oeni

Oenococcus oeni, often referred to simply as oeni in wine microbiology, is a lactic acid bacterium best known for its central role in malolactic fermentation (MLF) of wine. It is a Gram-positive, non-spore-forming, non-motile bacterium that typically appears as cocci or short rods and is adapted to the acidic, alcohol-rich conditions found in wine.

Physiology and metabolism of oeni enable it to grow at low pH and tolerate ethanol. It is a facultatively anaerobic organism with a metabolism geared toward malolactic fermentation, in which malate is decarboxylated to lactate and carbon dioxide. This conversion reduces overall acidity and can influence mouthfeel and flavor. Some strains also metabolize citrate, which can lead to minor diacetyl or buttery notes under certain conditions.

Ecology and industrial relevance are closely tied to winemaking. O. oeni populations are found on grapes, in grape must, and in winery environments, but MLF is often initiated deliberately using commercial starter cultures to ensure predictability. The bacterium typically conducts MLF after alcoholic fermentation and under controlled temperatures. Its performance is influenced by pH, temperature, sulfur dioxide levels, and residual nutrients; overly acidic must or high SO2 can inhibit growth, risking a stuck or incomplete MLF. Diacetyl production by certain strains can contribute to aroma compounds but may be undesirable if excessive.

Taxonomy and genomics place O. oeni within a wine-adapted group of lactic acid bacteria. The genome is compact, around 1.8 to 2.0 megabase pairs, frequently with multiple plasmids, reflecting adaptation to the wine niche. Strain diversity underpins differences in MLF kinetics, SO2 tolerance, and sensory impact, making selection important for winemaking goals.

See also: malolactic fermentation, wine microbiology, lactic acid bacteria.