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A decomposer breaks down dead organic material and absorbs products of that breakdown; many bacteria and fungi perform this role. Decomposers connect detritus from different trophic levels to material recycling, so placing all decomposers at one final step of every food chain is misleading. See Clark, Douglas and Choi, Biology 2e, 46.1: grazing and detrital food webs.

A fungus adapted to infect keratinised tissues. Malassezia associated with pityriasis versicolor is not thereby a dermatophyte.

A eukaryote is an organism belonging to the lineage characterized by a membrane-bounded cell nucleus and extensive cellular compartmentalization. A nuclear envelope surrounds the nuclear genome; nuclear chromosomes are typically linear. Ribosomes perform biological translation outside the cell nucleus. See OpenStax Microbiology, 3.4: nuclear organization and protein synthesis. Specialized biological cells can lose their cell nucleus, as mature mammalian red blood cells do, while remaining parts of eukaryotic organisms; see OpenStax Biology 2e, 40.2: mature red blood cells.

Yeasts, plants and animals are all eukaryotes. Typical cells in these groups have mitochondria, but chloroplasts belong to photosynthetic plant and algae lineages, not to every eukaryote. A cell wall occurs in typical plant and fungus cells but is absent from animal cells. These are comparisons of representative cells, not a checklist every specialized cell must satisfy. See OpenStax Biology 2e, 4.3: nucleus, organelles and representative cells.

The interesting feature is controlled separation: cell membranes and organelle membranes let different cellular processes occupy distinct compartments while membrane transport connects them. The evolution of eukaryotes also involved partnership: mitochondria descend from a bacterial endosymbiosis. Modern phylogenetics supports an archaeal ancestry for the nuclear lineage, alongside this bacterial contribution; see two-domain system. Thus the history of a whole eukaryote cannot be read as the history of just one of its genes.

A fungal cell wall contains structural polysaccharides including chitin and glucans. Its composition and organization vary among fungi; the diagram illustrates a composite rather than a pure chitin shell. A fungus sharing a cell wall with a plant does not imply shared wall chemistry.^[ref]

A fungus is a eukaryote that obtains nutrients by absorption rather than photosynthesis. Fungi include unicellular yeasts and multicellular forms. Their cell walls differ from bacterial peptidoglycan walls; having a cell wall does not make a fungus a plant. See OpenStax Biology 2e, 24.1: fungal nutrition and organization.

A genus of fungi including skin-associated yeasts.

The yeast plant is a fungus, whereas the characteristic vinegar-forming organisms are acetic acid bacteria. Calling both vegetable growths reflects an older classification of microscopic organisms; it does not make them green plants or two stages of one species.

The familiar brewing and baking yeast is Saccharomyces cerevisiae. Its oval or rounded cells possess a cell wall, cell membrane, cytoplasm, cell nucleus and other eukaryotic structures. Ordinary multiplication is by budding: a small outgrowth enlarges, nuclear division supplies it, and separation leaves a scar or a temporary attached cluster. Under suitable conditions, the life cycle also includes mating between compatible haploid cells and formation of spores by diploid cells. Those processes are distinct from simply breaking a chain of accidentally adherent cells. Neither roots nor a leaf-like apparatus is required for its growth.^[ref]

Yeast obtains organic nutrients from its surroundings. In alcoholic fermentation it converts fermentable sugar into ethanol and carbon dioxide; the idealized glucose balance isA real culture also uses material to build cells and produces additional compounds. The process does not require oxygen as a reactant in that net balance, although yeast can respire and its growth and product yields depend on conditions. In bread the gas expands the dough; in a fermenting liquid both gas evolution and chemical change can be followed. Gas bubbles alone do not identify the organism responsible.

The historical vinegar plant or mother of vinegar appears as a surface film or gelatinous mass in a suitable alcoholic liquid. Louis Pasteur used the name Mycoderma aceti for the film-forming agent in his investigations of acetification. In modern terms, members of several acetic-acid-bacterial groups can occur in vinegar cultures. Rod-shaped bacterial cells reproduce by binary fission; some form an extracellular, often cellulose-rich, matrix that helps produce a visible pellicle. The gross mother is therefore a growth structure or microbial community, not a single large cell and not necessarily a pure culture of one modern species.^[ref]

Its essential chemical role is oxidation of ethanol to acetic acid using atmospheric oxygen:Thus the historical term acetic fermentation denotes an oxidative microbial process, unlike the oxygen-independent net alcoholic-fermentation reaction above. Surface access to air favours the process. With suitable organisms and conditions, further oxidation of the acid can occur, so indefinitely leaving an exhausted culture exposed to air does not guarantee indefinite accumulation of vinegar.

Louis Pasteur's Mémoire sur la fermentation acétique by Louis Pasteur, published in 1864, supplies contemporary experimental evidence appropriate to this paper. Inoculating an appropriate alcoholic medium with material from an active film, observing film development and measuring the rise of acidity connects the growth with the chemical process. Comparisons with uninoculated or differently aerated preparations distinguish mere atmospheric exposure from the action of the growth. His experiments also examined the effects of submergence and the conditions under which the film performed acetification.^[ref]

Wine turning to vinegar therefore illustrates succession: yeast can first supply an alcoholic substrate from sugar, and acetic acid bacteria can then oxidize that substrate when oxygen is available. A white surface growth is not automatically the active vinegar agent; other yeasts and moulds can form films or spoil the liquid. Microscopy, controlled transfer and chemical measurement together provide a natural-history account stronger than identifying every surface skin by its household name.

Muguet denotes the adherent white fungal coating of oral thrush, now oral candidiasis; it is not simply any white tongue or any mouth ulcer. Patches occur on the tongue and oral mucosa and may merge into broader plaques. The underlying surface can be red, sore and liable to bleed when a plaque is disturbed. An infant may feed poorly because the mouth hurts. Milk residue lacks the same persistent inflamed adherent lesion, while an aphthous ulcer is a focal ulcer rather than a confluent fungal plaque. ^[ref]

Cause and nature. The modern explanation is opportunistic overgrowth of Candida, commonly Candida albicans, on a susceptible mucosal surface. An immature or impaired immune system, illness and altered local conditions favour it. Antibiotics, corticosteroids, diabetes and other causes of reduced host defence are relevant modern risk factors. These were not all understood in 1865, and exposure to the organism alone does not explain every case. Local oral candidiasis also does not, by itself, establish invasive fungal disease. ^[ref]

Evidence available in the nineteenth century. Fungal forms could already be observed with a microscope. A contemporary 1849 review reports Charles West's suggestion that the organisms could be transferred by shared cups and spoons, while the reviewer preferred an explanation involving decomposition and acidity. The review is evidence of the disagreement, not authority for spontaneous generation of a fungus. It also illustrates why historical “aphthae”, ulceration and Muguet should not all be treated as identical modern diagnoses. ^[ref]

Diagnosis and treatment. Clinical inspection is often sufficient; when uncertain or atypical, material from the lesion can be examined microscopically for fungal forms, with further laboratory testing when needed. Repeated or unusually severe disease calls for attention to underlying illness and host defence, rather than simply repeating local cleansing. ^[ref]

The historically defensible measures include maintaining adequate feeding, gentle mouth care and clean feeding utensils. Harsh scraping and treating every white patch as the same disease are inappropriate. Present-day treatment adds an age-appropriate antifungal drug, such as a suitable topical nystatin preparation; more extensive or resistant disease may require systemic treatment such as fluconazole. Drug choice and formulation depend on the patient's age and clinical state. Those agents are later additions and must not be placed in an 1865 prescription. Hygiene and correction of predisposing factors accompany, rather than replace, effective antifungal treatment. ^[ref]

The historical name pityriasis grouped finely scaling eruptions; its varieties were not all one fungal infection. A useful period list is simple or scalp pityriasis, pityriasis rubra, pityriasis versicolor and pityriasis nigra. The names describe distribution or colour, and classifications differed. In particular, “nigra” was sometimes a very dark form of “versicolor,” sometimes a pigmentation disorder with little true scaling. It cannot safely be translated into one present species of fungus.^[ref]

Simple pityriasis, often called pityriasis capitis on the scalp, gives loose, whitish, bran-like scales with little or variable redness. Microscopy of a scale principally shows flattened keratinised epithelial cells and debris. An oily or inflamed scalp can also contain organisms normally living there; this does not make every case identical to pityriasis versicolor. Present descriptions of dandruff and seborrhoeic dermatitis explain many such appearances, but the historical label cannot retrospectively specify the cause of each case.^[ref]

Pityriasis rubra denoted a conspicuously red, scaling eruption, sometimes extensive. Detached scales are again principally epidermal material. A modern skin biopsy may show an inflammatory pattern, but there is no single microscopical organism that defines the historical category. It must not automatically be renamed pityriasis rubra pilaris, a distinct follicular disorder. An extensive red and peeling skin is a pattern requiring causal diagnosis, rather than proof of one infection.^[ref]

Pityriasis versicolor produces irregular pale, yellowish-brown or brown patches, commonly on the upper anatomical trunk, with a fine surface scale. Here the microscopy is especially discriminating: a superficial scraping, cleared with potassium hydroxide, shows clusters of round yeast forms and short curved hyphae within the stratum corneum. Modern classification places the organisms in Malassezia. The older “Microsporon furfur” terminology should not be read as identification of a modern Microsporum dermatophyte. On a histological section the organisms occupy the horny surface layer and are made conspicuous by suitable fungal stains.^[ref]

Pityriasis nigra calls first for verification of what is actually being described. If it is dark pityriasis versicolor, the same superficial yeast and hyphae may be found; if the lesion is pigmentation without a fungal eruption, this finding cannot be assumed. A contemporary discussion explicitly recorded the overlap of nigra and versicolor and disputes over the significance of the supposed parasite.^[ref] Thus epidermal scales are common to the description, but demonstrable fungal structures particularly distinguish pityriasis versicolor. Examine actual scrapings with a optical microscope, compare several fields and use appropriate controls before attributing a different scaling disorder to a fungus.

The main differences concern cellular construction and the usual modes of obtaining food, movement and development; the main interdependence concerns transfers of matter and energy through ecosystems. The nineteenth-century animal–vegetable division is useful as a comparison of familiar forms, but it is not a complete modern classification of life.

A typical plant cell has a cell wall, a large central vacuole and, in photosynthetic tissue, chloroplasts. An animal cell lacks a cellulose wall and chloroplasts and has a different extracellular and tissue organization. Both possess a cell membrane, cytoplasm, a cell nucleus and, in their usual aerobic cells, mitochondria. Roots and other non-green parts of a plant need not contain functional chloroplasts; an erythrocyte's loss of its cell nucleus is likewise not a definition of all animal cells.^[ref]

Most green plants acquire carbon through photosynthesis, taking carbon dioxide from their surroundings and mineral nutrients and water chiefly through roots. Familiar animals obtain preformed organic material by feeding and digesting it or absorbing material derived from a host. Animals generally have conspicuous coordinated movement, and many have nervous systems and muscles; higher plants regulate growth and movement without such organs. These are useful generalizations with exceptions. Some plants depend on other organisms for carbon; some animals are permanently attached; and neither lack of locomotion nor a green appearance alone settles every microscopic classification. Fungi and bacteria should not be forced into one of these two kingdoms in a modern account.

One major interdependence is nutritional. Photosynthesis converts light input into chemical energy stored in organic material. Plant material feeds herbivores and, through them, carnivores; it also enters detrital food webs. An animal that eats an animal can therefore remain indirectly dependent on primary production by plants, algae or other photosynthetic organisms. Chemical elements pass between organisms, but energy is progressively dissipated as heat; it is not recycled in the same closed manner as an element.^[ref]

A second relation is gas exchange. In its simplest overall form, oxygenic photosynthesis may be represented byAerobic utilization of the resulting glucose has the complementary overall balanceThese balanced equations show complementary material transfers without making the underlying pathways literal reversals of one another. Plants also perform cellular respiration, by day as well as by night. They can therefore release carbon dioxide while consuming oxygen; a sufficiently illuminated leaf may simultaneously show net uptake of carbon dioxide because photosynthesis exceeds respiration. Stating that plants only purify air and animals only spoil it would erase this essential distinction between gross processes and net exchange.

Other relations close material cycles. Animals return organic matter through excretion and death; microbial decomposition and mineral transformations return nutrients in forms available for plant uptake. Most atmospheric nitrogen is not directly available to ordinary plants merely because animals have breathed or excreted near them: nitrogen fixation, ammonification and often nitrification involve particular microorganisms. Plants and animals also interact through pollination, seed dispersal, grazing, parasitism and shelter. Such relations may benefit both partners, one partner at the other's expense, or different partners under different conditions; interdependence is not universal mutual benevolence.

A closed illuminated aquarium containing plants and animals gives an instructive experiment, provided gas exchange with air, illumination, nutrient supply and organismal amounts are specified. Dissolved oxygen and carbon dioxide should be followed over light and dark periods. Survival alone is an ambiguous readout: microorganisms, stored food and air exchange may support it temporarily. A more controlled comparison includes illuminated and dark plant vessels and animal-only vessels. The expected alternation of net gas exchange makes the physiological distinction visible while exposing why no simple rule guarantees indefinite balance in any arbitrarily stocked closed jar.

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:

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]

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]

A ring-shaped patch is a clinical shape, not one histological disease. The historical category included several named scalp and skin affections whose relationship was disputed. In particular, old claims that alopecia areata was a fungal disease cannot be carried into modern pathology. A contemporary review actually records competing classifications of favus, scalp ringworm, circinate eruptions, beard disease and patchy baldness, and disagreement about whether their fungi were identical.^[ref]

The microscopic distinctions should be organized by the structure infected or injured:

Annular eczema or psoriasis supplies a further clinical mimic: an inflammatory epidermal pattern alone is not evidence of an invading fungus. For an actual microscopic investigation, collect scale from an active margin and affected hairs when hair is involved, mount the material on a microscope slide with potassium hydroxide to clear keratin, and examine under a microscope. A modern biopsy can be examined with Periodic acid-Schiff staining to highlight fungal structures. Distribution within the surface layer, hair shaft, follicle or bulb matters as much as the presence of particles. Fungal culture or molecular identification can resolve species when morphology cannot.^[ref] Distinguish the organism, its anatomical location and the host reaction; do not diagnose all circular lesions as one infection.

Yeasts are fungi whose growth is predominantly unicellular, commonly through budding or division into two cells. The word describes a growth form across different fungal lineages, rather than one species or an exclusive clade; some fungi can switch between unicellular and filamentous growth. Thus Saccharomyces cerevisiae is a yeast, but yeast is not another name for that one species. See Kurtzman, Fell and Boekhout, The Yeasts, chapter 1 (2011): definition and classification.

A yeast cell has eukaryotic organization, including a cell nucleus, mitochondria and a cell wall. Small size and a unicellular lifestyle do not make it a member of bacteria. These compartments let an experimentally accessible biological cell retain much of the organization shared with other eukaryotes. See Clark, Douglas and Choi, Biology 2e, 24.1: fungal cells and yeast growth.

A motivating experiment: can fermentation continue without intact yeast cells? In 1897, Eduard Buchner ground brewers' yeast with quartz sand and diatomite, added water, and pressed out a liquid extract. Adding concentrated cane sugar produced carbon dioxide after 15–60 minutes. A cell-retaining filter still yielded an active filtrate: with sterilized cane sugar in an icebox, fermentation began after about a day. Saturated lactose gave no observed fermentation. These are reported observations, not invented control results. See Buchner, Alcoholic fermentation without yeast cells (1897), translated excerpt hosted by Carmen Giunta.

The filtration comparison addresses the objection that surviving intact yeast cells caused the activity. The lactose comparison tests substrate dependence; it is not a no-sugar blank, nor a comparison at matched sugar concentrations. A proposed modern follow-up would also compare the same extract with and without added sugar, accounting for material already in the extract. That is an experimental suggestion, not a result attributed to Buchner. His report specifies brewers' yeast; it does not identify the preparation here as a pure Saccharomyces cerevisiae strain.

Heat treatment asks a different question: does destroying activity in the extract stop fermentation? Buchner's preliminary report described weak or absent activity after heating and filtration, rather than one clean universal threshold. Harden's 1914 account of the subsequent work summarized loss of activity on heating the juice to $50{}^{\circ }\mathrm{C}$. Harden also described requirements for both enzymes and coenzymes. Cell-free therefore means neither chemical-free nor a preparation containing one purified enzyme. See Arthur Harden, Alcoholic Fermentation (1914), chapters II and VI: cell-free activity and the fermenting complex.

The inference is that extracted cellular machinery can sustain these chemical reactions without an intact living biological cell. This does not show that the extract reproduces, grows or maintains itself like a yeast. Moreover, gas evolution alone does not identify ethanol: cellular respiration can also release carbon dioxide. Product identification is a separate measurement; Harden's account describes both ethanol and carbon dioxide as products of cell-free alcoholic fermentation. The beautiful separation is between the organism that produces and maintains the machinery and the machinery that catalyzes a particular transformation.