Sources and sinks of the air microbiome

• Main Author: Lau, Kenny Jia Xu
• Other Authors: Stephan Christoph Schuster
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Biological sciences::Microbiology
• Online Access: https://hdl.handle.net/10356/153351
• Institution: Nanyang Technological University

Microbial source tracking is necessary to study, model and predict the movement of airborne microorganisms, especially in the context of respiratory diseases, agricultural pathogens, food spoilage and mould growth in buildings. The indoor built environment, human lung, and plant surfaces were investigated as potential sources of the air microbiomes. The findings in this thesis are expected to help identify the sources and map the fluxes of bioaerosols between the built environment, humans, and plants. Source tracking was conducted at two indoor offices with poor air quality. Metagenomics was used to characterise both the indoor air and surface microbiome. Bayesian inferential analysis was performed to quantify probabilities of transfer between the air and surfaces. Computational fluid dynamics modelling was also used to examine airflow patterns within the indoor space. Results from the metagenomic, probabilistic and airflow models reveal that cellulosic, gypsum and textile surfaces are likely reservoirs of indoor air microbes in moisture-damaged buildings. Secondly, to study the microbial exchanges between the human lung and environment, source tracking was also performed on the sputum, inhalers of respiratory patients, including the air and surfaces of their residential homes. Results suggests that there was a significant amount of microbial transfer between the lung and inhaler. Inhaler surfaces harbour antibiotic resistance genes such as macrolide, beta-lactam, and multi-drug resistance genes via contact transfer with the human oral cavity. Given the similarity between the lung and inhaler microbiomes, a swab sample of the inhaler may be a good surrogate of the lung microbiome. Further findings suggest that the aircon, air purifiers and fans in homes are potential microbial sources exhibiting a modest transfer of 5-10% to the lung microbiome, whereas the inhaler shows a transfer probability of more than 80% in microbial exchanges with the lung. Lastly, plants have large surface areas of leaves that may be potential sources to the air microbiome. Source tracking investigation was performed between the leaves of outdoor plants and outdoor air. Results suggest that leaves contribute to less than 5% of the outdoor air in Singapore. Further experiments revealed that the adaxial and abaxial of the leaf surfaces harbour different microbiomes. Metagenomics profiling showed that the abaxial leaf surfaces have lesser bacteria than the adaxial counterpart. This could potentially be explained by the microbe-plant interactions where plants could deploy defence strategies to guard their stomata from phytopathogens. The leaves of two species of garden plants were sampled. Rhapis excelsa showed increased in reactive oxygen species production while Cordyline fruticosa has a hydrophobic surface nanostructure that may explain the reduction in bacteria observed on the abaxial surface. In summary, the study of sources and sinks of the air microbiome requires a combinatorial approach of metagenomics, Bayesian probability, Markov Chain and computational fluid dynamics modelling. Results from the combined analyses can better inform indoor occupants of the sources of contamination so that relevant disinfection actions can be taken. Findings in this thesis also showed that inhalers are potential reservoirs for antibiotic-resistant microbes. It was also found that the fan blades, aircon panels and air purifiers in residential homes are potential environmental sources of microbes in indoor air. Lastly, the leaf surfaces of outdoor plants only contributed to less than 5% the outdoor air, which is likely due to the low wind speeds in Singapore unlike studies conducted in temperate and coastal sites. This work can provide a basis for modelling of bioaerosols transport and their impact on public health.