The Microbiome in Wine: Beyond Yeast

The Microbiome in Wine: Beyond Yeast

For most of the history of enology, the microbiology of wine was understood primarily through the lens of a single organism: Saccharomyces cerevisiae, the yeast that drives Fermentation. Its role was foundational, and understanding it well seemed sufficient. But over the past two decades, advances in DNA sequencing technology — particularly next-generation sequencing methods that can rapidly identify all microbial species in a complex sample without the need to culture them — have revealed that wine's microbial world is vastly more complex, diverse, and interesting than previously imagined.

The Vineyard as Microbial Ecosystem

The journey of wine's microbiome begins long before grapes enter the winery. The vineyard soil, vine surfaces, grape berries, and winery environment together constitute a complex and interconnected microbial ecosystem spanning bacteria, fungi, viruses, and archaea.

Soil microbiome: Vineyard soils harbor extraordinarily diverse microbial communities. A gram of healthy vineyard soil may contain a billion individual microorganisms representing thousands of species — bacteria (including Proteobacteria, Actinobacteria, Firmicutes, and Acidobacteria), fungi (including both saprophytic decomposers and mutualistic mycorrhizal species), and archaea (often involved in nitrogen cycling). These communities are shaped by soil type, pH, temperature, rainfall, management practices (Organic Wine vs. conventional, tillage vs. cover crop), and the specific vine species growing above them.

Mycorrhizal fungi — organisms that form symbiotic relationships with vine roots, extending the root's effective absorptive surface area in exchange for photosynthate — are particularly important components of the vineyard soil microbiome. AMF (arbuscular mycorrhizal fungi) from genera including Rhizophagus, Glomus, and Funneliformis colonize the root cortex and create extensive hyphal networks in the soil. These networks improve water and phosphorus uptake, particularly under drought stress, and may also influence the vine's resistance to soil-borne pathogens. Management practices that disrupt soil (tillage, synthetic fertilizer application) tend to reduce AMF diversity, while organic and biodynamic management supports more diverse mycorrhizal communities.

Grape berry surface: The surface of a grape berry at harvest supports its own distinct microbial community. Bacterial genera including Gluconobacter, Gluconacetobacter, Erwinia, and lactic acid bacteria (Leuconostoc, Lactobacillus, Pediococcus) coexist with yeasts including Hanseniaspora uvarum, Starmerella bacillaris, Metschnikowia pulcherrima, and many others. The composition of this community differs by variety, vintage, and geographic origin — and has been proposed as one mechanism through which Terroir expresses itself in wine.

Studies comparing berry microbiomes from different Burgundy appellations have found statistically distinguishable microbial community structures — a result consistent with the proposition that place-specific microorganisms contribute to place-specific wine character. However, the extent to which these microbial differences persist through winemaking and express as flavor differences remains an active area of research.

Lactic Acid Bacteria: The Second Fermentation

After S. cerevisiae completes primary alcoholic fermentation, a second microbial community takes center stage: the lactic acid bacteria (LAB) responsible for Malolactic Fermentation (MLF). This transformation — the conversion of tart Malic Acid to softer lactic acid by bacterial metabolism — is one of the most consequential microbial events in winemaking.

The dominant LAB species in wine is Oenococcus oeni, a remarkably stress-tolerant organism adapted to the extreme conditions of wine: high alcohol (10–15%), low pH (3.0–3.8), low nutrient availability, and the inhibitory presence of Sulfur Dioxide used to protect wine from spoilage. O. oeni survives these conditions through multiple adaptations including fatty acid membrane remodeling, malate decarboxylase enzyme activity for energy generation from malate, and effective sulfite resistance mechanisms.

But O. oeni is not the only LAB species active in wine. Species of Lactobacillus, Pediococcus, and Leuconostoc can also drive MLF under certain conditions — and these organisms are less desirable than O. oeni because they are more likely to produce off-flavors. Lactobacillus species are associated with the production of biogenic amines (histamine, tyramine) — compounds that cause adverse reactions in sensitive individuals and that are subject to regulatory limits in some countries. Pediococcus species can produce exopolysaccharides that give wine an undesirable "ropy" texture, a condition known as "ropiness" or "graisse."

The interplay between yeast and LAB is managed carefully by winemakers through Sulfites additions, temperature control, and timing decisions. In natural and Natural Wine production, where sulfite additions are minimal or absent and spontaneous fermentation is embraced, the microbial dynamics are more complex and less predictable — which is both the appeal and the risk of this approach to winemaking.

Acetic Acid Bacteria and Volatile Acidity

Among the most significant bacterial spoilage organisms in wine are the acetic acid bacteria (AAB) — members of the genera Acetobacter and Gluconobacter that oxidize ethanol to acetic acid under aerobic conditions. The result is Volatile Acidity: primarily acetic acid and its ester ethyl acetate, which impart vinegary aromas detectable in wine at relatively low concentrations (threshold around 600 mg/L acetic acid for most tasters).

AAB are naturally present on grape surfaces and in winery environments. They are strictly aerobic: they require oxygen to grow and can only be active where wine is exposed to air. This makes managing oxygen exposure (through tank headspace, barrel maintenance, and transfer practices) a primary strategy for controlling volatile acidity. Wineries using whole-cluster press and early SO₂ addition can largely suppress AAB activity during winemaking; those pursuing natural fermentation with minimal intervention must accept somewhat elevated volatile acidity as a potential consequence.

Winery and Cellar Microbiomes

The winery itself harbors a distinct microbial community that interacts with wine over the course of production and aging. Wood — in barrels, tanks, and structural elements of old stone cellars — is a particularly rich reservoir of microorganisms.

Oak barrels are not microbiologically inert surfaces. The porous wood harbors communities of bacteria and fungi in its deeper layers, even after toasting during cooperage. Brettanomyces bruxellensis — the notorious spoilage yeast responsible for "Brett" character in wine — can colonize and survive in oak barrel wood, resisting cleaning and sanitation attempts. Brett produces phenolic compounds including 4-ethylphenol (band-aid, medicinal), 4-ethylguaiacol (smoke, spice), and isovaleric acid (sweaty, rancid) that dramatically alter wine character. At low levels, some winemakers and critics consider Brett character to add complexity; at higher levels, it is universally recognized as a defect.

The famous cellars of historic Burgundian estates, Champagne houses, and Rioja bodegas harbor what might be called "house microbiomes" — microbial communities established over decades or centuries in the stone walls, old wood, and cellar air that interact with wines aging in that environment. The extent to which these cellar-specific microbiomes contribute identifiable character to wine remains scientifically speculative but is enthusiastically promoted by producers who age in historic caves and cellars.

The Microbiome and Terroir

Perhaps the most scientifically controversial and culturally resonant dimension of wine microbiology is its relationship to Terroir. The traditional concept of terroir — the complete natural environment of a wine, including soil, climate, and topography — is increasingly being expanded to include the microbial community of the place.

Studies have now documented geographic structuring of wine-relevant microbiomes at multiple levels: vineyards in different appellations within the same region have distinct microbial community compositions; wines fermented with native microorganisms from different sites show different chemical profiles; and in some blind tastings, wines from spontaneous fermentation are more reliably identified with their geographic origin than inoculated wines.

The mechanistic hypothesis is straightforward: if a place has a distinctive microbial community, and if native fermentation incorporates that community into the fermentation process, and if different microbes produce different fermentation metabolites, then wines fermented natively should reflect their microbial geography. This is, in essence, a microbial extension of the terroir concept.

The counterargument — that S. cerevisiae comes to dominate all fermentations regardless of starting microbiome, and that non-Saccharomyces organisms make metabolic contributions too small and transient to reliably distinguish place — is also scientifically defensible. The research on both sides is ongoing and not yet conclusive.

Microbiome in Bottle Aging

Wine is not a sterile product. Even bottled wine with SO₂ protection may contain viable microorganisms at very low populations, including Brettanomyces, O. oeni, and various bacteria. Under normal bottling conditions, these populations do not grow significantly. However, in wines with very low SO₂ (particularly Natural Wine and Organic Wine bottlings), microbial activity in bottle can continue slowly, contributing to the complex evolution of aged wines.

The interaction between microbial metabolites produced during fermentation and chemical reactions occurring during bottle aging is the subject of active research. The Lees particles present in unfiltered wines contain yeast and bacterial cells that contribute enzymes capable of hydrolyzing wine compounds (esters, glycosides) and releasing precursors for flavor compounds. This "posthumous" microbial contribution to bottle aging may be one mechanism through which minimal-intervention wines develop distinctive aged character.

The microbiome of wine is, ultimately, a lens through which the false distinction between "natural" and "made" becomes visible. Every wine, no matter how technically managed, is a product of microbial life. The choice is not whether microbes will shape the wine, but which ones, under what conditions, and guided by whose intention. The most effective winemakers of the future will be those who can read and collaborate with this invisible biological community — steering it toward the specific expressions of place and grape that make wine inexhaustible in its diversity and fascination.