"Infusorian Animalcules Living Without Free Oxygen
and Determining Fermentation."

Comptes rendus de l'Académie des sciences 52 (February 25, 1861): 344-47;
Reprinted in Oeuvres de Pasteur, vol. 2, pp. 136-38. Translation by A.S. Weber.

The variety of products formed by the so-called lactic fermentation are well known. Lactic acid, a gum, mannite, butyric acid, alcohol, carbonic acid and hydrogen appear simultaneously or successively in extremely variable and quite unexpected proportions. I have slowly realized that the vegetable ferment which transforms sugar into lactic acid is different from the one or ones (because there are two of them) which determine the production of the gummy material, and that these ferments do not produce lactic acid. Moreover, I have also recognized that these various vegetable ferments, if they are perfectly pure, can in no way give rise to butyric acid.

Therefore there must be a specific butyric ferment. I have focussed my attention on this point for a long time. The communication which I have the honor of addressing to the Academy today precisely concerns the origin of butyric acid in the so-called lactic acid fermentation. I will not enter here into all the details of this research. I will first limit myself to announcing one of the conclusions of my work: that is, the butyric ferment is an infusorian.

I had been prepared not to expect this result, to such a degree that for a long time I felt compelled to prevent the appearance of these little animals, for fear that they were not drawing nourishment from the vegetable ferment which I had assumed to be the butyric ferment, the same vegetable ferment which I was searching to discover in the liquid media that I was using. But unsuccessful in uncovering the cause of the origin of the butyric acid, I was in the end struck by the coincidence, that my analyses showed me to be inevitable, between the acid and the infusoria, and conversely between the infusoria and the production of the acid, a fact that I had previously attributed to the favorable and suitable environment that the butyric acid provided to these animalcules. Since then, a great number of experiments have convinced me that the transformation of sugar, mannite, and lactic acid into butyric acid is due exclusively to these infusoria, and that it is necessary to consider them as the true butyric ferment.

The infusoria can be described as follows: they are little cylindrical rods, rounded at the ends, ordinarily straight, and either isolated or joined in chains of two, three, four or sometimes even more divisions. On average, their size is 0.002 mm. The length of one of the isolated divisions varies from 0.002 mm to 0.015 mm to 0.02 mm. These infusoria propel themselves by gliding. During this movement, their bodies stay rigid, or undergo light undulations. They pirouette, balancing themselves where the anterior or posterior part of their bodies vibrates rapidly. The undulations of their movements become very evident as soon as their length reaches 0.015 mm. Often they are bent back towards one of their extremities, sometimes to both. This peculiarity is rare at the beginning of their lives.

They reproduce by fissiparity [division]. The chains of divisions which some of their bodies join into are evidently a result of this mode of reproduction. A single infusorian which drags other bodies after itself sometimes rapidly shakes as if to detach itself.

Although the bodies of these vibrios have a cylindrical appearance, one might say that they are often formed from a series of particles or very short, scarcely begun divisions which are without doubt the rudimentary beginnings of these little animals.

One can sow these infusoria just like one would sow beer yeast. They multiply if the environment is adapted to their nutrition. But it is essential to remark here that one can sow them in a liquid containing only sugar, ammonia, and phosphates; that is to say, crystalline and completely mineral substances, and they reproduce themselves in correlation with the butyric fermentation, which appears very evident. The weight formed from this process is notable, although always small when compared to the total quantity of butyric acid produced, just as in all ferments.

The existence of infusoria possessing the character of ferments is already a fact which certainly seems worthy of attention; but one unique peculiarity accompanies this fact-these infusorian animalcules live and multiply without limit without the necessity of providing them with the least quantity of air or free oxygen.

It would be tedious to recount here how I absolutely excluded oxygen from the interiors and surfaces of the liquid media where these infusoria live and swarm by the millions, since I have carefully established this elsewhere. I will only add that I did not want to present my findings to the Academy without calling several of its members to witness, who appeared to acknowledge the rigor of the experimental proofs which I placed before their eyes.

Not only do these infusoria live without air, but the air kills them. If one passes a stream of pure carbonic acid for an unspecified amount of time through the liquid in which they live, their life and reproduction are in no way affected. If, on the contrary, a stream of atmospheric air is substituted for the carbonic acid under exactly the same conditions, in only one or two hours the infusoria all die, and the butyric fermentation connected to their existence is soon stopped.

Thus we arrive at this double proposition:

This is, I believe, the first known example of animal ferments, and also of animals living without free oxygen gas.

The comparison of the way of life and properties of these animalcules with the way of life and properties of the vegetable ferments who live equally without the aid of free oxygen, is selfevident, along with the consequences that may be deduced from it relating to the cause of fermentations. However, I would like to reserve the ideas which these new facts suggest until I have submitted them to the light of experiment.