It has been estimated since 1850s that rainwater also contains many microorganisms but the study of rainwater was difficult for researchers, as they could not collect the rainwater directly without any contamination.
At the end of 19th century it was revealed that rainwater also contains many microbes. Experiments were conducted in which rainwater was collected in a platinum crucible that was kept at about 1.7 m from the ground on a wooden frame. Then raindrops were inoculated onto a medium. Bacterial growth in the Petri dish was noticed after incubation.
Later rainwater air spora was investigated and bacteria and pink yeasts and Penicillium were identified. The Scripps Institute of Oceanography in California conducted rainwater analysis and they identified about 110 colonies of bacteria/m3 of air. Gregory et al. (1952) analyzed rainwater air spora on a special basis by using two funnels of 20 mm diameter. One of the funnels was covered by keeping an asbestos plate board 25 cm from the funnel.
The funnel which was kept closed (covered), helped in the dry deposition and the other which was not covered, helped in the water collected as it was kept at a height in order to avoid the splash droplets from the ground to enter to the rain water collected in the flask. Then they observed that the rainwater contained bacteria and moulds, such as Cladosporium, Alternaria, Pneumococcus and Micrococcus. Later Suzuki collected rainwater in a field affected with rice blast disease and the collected water, which contained eight times to the amount of microbes than those collected from the disease-free field.
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