Abundance and viability of particle-attached and free-floating bacteria in dusty and nondusty air

Airborne bacteria are widespread as a major proportion of bioaerosols, and their coexistence with dust particles enables both bacteria and dust particles to be more active in ice cloud formation and to be harmful to public health. However, the abundance and viability of particle-attached and free-floating bacteria in dusty air have not been quantitatively investigated. We researched this subject based on the fact that airborne bacterial cells are approximately 1 mu m or smaller in aerodynamic diameter; therefore, particle-attached bacteria should occur in aerosol samples of particles larger than 1 mu m, and free-floating bacteria should occur among particles smaller than 1 mu m. Our observations at a coastal site in Japan in spring, when the westerlies frequently transported dust from the Asian continent, revealed that particle-attached bacteria in dust episodes, at the concentration of 3.2 +/- 2.1 +/- 10(5) cells m(-3) on average, occupied 72 +/- 9% of the total bacteria. In contrast, the fraction was 56 +/- 17% during nondusty periods, and the concentration was 1.1 +/- 0.7 +/- 10(5) cells m(-3). The viability, defined as the ratio of viable cells to total cells, of particle-attached bacteria was 69 +/- 19% in dust episodes and 60 +/- 22% during nondusty periods on average, both of which were considerably lower than the viabilities of free-floating bacteria (about 87 %) under either dusty or nondusty conditions. The presented cases suggest that dust particles carried substantial amounts of bacteria on their surfaces, more than half of which were viable, and spread these bacteria through the atmosphere. This implies that dust and bacteria have important roles as internally mixed assemblages in cloud formation and in linking geographically isolated microbial communities, as well as possibly having a synergistic impact on human health.