The original French article appears in the Appendix, pages 267-268.

1861, Louis Pasteur
 

The wide variety of products that are formed during the lactic acid fermentation are well known. Lactic acid, a gum, mannitol, butyric acid, alcohol, carbon dioxide and hydrogen all appear simultaneously or successively in highly variable proportions and in a quite capricious manner. I have gradually been led to realise that the plant ferment which converts sugar into lactic acid is different from the one(s) which bring about the production of the gummy material, and that the latter in turn do not produce lactic acid. On the other hand it is equally true that none of the various plant ferments can give rise to butyric acid, if they are separated from the other forms.
Therefore there has to be a distinct butyric acid ferment. I have been occupied with this idea for a considerable length of time. I would like to address myself to the Academy today on the origin of butyric acid in the so-called lactic acid fermentation.
I will not go into all of the details of this research. I would like to indicate first one of the conclusions from my work. This is that the ferment which produces butyric acid is an infusorium.
For a long time I was prevented from discovering this fact because I had devoted my efforts to eliminating these small animals which I feared would feed on the plant ferment which I supposed to be the cause of the butyric acid fermentation and which I was looking for in the liquids which I was studying. But after unsuccessfully looking for the cause of the butyric acid fermentation, I was finally struck with the correlation . . . between this acid and the infusoria, and inversely between the infusoria and the production of this acid, circumstances which I had attributed to the necessity of butyric acid for the life of these small animals.
Since then a large number of experiments have convinced me that the conversion of sugar, mannitol, and lactic acid into butyric acid is due exclusively to these infusoria, so that it is necessary to consider them to be the true butyric acid ferment.
Here is their description: They are small cylindrical rods, rounded at the ends, generally straight, singly or in chains of two, three or four, and sometimes more. They average about 0.002 mm in diameter, varying in length from 0.002 to 0.015 or 0.02 mm. They move with a gliding motion. During this movement, their bodies remain rigid or make slight undulations. They spin, balancing or quivering actively the two ends of their bodies. The undulatory nature of their movements becomes very obvious when they are longer than 0.015 mm in length. Frequently they are bent at one end, sometimes at both ends. This latter is seldom seen when they are young.
They reproduce by binary fission. It is apparently because of their mode of reproduction that they occur often in chains. One of the units attached to others may move quickly several times in order to detach itself.... [These organisms are not animals, as Pasteur supposed, but small motile bacteria.]
These infusoria can be inoculated in the same way as beer yeast. They multiply if the medium is suitable for their nutrition. But it should be stated that they can be inoculated into a medium containing only the crystallizable and mineral substances sugar, ammonium, and phosphates, and they can reproduce simultaneously with the rapid appearance of the butyric acid fermentation. The weight of cells that are formed is significant, although always small compared to the total quantity of butyric acid formed, but this is true for all ferments.
The existence of infusoria which are able to bring about fermentations is already a notable fact. But in addition another unusual aspect should he mentioned. This is that these infusorial  animals are able to live and multiply
indefinitely in the complete absence of air or free oxygen These infusoria can not only live in the absence of air, but air actually kills them. If a stream of carbon dioxide is passed through a medium in which they are multiplying, their viability and their reproduction are not affected in the least. On the contrary, under the same conditions, if one substitutes a stream of air for the carbon dioxide, in one or two hours they all die, and the butyric acid fermentation which requires their viability is  stopped immediately.
 We have arrived therefore at the following, double proposition:
 1. The ferment which produces butyric acid is an infusorium.
 2. This infusorium lives in the complete absence of free oxygen.
 I believe this is the first example known of an animal ferment, and also
the first example of an animal living in the absence of free oxygen.
We will have to consider how the relationship of the mode of life and
the properties of these small animals, together with the same aspects of the
plant ferments which can also live without free oxygen, are related to the processes of fermentation. In the meanwhile I would like to make no further comment on the ideas which these new facts suggest until further research has been done.

Commentary by Thomas Brock
 This is the first report that any organism can live and reproduce in the complete absence of free oxygen. cal processes to the presence or absence of oxygen. This discovery is quite important for general biology, since it shows that oxygen gas is not a requisite for life. This discovery opened up for Pasteur a new field of study, relating fermentative and biological processes to the presence or absence of oxygen. It led to his discovery that yeast can live either aerobically or anaerobically, and that the yeast differs in  function under these two conditions.
There are a large number of organisms now known that will grow under anaerobic conditions. Some of these are pathogenic, while others seem to be saprophytic. So far as is known, oxygen is not toxic to these organisms in itself, but its  presence brings about certain oxidation processes which allow the accumulation  of hydrogen peroxide, which is toxic. Organisms that can grow either in the presence or absence of free oxygen are called facultative organisms. Those which will grow only in the absence of oxygen are called obligate anaerobes. Their culture requires special procedures that eliminate oxygen gas, and consequently they are not studied as often as are aerobic and facultative organisms.