THE UPPER-AIR SPORA 



air and sampling about 28 litres per minute. The catches were examined 

 both microscopically and by culturing. Bacteria averaged 12 per cubic 

 metre on all flights, and 9 per cubic metre at 6,100 metres or higher. 

 There was sometimes evidence of a zone of greater concentration at a 

 height of several thousand metres. Moulds were usually less numerous 

 than bacteria, but the nutrient agar on which filter washings were plated 

 was recognized as unfavourable to mould growth. Bacteria were mostly 

 spore-formers and those identified included species of Bacillus, Achro- 

 mobacter, and Micrococcus. Among moulds, Aspergillus and Penicillium 

 predominated, occurring with some other Fungi Imperfecti including 

 Cladosporium {Hormodcndrum) and Fusarium, as well as Mucoraceae, 

 Actinomycetes and, occasionally, yeasts. Pollen was found on only three 

 flights (Proctor, 1934, 1935). 



The highest mould count obtained in the M.I.T, studies occurred at 

 an altitude of 200 to 300 metres in May over a wooded area, where 22 

 bacteria and 260 moulds per cubic metre were recorded. Particularly 

 large counts of bacteria and moulds occurred during a dust storm which 

 apparently came from Nebraska and South Dakota — the same dust- 

 storm during which Soule (1934) recorded mass invasion of his labora- 

 tories in Michigan by Bacillus megatherium. During this dust-storm, at an 

 altitude of 1,500-3,300 metres over the Boston area, bacteria totalled 

 140, moulds 44, and dust particles 2,800, per cubic metre, respectively. 

 However, during the whole survey, dust particles were over 100 times as 

 numerous as viable microbes — suggesting that much of the dust came from 

 industry and combustion rather than from the soil (Proctor & Parker, 



^938)- ... 



Petri dishes of nutrient agar w ere exposed during flights at from 300 



to 3,250 metres over Nashville, Tennessee, during winter by Wolf (1934). 

 On this medium bacteria outnumbered moulds, the bacilli contributing 

 37-7 per cent, non-spore-forming rods 24-6 per cent, and cocci the 

 remaining 37-7 per cent, of the total bacterial count. The bacteria were 

 very similar to those found by Proctor (though with a smaller percentage 

 of spore-formers) and further study supports the general conclusion 

 that aerial bacteria are of types commonly found in soil and water, are 

 generally unable to ferment common sugars with the production of gas, 

 and are unable to produce indole. 



From these flights by Wolf, Actinomyces griseolus was isolated twice, 

 at 700 and 1,400 metres, and A. phaeochromogenus once, at 620 metres. A 

 pink yeast was found at 1,750 and 3,050 metres. Fungi isolated, with their 

 percentage frequencies, included: Fusarium, 29; Alternaria, 22; Clado- 

 sporium {Hor?nodejidru?ii), 20; Verticillium, 5; Aspergillus, 3; Penicillium, 

 1-6; and among others w^ere Acladium, Brachysporium, Cephalothecium, 

 Chaetomium, Hebninthosporium, Macrosporium, Mucor, Oospora, Pleno- 

 zythia, and Scopulariopsis. The large numbers of Fusarium spores and 

 small numbers of Aspergillus and Penicillium spores are remarkable. 



139 



