Importance ubiquitous bacterial genus with dominant occurrence on the platform and lab outlet (Figure 3a-c) . 124
In addition, while the total number of detected genus are similar among lab outlet (76), public area 125 (81) and platform (79), the number of shared genus is largest in public area (39), and smallest in lab 126 outlet (22) (Figure 3d-f) . Moreover, for either of the sampling sites including lab outlet, public area 127 and platform, PL has much less laboratory-specific genus compared to AL and ML (Figure 3d-f) . 128 Therefore, we speculated that while public areas shared by experimenters might have largest 129 number of shared genus, key sites such as lab outlet and platform has their specific sets of genus as 130 potential contaminations. 131
We next compared the relative abundances of representative genus from three main sites within the 132 laboratory. Pseudomonas, Acinetobacter and Enterobacter were most abundant among all sampling 133 sites (Figure 4a-c) . In addition, the number of all identified genus in AL (84) and ML (87) were 134 much more than those in PL (53) (Figure 4d-f) . Moreover, the platform of AL has the highest 135 number of site-specific genus (Figure 4d-f) . These results again confirm that the richness of 136 microbial communities of platform and lab outlet depended heavily on the type of laboratory. 137 138
We then performed literature mining to identify the possible sources of these microbial 140 contaminations, referencing varies sources. We categorized the sources into laboratory reagent 141 microbe, human-introduced microbe, and basic environmental microbe. Interestingly, laboratory 142 reagents and human daily activities might play very important roles in introducing these possible 143 microbial contaminations ( Table 1) . 144
To obtain a characteristic set of microbial contaminants, or biomarkers, for each type of biological 145 laboratory, we used LDA Effect Size (LEfSe) to discover the biomarkers at each taxonomic level. 146 29 taxa (7, 15 and 7 taxa from AL, ML and PL respectively; Figure 5 ) were detected with high 147 LDA scores. For samples from AL, Becateroidetes, Flavobacteriaceae and Gemmata were 148 identified as biomarkers. Enterobacteriales and Enterobacteriaceae were identified as biomarkers 149 for ML. Pseudomonas, Pseudomonadaceae and Pseudomonadales, which belong to Pseudomonas, 150 were identified as biomarker for PL with high confidence (Figure 5a) . The evolutionary 151 relationship between these bacteria at different taxonomic levels is shown in Figure 5b . 152
To further explore the characteristics of the biomarkers for different laboratories, we screened the 154 genera with a relative abundance of > 1/1000 within the same type of laboratory. This identified ML 155 to contain a greater variety of bacteria (65 genera) than AL (59) and PL (48). The population of the 156 overlap between the detected genera of the three types of laboratories was 39 (Table 1) , the highest 157 was found between ALs/MLs (9 shared genera) and followed by ALs/PLs (3) and MLs/PLs (1) Psychrobacte of AL is a probiotic of fish, and its highest diversity was detected in sample A1B1, 175 corresponding to the incubator of the zebrafish laboratory by backtracking analysis. Buchnera of PL, 176 a symbiotic bacterium of aphids is specifically associated with the tissue culture process. 177
Flavisolibacter of ML, which improves nitrogen fixation in rhizosphere of plants, has the highest 178 abundance in sample M1W12, which was from cultivated plants on the windowsill in the M1 179 laboratory. Together, these results showed high concordance between the characteristics of the 180 laboratory and the sampling site, demonstrating that the compositions of microbial communities 181 have profound association with their hosting laboratories. 182
As already known, the present of these contaminants can bring inconvenient for our experiment 183 more or less, so caution and preciseness must be followed throughout the whole experiment. And 184 the use of blank control during sampling, DNA extraction and sequencing is also necessary for 185 detecting contamination. Furthermore, he contaminants are associated with the use of different kits to use the same kits in one experiment and disentangle batch effects. Additionally, we should 188 catalogue the laboratory microbial contaminants better, and thus, as if we know the contaminants, 189 antibiotic treatment can be executed before experiment to mitigate the experimental bias caused by 190 these microbial contaminants. These microbes are intimately connected with the experimental materials, and will also assert 215 negative effects on the experiment process as well as on experimenters. 216 are needed. They would help in devising countermeasures to mitigate the experimental bias caused 219 by these microbial contaminants. Second, we hope that longitudinal studies would help to confirm 220 our findings, since our samples were collected from the same building in summer and may not 221 reflect the seasonal dynamics of the microbial communities. Supplementary Table 5 . During sampling, all the 236 staff and devices were in full operation (normal status). After sampling, the swabs were kept at 4 °C. 237
Afterwards, the genome of all samples extracted by using biological sampling kit was stored at -238 20 °C. 239 DNA extraction. First, 1 mL sample, in total, and 1 mL buffer was added into centrifuge tube, and 240 the mixture was stirred gently. After water bath of 2 h 65 °C, mixing by hand every 30 min, the 241 suspension was vortexed for 10 seconds. The tube was placed on ice for 10 min and centrifuged 242 afterwards (100 g, 5 min, 4 °C). The supernatant was transferred into another tube, and an equal 243 volume of phenol: chloroform: isoamyl alcohol in a ratio of 25:24:1 was added. The suspension was 244 mixed gently and centrifuged at 15°C/1000g for 5 min. The aqueous layer was transferred into a 245 new tube. Then, the same volume of isopropanolis was added to cause the DNA to precipitate out of 246 the aqueous solution. After incubation at -20 °C overnight, the suspension was centrifuged at 247 4°C/13500g for 30 min. After removing the supernatant, the precipitate was rinsed with 1 mL of 70% ethanol, and centrifuged repeated at 4 °C/13500 g for 30 min until the precipitate was 249 completely dried and re-dissolved in 20 μL of PCR-grade water for easy handling and storage. 250 