Indoor Airborne Microbial Community And Influencing Factors In Schools In Nanjing

Indoor airborne microbial pollution is strongly associated with many respiratory diseases and allergic symptoms.As public studying places,schools usually harbor many students for a long time in classrooms,dormitories and etc.And schools have the characteristics of high personnel density,narrow space and poor ventilation,which easily results in microbial pollution in indoor air.Therefore,it is of great significance to study the characteristics of indoor airborne microbial communities in schools.Moreover,many factors such as relative humidity,occupancy and ventilation in the built environment have been proven to play important roles in structuring microbial communities indoors.So the analysis of influencing factors of indoor airborne microorganisms could help formulate preventive measures to bring microbial pollution under control in schools.In this study,the characteristics of indoor airborne microbial communities were investigated at four representative schools,i.e.kindergarten,primary school,middle school and university in Nanjing by both cultivation method and high-throughput sequencing method.First,the six-stage Anderson sampler was used to survey indoor airborne culturable microbial concentration and size diatribution by the cultivation method.Results showed that airborne microbial concentration in summer was significantly higher than in winter for both bacteria and fungi.Differences in microbial pollution were also observed among different schools.The highest bacterial and fungal concentration in summer were found in the university（990CFU/m³）and the primary school（681 CFU/m³）respectively.In winter,among these schools,the kindergarten had the maximum concentrations of airborne bacteria and fungi with the mean values of 575 CFU/m³ and 666 CFU/m³,respectively.In respect to the size distribution,the peak concentrations appeared at either stage 4（2.1μm～3.3μm）or stage 5（1.1μm～2.1μm）.The correlation analysis was conducted between indoor tempertature,relative humidity,carbon dioxide,PM_2.5and indoor culturable microorganisms.It was found that temperature,relative humidity and PM_2.5 were strongly correlated with airborne microbial concentration,while the relationship between carbon dioxide and microbial concentration was relatively weak.Then,the High Bio Trap sampler was employed to investigate the community structures of indoor and outdoor airborne microorganisms in the kindergarten,the primary school and the middle school by the high-throughput sequencing method.Results showed that there were significant differences in the microbial community structures between summer and winter.In addition,indoor environment in schools was affected by outdoors,making indoor and outdoor microbial community structures similar.The community structures of indoor airborne microorganisms in these different schools were similar.The dominant bacterial genera in schools included Bacteroides and Bifidobacterium.And the dominant fungal genera included Fusarium and Guehomyces.Apart from this,the relative abundance of some human-associated microbial genera detected in this study was low.Pathogenic microorganisms were also detected in schools.The airborne pathogenic bacteria in schools included Streptococcus＿pneumoniae,Bacteroides＿fragilis and Bacillus＿cereus.The detected pathogenic fungi were Fusarium＿oxysporum,Candida＿albicans and Aspergillus＿flavus.The VPA analysis indicated that relative humidity had more impact on community structures of indoor airborne microorganisms than temperature,carbon dioxide and PM_2.5.Finally,the effects of air conditioners and natural ventilation on indoor airborne microbial communities in schools were analysed.Compared with the rooms where air conditioners were turned off,concentrations of airborne culturable microorganisms in the rooms with air conditioners turned on were higher.On the other hand,in the condition of long-term natural ventilation,indoor airborne culturable bacterial concentration could be diluted,but culturable fungal concentration might not necessarily decrease.The reason is that the source environment of indoor bacteria and fungi is different.In addition,the use of air conditioners and natural ventilation had no impact on the composition of dominant genera in the microbial communities.But the relative abundance of these dominant genera was indeed affected by the above two factors,and the variation trend of relative abundance depended on different species.