THE MICROBIOLOGY OF THE ATMOSPHERE 



colony). Diurnal periodicity showed maximum numbers near midnight 

 (210 colonies per cubic metre) and a minimum in the morning (30 col- 

 onies). Similar, but fewer, colonies were found regularly in London and 

 Rothamsted. They show^ed no pronounced seasonal trend, according to 

 Hamilton (1959), who also reported the rare occurrence of diatoms and 

 desmids. Evidently microscopic algae are widely prevalent in the at- 

 mosphere in numbers varying from a few to a few hundred per cubic 

 metre, and occasionally they may be heavily deposited on the ground 

 (D.S.I.R., 1931). 



(iii) Ferns. For ferns the reports are few. At Rothamsted — with no 

 large areas of bracken within several kilometres, and only small quantities 

 within I km. — spores of the Pteridimn type occurred frequently in warm, 

 dry weather from late July to mid September. They averaged 4 per cubic 

 metre, with a maximum concentration of 36 per cubic metre (Gregory & 

 Hirst, 1957; Hamilton, 1959). 



Miquel's Work on Bacteria and Moulds 



Recognizing the paucity of information on airborne microbes, Pierre 

 Miquel made daily counts in Paris during the last quarter of the nineteenth 

 century (cf. p. 9). Miquel's 'contribution to the microscopic flora of the 

 air' is probably the most sustained series of volumetric measurements 

 of the microbial population of the outdoor air ever attempted. Daily 

 observations in the Pare Montsouris, about 5 km. south of the centre of 

 Paris, served him as a standard for comparison with the polluted air in 

 the densely populated city. The bacteria of the outdoor air were classified 

 in the following percentages as: Micrococcus 66, Bacillus 25, Bacterium 6, 

 Vibrio 1-2. 



Miquel (1899) shows a seasonal variation in total bacterial and mould 

 concentrations (Table XVIH). Most of the samples were taken with a 

 form of the Pasteur trap {see p. 5), using a sterile plug of powdered an- 

 hydrous sodium sulphate as a filter. This was dissolved after exposure 

 and inoculated to flasks of filtered saline beef extract. At the Pare Mont- 

 souris, bacteria were nearly three times as numerous in summer as in 

 winter, but moulds fluctuated rather less. Near the Hotel de Ville in the 

 centre of Paris, bacteria showed a similar seasonal variation but were 2| 

 times as many as in the Pare; moulds were 10 times as numerous but 

 showed little seasonal variation. At first Miquel argued that, as only 

 one-tenth could have been blown in from the country to the centre of the 

 city, the rest must have come from houses. But after the year 1881 he 

 noted a steady annual decline and he attributed this to improved street 

 cleaning and washing to lay dust which, we may suppose, consisted 

 largely of soil enriched with horse droppings. Data are also given for a 

 narrow, unhygienic street, and for one of the main sewers of Paris. The 

 air of sewers was no more highly contaminated than the outside air, and 

 was often surprisingly pure (Chapter XH). 



no 



