Diurnal distribution of total and culturable atmospheric bacteria at a rural site

During the summer and fall of 1996 ambient air samples were collected using a high-volume wet cyclone sampler positioned 2 m above ground level in an agricultural area near Corvallis, OR. Samples were collected for approximat ely 2-hr periods from 0500 to 2100 hr on 11 days, Total and culturable atmo spheric bacterial (TAB and CAB) concentrations in the samples were determin ed using epifluorescence microscopy and filtration-culture methods. It was found that the CAB concentration in the atmosphere was the lowest at dawn, gradually increased from sunrise to reach a maximum in the afternoon, and f inally decreased in the evening, The TAB load had a similar but much less p ronounced trend. The geometric mean of TAB concentration was about 27 to 22 2 times greater than that of CAB concentration in the atmosphere, A much hi gher TAB determination suggests that atmospheric bacteria might play a grea ter role than expected when the culturable determination is used as an inve stigation method in allergic disease and ecological research, This finding also indicates the necessity for using nonculturable airborne pathogen dete ction methods such as those targeted on nucleic acid or other macromolecule s, rather than the classical culturable methods currently used in airborne epidemiological investigation. The CAB to TAB ratio in the atmosphere was highest in the afternoon, coinci dent with the maximum CAB concentration. This could be explained by either or both of the following reasons: (1) the maximum live bacterial flux from the ground at that time does not have sufficient time to be rendered noncul turable before reaching the sampler's position; and (2) the prevalence of l arge, protected bacterial particles in the afternoon and a high collection efficiency of the cyclone sampler for these particles.