The bacterial composition of ventilation filter dust in Norwegian pre-school nurseries

The microbial content of dust collected from intake and exhaust air filters in six Norwegian nurseries was determined using 16S rRNA pyrosequencing and plate count analyses. The concentration of endotoxins was also estimated. About 96% of the sequences were classified as Actinobacteria, Proteobacteria, Firmicutes, Bacteroidetes and Cyanobacteria. At the genus level, about 30% of sequences from the exhaust filter were classified as bacteria of probable human origin, such as Streptococcus and Corynebacterium species. These were close to absent in intake dust samples (<1%). This suggests that occupancy shapes the indoor microbiota, creating an environment relatively rich in genera of potential health significance. There were significantly greater counts of culturable bacteria and fungi in exhaust samples, indicating that passage of air through the nursery causes deterioration in the general air quality. Although there was more endotoxin in exhaust dust, the endotoxin levels per colony forming unit were similar in both samples. This study explores, for the first time, the bacterial composition of ventilation filter dust in Norwegian nurseries, and is important as it reveals what types of microorganisms nursery users are exposed to. In addition to possible direct health issues, the nature of our early exposure to microbes may be significant in the development of immunological conditions.