The names are historical microscopic groupings, and the central dispute is whether the organisms cause the chemical changes, arise as their products, or merely grow in the altered liquid. Those are different causal propositions. Seeing small moving bodies in a fermenting infusion does not by itself choose among them.
Infusoria were organisms encountered in infusions of organic material. Many recognizable examples are now classified as ciliates or other unicellular eukaryotes, with a cell nucleus, organized feeding structures and locomotory cilia or flagella. Historical usage could extend much more widely than a modern taxonomic group. Some organisms found in an infusion feed on the bacteria already growing there; their later appearance can thus reflect ecological succession rather than production of the first chemical change.
Vibriones commonly denoted minute actively moving, often elongated, curved or sinuous forms. It does not follow that every historical vibrion belongs to the modern genus Vibrio. Bacteria, in some nineteenth-century classifications, meant a narrower set of minute rod-like bodies than the present bacterial domain. In modern terms bacteria are prokaryotes, lacking a membrane-bound cell nucleus; they generally multiply by binary fission. Shape and apparent movement alone do not establish their species. Brownian motion of nonliving particles also must not be mistaken for independent locomotion.
Three positions illustrate the debate available in 1866:
- Chemical decomposition transmitted by contact. Justus von Liebig treated a ferment as unstable nitrogenous material whose own alteration communicates decomposition to fermentable substances. On such an account, multiplication of a definite living organism is not the necessary primary explanation; organisms found in the altered material can be secondary accompaniments or consequences. This is a claim about the initiating cause, rather than merely the true observation that fermentation has chemical reactants and products.[ref]Organic Chemistry in its Applications to Agriculture and Physiology by Justus von Liebig. Part II, chemical processes of fermentation, decay and putrefaction; contact with decomposing nitrogenous matter, 1840
- A specific living ferment. Louis Pasteur's work connected particular transformations with the growth of particular microscopic agents. His lactic-fermentation account describes inoculating a suitable medium with a small amount of ferment, observing its multiplication and the associated chemical change. For alcoholic fermentation the familiar agent is yeast, now classified among fungi, not an infusorian animal or a bacterium. Different products can indicate different microbial processes rather than interchangeable effects of any decomposing material.[ref]Mémoire sur la fermentation appelée lactique by Louis Pasteur. §II, Fermentation lactique; inoculation, growth and the concluding contrast with the chemical-contact interpretation, communicated 1857
- Spontaneous generation or heterogenesis. Félix Archimède Pouchet defended the production of microscopic life from suitable organic materials without the introduction of pre-existing organisms of the same kind, a position developed in his 1859 Hétérogénie by Félix Archimède Pouchet. That interpretation makes some apparent organisms products of the putrefying preparation. It must be separated from the question whether, once present, they can accelerate further decomposition. Louis Pasteur opposed this explanation of ordinary laboratory infusions through controlled exclusion of contaminating particles.[ref]Sur les corpuscules organisés qui existent dans l'atmosphère by Louis Pasteur. Historical discussion of heterogenesis and experiments excluding atmospheric dust, 1861
The decisive experimental question is not whether fresh air is present, but what accompanies it. In a curved-neck flask, a boiled nutrient liquid remains in communication with air while much airborne dust settles in the neck. An otherwise comparable preparation can be exposed by breaking the neck or bringing trapped material into contact with the liquid. The comparison tests whether access of contaminating material, rather than air as such, permits growth and putrefaction. Heat treatment, the preparation's composition, possible surviving resistant forms and accidental contamination all affect interpretation. A negative culture under one condition is not a universal proof about the origin of life.[ref]
Sur les corpuscules organisés qui existent dans l'atmosphère by Louis Pasteur. Curved-neck experiments: unaltered liquid with air access; alteration following removal of the neck or disturbance admitting particles
Modern fermentation is an enzyme-mediated metabolic process; putrefaction commonly involves microbial degradation of nitrogenous organic matter, with mixed and sometimes foul-smelling products. Organisms can be causal agents, and their products can subsequently feed other organisms. Thus the two directions of influence need not be mutually exclusive in a mixed infusion. The mere coexistence of bacteria, ciliates, gas and odour supplies much weaker evidence than controlled inoculation, time-resolved microscopy and chemical measurement.
A later qualification is essential. Eduard Buchner's cell-free fermentation experiments, first reported in 1897 and described in his 1907 lecture, showed that an extract of yeast could carry out alcoholic fermentation without intact living yeast cells. This supports chemical machinery made by organisms; it does not restore the earlier indiscriminate decomposition-by-contact hypothesis. These later findings must not be put into an 1866 candidate's mouth.[ref]
Cell-free fermentation by Eduard Buchner. 1907 lecture, preparation of pressed yeast juice and demonstration of fermentation; historical account of the 1897 discovery
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