The microorganisms Escherichia coli DH5␣ and Bacillus thuringiensis HD-1 show an increased tolerance to freeze-drying when dried in the presence of the disaccharides trehalose and sucrose. When the bacteria were dried with 100 mM trehalose, 70% of the E. coli and 57% of the B. thuringiensis organisms survived, compared with 56 and 44%, respectively, when they were dried with sucrose. Only 8% of the E. coli and 14% of the B. thuringiensis organisms survived drying without the sugars. Fourier transform infrared spectroscopy was used to investigate the role of membrane phase transitions in the survival of the organisms during drying and rehydration. Both E. coli and B. thuringiensis showed an increase of 30 to 40؇C in the temperature of their phospholipid phase transition when dried without the sugars, while phase transition temperatures of those dried with the sugars remained near those of the hydrated cells. A Fourier transform infrared spectroscopy microscope made it possible to investigate the effects of drying on the protein structure in the intact cells. The amide II peak shifts from 1,543 cm ؊1 in the hydrated cells to about 1,533 cm ؊1 in the cells dried without sugar. There is no shift in the amide II peak when the cells are dried with trehalose or sucrose. We attribute the increased survival to the sugars' ability to lower the membrane phase transition temperature and to protect protein structure in the dry state. In addition to increasing the immediate survival of both species, the addition of trehalose protected the cells from the adverse effects of exposure to light and air while dry. E. coli dried with trehalose and exposed to light and air for 4 h had an increase in CFU of between 2,000 and 4,000 times the number obtained with E. coli dried without trehalose. B. thuringiensis showed an increase in CFU of 150% in samples dried with trehalose compared with samples dried without trehalose. The cells dried with sucrose did not show an increased tolerance to exposure following drying.

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This MiniReview is concerned with the sources, flux and the spacial and temporal distributions of culturable airborne bacteria; how meteorological conditions modulate these distributions; and how death, culture media, and experimental devices relate to measuring airborne bacteria. Solar radiation is thought to be the planetary driver of the annual (seasonal, where it occurs) and diurnal natural alfresco atmospheric bacterial population cycles. Long‐term climatological and short‐term meteorological events such as storms also ‘randomly’ modulate the populations. The annual cycle may be due largely to the seasonal events of vegetation growth, drying conditions, rainy season, and freezing winter temperatures. The diurnal cycle may also be influenced largely by solar radiation in that the sunrise peak that has been frequently observed may be due to convective updrafts entraining epiphytic and soil bacteria into the atmosphere. At this time they are initially concentrated in a slowly deepening mixed layer but then, as the layer deepens more rapidly in mid‐morning, they become increasingly dilute. When the layer formation slows in the early afternoon, and forms an inversion cap, the bacteria slowly accumulate in the deeper mixed layer until sundown. At sundown atmospheric cleansing processes rid the atmosphere of the larger bacteria associated particles. These processes may include gravitational settling, death due to desiccation, and upward displacement of the warm, light air by clean, cold, heavier air.

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