Seasonal variations of airborne bacteria in the Mogao Grottoes, Dunhuang, China

Airborne bacteria are important biological components of bioaerosol and play an important role in the conservation of cultural heritage. High concentration of bacteria in the atmosphere can result in biological air pollution and potentially diseases. In this study, a systematical survey of culturable airborne bacteria was carried out in Mogao Grottoes of Dunhuang, China at four sites (one cave is open to visitors, another is semi-closed, the third one is closed to visitors, and the fourth site is the entrance gate). Airborne bacteria were collected using a six-stage culturable FA-1 sampler monthly from September 2008 to August 2009. The populations of culturable bacteria were in a range from 1.01 × 102 colony forming units (CFU/m3) to 3.8 × 103 CFU/m3. The mean was 1.30 × 103 ± 145 CFU/m3, 1.1 × 103 ± 279 CFU/m3, 4.4 × 102 ± 67 CFU/m3 and 4.4 × 102 ± 77 CFU/m3 in the Closed Cave, Open Cave, Semi-closed Cave and the Entrance, respectively. The particle size of airborne bacteria were mainly distributed in stage 4 of sampling with diameters approximately 2.1–3.3 μm. The correlation was found between airborne bacterial concentrations and environmental temperature, relative humidity, as well as tourists number. The results suggested that the number of visitors has an obvious influence on both the concentrations and the compositions of ambient bacteria in Mogao Grottoes.