Quantitative Sampling of Indoor Air Biomass by Signature Lipid Biomarker Analysis: Feasibility Studies in a Model System

Abstract

Standard methods for characterizing the microbial content of indoor air rely on detection of viable microbes that are collected in water or impacted onto growth substrata. Viable counts consistently underestimate microbial numbers in environmental samples by 90–99.9%. Assays of biochemical components characteristic of all cells provide an assessment method independent of the ability to culture the organisms. This article provides evidence that lipid analysis provides quantitative recovery of known volumes of culturable bacteria. Monocultures of Escherichia coli, Bacillus subtilis, and Legionella bozemanii and two mixtures of these organisms were deposited onto glass fiber filters using an air test stand constructed as a modification of the ASTM 1215 standard. Filter deposited biomass was determined by three methods: (1) viable counts of bacteria sampled using an impinger, (2) phospholipid ester-linked fatty acid (PLFA) analysis, and (3) hydroxy fatty acid (OH FA) analysis. PLFA and OH FA were quantified by gas chromatography-mass spectrometry. Bacterial counts determined as colony forming units/filter following impinger sampling ranged between 25.6% (B. subtilis) to 18.5% (L. bozemanii) of the counts determined by PLFA analysis. Sampling efficiency and mechanical and dehydration stresses to the bacteria during aerosolization could have caused the decreased culturable viable counts in comparison to PLFA analysis of viable cells. Signature lipid analysis provides insight into the community composition. Analysis of the glass filters after aerosolization showed that capture was not selective, and monocultures and mixtures gave the expected signature lipid patterns enabling differentiation between species.