Elucidating airborne bacterial communities and their potential pathogenic risks in urban metro environments

Metros are the predominant mode of transportation for urban residents. Because of high passenger volume and pollutant concentrations, concern is growing regarding the potential health hazards of exposure to potential pathogenic airborne bacteria in metros. However, the risks of airborne bacterial communities in metros have not been assessed. Therefore, this study was conducted to explore the airborne bacterial communities and potential pathogenic risk of bacteria in the inner metro train (IM) and metro stations (MS) in Busan, South Korea. The concentrations of culturable total airborne bacteria (CABs) and culturable total airborne Staphylococcus (CAS) were higher in the MS samples than in the IM samples. Bacterial community analysis revealed that although the overall metro environment was dominated by human-associated bacteria, such as Corynebacterium and Staphylococcus genera, the IM and MS samples exhibited significantly distinct core bacterial taxa despite their similar bacterial communities; this is a result of human activity rather than the presence of passengers. Through multilocus sequence typing (MLST), the isolated S. epidermidis from both the IM and MS samples was identified as a human pathogen with four sequence types (ST190, ST54, ST992, and ST817). Furthermore, the MLST results were significantly positively correlated with the CABs and CASs in both the IM and MS samples. The S. aureus infection pathway was predicted in all samples using PICRUSt2 and was significantly higher in the IM samples than in the MS samples. The findings of this study can serve as a reference for developing microbial public health provisions for metro systems.