HISTORICAL INTRODUCTION 



the microscopic spores of ferns, mosses, and fungi — as well as protozoa — 

 were commonly liberated into the air and transported by the wind. The 

 potential sources of the air-spora had been discovered and identified in 

 the main before the year 1 800, but their role remained obscure. 



Controversy on Spontaneous Generation * 



Leeuwenhoek had come to doubt the belief, dating from Aristotle, 

 that flies, mites, and moulds were generated spontaneously by decaying 

 animal and vegetable matter. To him it seemed likely that animalcules 

 could be carried by the air, and this provided an alternative explanation 

 to spontaneous generation. J. T. Needham {b. 1713, d. 1781) had claimed 

 that minute organisms would appear in heated infusions; but L. Spallan- 

 zani {b. 1729, d. 1799) showed, by a series of experiments, that when 

 organic materials were subjected to sufficient heat-treatment (with various 

 precautions against contamination) they would neither putrify nor breed 

 animalcules unless exposed to air. From this Spallanzani concluded that the 

 microbes were present in the air admitted experimentally to his sterilized 

 vessels. A rearguard action was fought to explain away these results. 

 J. Priestley (^. 1733, d. 1804) and L. J. Gay-Lussac {b. 1778, d. 1850) 

 claimed that heating the vessels drove out the air and that it was shortage 

 of oxygen., not lack of 'seeds', which prevented heat-sterilized materials 

 from generating a microbial population. 



Meanwhile, Appert (1810) put heat sterilization on a commercial 

 basis by applying it to food preservation ; but the controversy lingered on, 

 even into the present century, although the experiments and polemics of 

 Louis Pasteur were decisive. Pasteur showed that food could be conserved 

 in the presence of oxygen and that preservation depends on the destruction 

 by heat of something contained in the air. In 1859 F. A. Pouchet, of 

 Rouen, had raised the objection that a very minute quantity of air sufficed 

 to allow the development of numerous microbes in heated infusions, and 

 that the air would have to be a thick soup of microbial germs. 



In reply, Pasteur (1861) sterilized a series of evacuated flasks con- 

 taining nutrient medium. So long as the flasks remained unopened they 

 all remained sterile; but, even when they were opened and air was ad- 

 mitted, he found that one or two out of each batch would remain sterile 

 on incubation. Pasteur replied to Pouchet, denying that only a minute 

 quantity of air needs to gain access for a microbe population to develop 

 and for putrefaction to take place. On the contrary, the cause of the 

 phenomenon was discontinuous and a sample of 250 cc. of air might or 

 might not contain germs. 



Pasteur then showed, by opening batches of about forty such flasks 

 in various sites, that the quantity of airborne germs differed in different 

 places. In the open air in Paris he obtained bacteria, yeasts, and moulds ; 



* See also Bulloch (1938) and Oparin (1957). 



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