Bacterial community assemblages in classroom floor dust of 50 public schools in a large city: characterization using 16s rRNA sequences and associations with environmental factors

Characterizing indoor microbial communities using molecular methods provides insight into bacterial assemblages present in environments that can influence occupants’ health. We conducted an environmental assessment as part of an epidemiologic study of 50 elementary schools in a large city in the northeastern United States. We vacuumed dust from the edges of the floor in 500 classrooms accounting for 499 processed dust aliquots for 16S Illumina MiSeq sequencing to characterize bacterial assemblages. DNA sequences were organized into operational taxonomic units (OTUs) and identified using a database derived from the National Center for Biotechnology Information. Bacterial diversity and ecological analyses were performed at the genus level. We identified 29 phyla, 57 classes, 148 orders, 320 families, 1,193 genera, and 2,045 species in 3,073 OTUs. The number of genera per school ranged from 470 to 705. The phylum Proteobacteria was richest, followed by Firmicutes, Actinobacteria, Bacteroidetes, and Cyanobacteria while Firmicutes was most abundant. The most abundant order included Lactobacillales, Spirulinales, Clostridiales, Bacteroidales, Pseudomonadales, and Micrococcales. Halospirulina was the most abundant genus (the only genus within the order Spirulinales), followed by Lactobacillus, Streptococcus, Sphingomonas, Clostridium, and Pseudomonas. Gram-negative bacteria were more abundant and richer (relative abundance=0.53;1,632 OTUs) than gram-positive bacteria (0.47; 1,441). The Bray-Curtis dissimilarity index ranged from 0.22 to 0.63, with a median of 0.40. Effects of school location, degree of water damage, building condition, number of students, air temperature and humidity, floor material, and classroom’s floor level on the bacterial richness or community composition were statistically significant but subtle. Our study indicates that classroom floor dust had a characteristic bacterial community represented by more gram-negative bacteria, in comparison to typical house dust that is represented by more gram-positive bacteria. Health implications of exposure to the microbiomes in classroom floor dust may be different from those in homes for school staff and students.