19The number of culture-independent indoor microbiome study has increased 20 remarkably in recent years, but microbial composition among different sampling 21 strategies remains poorly characterized and their impact to downstream microbiome 22 analysis is also not clear. In this study, we reported a case study of microbial 23 composition of floor and air dust in 87 dormitory rooms of Shanxi University, China. 24 Floor and air dust were collected by vacuum cleaner and petri-dish, respectively, and 25 the bacterial composition was characterized by 16S rRNA sequencing. The 26 composition of floor and air dust differed significantly (R 2 = 0.65, p < 0.001, Adonis), 27 and Pseudomonas dominated in floor dust (75.1%) but was less common in air dust
28(1.9%). The top genera in air dust, including Ralstonia (15.6%), Pelomonas (11.3%),
29Anoxybacillus (9.3%) and Ochrobactrum (6.2%), all accounted for < 1% abundance 30 in floor dust. The dominant Pseudomonas in floor dust swamped low frequency 31 organisms, leading to significant lower number of operational taxonomic units (OTUs) 32 compared with air dust in the same sequencing depth. The different microbial 33 composition of floor and air dust can lead to differences in downstream 34 bioinformatics analyses. We searched the dormitory microbiome against ~200,000 35 samples deposited in Microbiome Search Engine (MSE), and found that the 36 compositions of floor dust samples were similar to samples from building 37 environment and human nasopharynx, whereas the compositions of the air dust 38 samples were similar to mosquito tissues.39 40 Importance 41 42Increasing number of indoor microbiome studies has been conducted in recent years, 43 but the impact of sampling strategy is far from clear. In this study, we reported that the 44 floor and air sampling can lead to drastic variation in microbial composition and 45 downstream analyses in university dormitories, including microbial diversity 46 estimation and compositional similarity search. Thus, the bioinformatics analysis 47 48 necessary to collected samples from different sampling strategies to comprehensively 49 characterize the microbial composition and exposure to human occupants in an indoor 50 environment. 51 52 53 54 55 The development and application of the cost-efficient next-generation sequencing 56 technology in the past decade greatly facilitates the culture-independent researches to 57 characterize microbiome composition in various ecological niches, including human 58 gastrointestinal and respiratory tract, skin and indoor environment [1-6]. The first step 59 of microbiome study is to collect samples, and various approaches has been applied 60 for different sample types. The gut microbiome is commonly sampled from stool, 61 which represents well the microbial composition of colonic lumen and mucosa [7], 62but other sampling strategies such as mucosal biopsy, brushing or rectal swab are also 63 applied in various studies [8, 9]. Similarly, swab, spurum, lavage, aspirates and 64 brushing have all been used...