Role of Indoor Chemistry on Surface Bacterial Community in the Built Environment

Abstract

Understanding the factors that shape bacterial communities on indoor surfaces is crucial, as individuals spend approximately 90% of their time indoors and frequently interact with these surfaces. However, the impact of indoor chemistry on surface bacterial communities has been largely overlooked. To fill this gap, this doctoral thesis investigated the role of cooking-derived organic compounds, sulfur dioxide (SO₂) and ozone (O₃) on surface bacterial communities under varying indoor environmental conditions through a combination of field studies and laboratory experiments.

First, since the temporal relationships between chemical and biological constituents remain unclear as most field studies have relied on single-time snapshots and seldom examined the interplay between chemical and biological dynamics, a month-long spatiotemporal field study across 20 households in Hong Kong was conducted to investigate the factors influencing chemical and biological constituents on common indoor surfaces. Among the 16 household- and occupant-related factors analyzed, routine oil-based cooking was the primary driver of microbial diversity and composition on indoor surfaces. Surfaces in kitchens with frequent cooking exhibited elevated total organic carbon levels, which were linked to increased bacterial abundance. A focused analysis of six kitchens with well-controlled frequencies of oil-based cooking revealed that cooking-derived organic compounds, particularly alkanes, promoted bacterial abundance while reducing microbial diversity. Network analysis further revealed strong interactions between these organic compounds and bacterial taxa, especially those within the Proteobacteria and Firmicutes phyla. These findings highlight the impact of routine household activities on indoor chemical–biological interactions, enhancing our understanding and informing strategies to improve indoor environments and occupant well-being.

Next, while high concentrations (≥100 ppm) are known to act as disinfectants, the effects of typical indoor concentrations (≤100 ppb) remain unclear. This study investigated SO₂ impacts on bacterial viability, biofilm formation, and community composition across nutrient gradients and relative humidity (RH, 20%–97%) using controlled chamber exposures with Escherichia coli and real-world kitchen surface communities. Bactericidal effects were strongest on loosely adherent E. coli under nutrient-poor, low-RH (20%) conditions and low cell density (10⁶ CFU/cm²), likely due to increased acidification and sulfate adsorption. At this density and nutrient level, ≥30 ppb SO₂ significantly reduced viability across all RH levels, while ≤100 ppb did not affect nutrient-rich surfaces at 97% RH or at higher densities (10⁷ CFU/cm²). Biofilm inhibition required 100 ppb, indicating greater resistance than loosely adherent cells. In kitchen surface communities, bacterial abundance declined at 30 ppb on cooking surfaces and at ≥10 ppb on non-cooking surfaces at ≤60% RH, with no effect at 97% RH. At 30 ppb, SO₂ reduced bacterial diversity and altered microbial composition, independent of surface type or RH. Ambient SO₂ evidently has an underrecognized impact on indoor-surface microbial communities.

Finally, ozone (O₃), a prevalent indoor oxidant, remains inadequately characterized in its influence on surface microbial communities. This study examined the effects of indoor-relevant O₃ concentration (≤70 ppb) on Escherichia coli surface viability and biofilm formation across relative humidity (20-97%), nutrient availability, and in the presence of squalene. Results demonstrate that ≥30 ppb O₃ significantly reduce both loosely adherent cell viability and biofilm formation, particularly at ≤60% RH on nutrient-poor surfaces. Biofilms exhibited greater resistance, requiring 24 hours at ≥30 ppb O₃ on nutrient-poor surfaces for complete inhibition, whereas loosely adherent cells were inactivated ≥16 hours under the same conditions. Water-soluble SqOz reaction products, generated even at ≥10 ppb O₃, showed pronounced bactericidal effect, with lipid and carboxyl-rich alicyclic molecules identified as key bactericidal compounds. At 20% RH and extended exposure (24 hours), these products adsorbed more effectively onto bacterial cells, resulting in complete inactivation. Concurrently, E. coli released diverse stress-response compounds when exposed to SqOz reaction products, indicating activation of adaptive mechanisms. These findings reveal that indoor-O₃ levels can substantially alter surface microbial communities through both direct stress and secondary chemistry, with important implications for indoor air quality management.

The findings of this thesis highlight the significant impact of cooking-derived organic compounds, SO₂ and O₃ on bacterial communities on indoor surfaces, enhancing our understanding of indoor microbiomes shaped under varying environmental conditions and providing valuable insights for improving indoor environments and promoting occupant well-being.

Date of Award	12 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Patrick Kwan Hon LEE (Supervisor)