33% in winter daytime and nighttime samples, respectively. We further characterized microbial composition and identified potentially pathogenic genera under different environmental conditions. Most pathogens were less abundant in winter. However, Streptomyces, Saccharopolyspora, and Nocardiopsis were enriched under low temperature and absolute humidity conditions, and their abundance further increased with high PM_2.5 levels, as supported by microbial community composition analyses. Co-occurrence network analysis further revealed more interconnected positive microbial associations in autumn, whereas winter communities exhibited greater modularity and increased negative correlations under colder and drier conditions. These findings provide insights into airborne microbial diversity, community assembly, community composition, and co-occurrence patterns across contrasting environmental conditions and their associations with air quality, with potential relevance for understanding airborne microbial exposure and respiratory health concerns in urban environments.