16S of Dust Microbiome
DNA extraction For DNA analyses, the samples were processed using DNeasy PowerFood Microbial Kit (QIAGEN, Singapore), with slight modification in the cell lysis procedure and DNA elution step as compared to the manufacturers protocol. Briefly, the samples were resuspended in 450 l Solution MBL and disrupted in PowerBead Tube using FastPrep-24 instrument (MP Biomedicals, USA) for 20 seconds at 4 m/s twice. DNA in samples were eluted with 70 l of elution buffer, then incubated at room temperature for 6 minutes prior to centrifugation. Total DNA was extracted and stored at  80 C until sequencing of bacterial 16S rRNA. A negative control - water (blank) was included for each sequencing batch and subjected to the same processing to rule out possible environmental contamination during processing. Bacterial 16S rRNA gene amplification and sequencing Bacterial V4-V6 hypervariable regions of the 16S rRNA gene were amplified using the forward primer 533F (5-GTGCCAGCAGCCGCGGTAA-3) and the reverse primer 1062R (5-TCACRRCACGAGCTGACGA-3). PCR amplifications were performed in a final volume of 25 l, containing 3 l of template DNA, 1.25 l of each primer (10 M), 14 l of RNase-free water, 5 l of 5x HotStarHiFidelity PCR buffer and 0.5 l of HotStarHiFidelity DNA polymerase (QIAGEN, Singapore). The PCR conditions were as follows: 95 C for 5 mins; 30 cycles of 94 C for 30 s, 55 C for 1 min, 72 C for 1 min; followed by 72 C for 10 mins. Amplicons were then purified and fragmented with the E220 Focused-ultrasonicator (Covaris, Woburn, Mass). The Qiagen GeneRead DNA Library Prep I Kit (Qiagen) was used for library preparation. The Multiplexing Sample Preparation Oligonucleotide Kit (Illumina, San Diego, Calif) was used to label DNA sequencing libraries. Multiplexed pair-end sequencing (2 x 76-bp reads) was performed with the Illumina HiSeq Rapid platform.