Airborne bacterial samples were collected using wet cyclone and cascade impact samplers 2 to 3 times a week at 2 agricultural sites in the mid-Willamette River valley, near Corvallis, OR during 1997. The concentrations of total (TB), culturable (CB), and particulate-associated culturable (PACB) bacteria in the ambietn atmosphere were measured usign epifluorescence microscopic and culture methods.All three categories of airborne bacteria were found to have major concentration peaks in the summer (e.g., from June to September), especially in July and August. This may reflect the greater summer flux of bacteria from agricultural sources and activities and dry/dusty soil condtions. The PACB had several smaller peaks scattered in the winter and autumn. Size analysis of the PACB showed that the summer PACB peak was composed primarily of larger bacterial particles, whereas the smaller peaks in other seasons were composed primarily of smaller bacterial particles that occur durign rainfall or storms events. The concentrations of TB and CB were positively correlated with temperature and solar radiation, but negatively with relative humidity. This is thought to reflect the contributions of agricultural activities and solar ground-heating effects outweighing the biologically damaging/lethal effect of solar radiation, high temperature, and dry conditions.It was shown that the cound median diameter (CMD) of the PACB varied durign the year with no obvious seasonal pattern. The ratios of CB to PACB and CB to TB concentrations in the atmosphere were greater in the summer than in the other seasons, while the TB to PACB ratio was the greatest in the spring. These observations could be interpreted as follows: mroe culturable bacteria, compared with the total, were aggregated in clumps or rafted on plant/soil debris in teh summer, while single or relatively fewer culturable bacteria were associated with the particles in other seasons. This may be caused by the newly exposed bacterial particles with a high proportion of culturable bacteria,short transport ime interval from aerosolization to deposition in the sampler, and/or more resistant baterial populations in/on the summer-time sources. Spring airborne bacterial populations may be more sensitive to, and/or extensively exposed to,environmental stresses (starvation, sunlight, etc.) and aerosolization, thus fewer culturable bacteria per particle might be expected.A seasonal mirostructure illustration of airborne bacteria particles is proposed that may be useful for the interpretation of aerobiological data, the investigation of health or ecological effect, and teh detection of the 3 categories of airborne bacteria defined herein.
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