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 (b. 1733, d. 18o4) and L. J. Gay-Lussac (b. 1778, d. 185o) 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 (181o) 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; but some flasks remained sterile. In cellars of the Observatoire, where the temperature was constant and the air still and dust-free, many more flasks remained sterile.
On 5 November 186o, Pasteur deposited at the office of the Academy no fewer than seventy-three quarter-litre flasks, some of which he had opened to the air in batches of twenty at various heights ranging from the foothills of the Jura to high up on Mont Blanc, as follows: The cause of this supposed `spontaneous generation' was not only discontinuous but, moreover, its concentration decreased with height.
F. A. Pouchet had admitted that among dust particles of vegetable origin there were some spores of cryptogams, but he held that these were too few to account for the phenomena of putrefaction.
Pasteur decided that he would abandon Pouchet's method, which relied on examining spontaneous deposits of dust on the surface of objects, in favour of a new method of studying the particles by collecting from actual suspension in the air. Pouchet had drawn invalid conclusions from surface deposits because, according to Pasteur, the light air-movements which constantly play over surface deposits would pick up and remove the extremely minute and light spores of microbes more readily than they would any coarser particles. (It now appears, however, that the small numbers of the lighter bodies in surface deposits is due to the extreme slowness with which they are deposited, rather than to their preferential removal after deposition.) Pasteur's apparatus for extracting the suspended dust in the air, for microscopic examination, was quite simple (Fig. 1). A tube of cm. diameter was extruded into the open air through a hole drilled in a window frame several metres above the ground. The rear part of the tube was packed with a plug of gun-cotton to catch particles. Air was drawn through the apparatus by means of a filter pump, and the volume of air was measured by displacement of water. Tests were made on air drawn from beside the Rue d'Ulm, and from the garden of the Ecole Normale in Paris. During aspiration, solid particles were trapped on the fibres of the gun-cotton plug. After use, the gun-cotton was dissolved in an alcohol ether mixture, the particles were allowed to settle, the liquid was decanted, and the deposit was mounted for microscopical examination.
Pasteur, as usual, had little interest in the specific identity of his or ganisms; he was no taxonomist. The particles exactly resembled the `germs' of lower organisms. They differed in volume and structure so much among themselves that they clearly belonged to very many species or even groups, including bacteria, moulds and yeasts. Their numbers contradicted the general conclusion that the smallest bubble of air admitted to a heat-sterilized medium is sufficient to give rise to all the species of infusoria and cryptogams normal to an infusion. This view was shown to be highly exaggerated, and Pasteur indicated clearly that it is sometimes possible to bring a considerable volume of ordinary air into contact with an infusion before living organisms develop in the latter.
Pasteur had demonstrated visually the existence of an air-spora, he had pointed out that it should be measured while in suspension and not after deposition on surfaces, and he had made the first rough visual measurements of its concentration in the atmosphere of the City of Paris: a few metres above the ground in the Rue d'Ulm, after a succession of fine days in summer, several thousands of micro-organisms were carried in suspension per cubic metre of air. He then abandoned the method—remarking, however, that it could doubtless be improved and used more extensively to study the effects of seasons and localities, and especially during outbreaks of infectious diseases.