Concurrent measurements of indoor and outdoor airborne bacteria in university classrooms at a tropical megacity: concentration and bacterial composition

Abstract

Airborne microbial concentrations in educational buildings are a growing concern due to their impact on student health and learning. This study determined atmospheric bacterial concentrations and composition in indoor and outdoor environments at a university in Ho Chi Minh City, Vietnam. Samples were collected from schoolyards, mechanically and naturally ventilated classrooms, during student presence and absence, March to December 2023. Plate Count Agar was used for bacterial concentration, while 16S rRNA sequencing determined bacterial composition. Indoor concentrations of cultured bacteria ranged 158–541 CFU/m³ , generally lower than outdoor levels 174–718 CFU/m³. Concentrations were higher in occupied rooms than vacant ones and lower indoors compared to outdoors. Most indoor samples were below standards such as the European Commission, WHO and Vietnam (less than 500–1000 CFU/m³). Temperature and relative humidity showed a positive and negative correlation with bacterial concentration (r = 0.348 and -0.628, respectively). Sequencing revealed 37 phyla, 1115 genera, and 1183 species, with Actinobacteriota (32.4%), Proteobacteria (30.6%), and Firmicutes (24.1%) dominating. Alpha diversity showed no significant difference between mechanically (10.36) and naturally ventilated (10.87) rooms. Our findings indicate that human activity may contribute to an increase in bacterial concentrations, whereas mechanical ventilation appears to mitigate this trend. Mechanical ventilation was effective in controlling bacterial concentrations; however, it did not result in significant changes to the bacterial community composition compared to natural ventilation. The findings in this study could contribute to the increasing awareness of the need to maintain optimal air quality in university classrooms, both in the region and nationwide.