Leuconostoc citreum is a microorganism that plays a significant role in food science and biotechnology. This bacterium belongs to the Lactic Acid Bacteria (LAB) group, which converts sugars into organic acids, thereby preserving and transforming foods. While often overshadowed by other LAB species, L. citreum is a powerhouse of fermentation responsible for generating unique flavors, textures, and beneficial compounds in a wide array of fermented products around the globe. It was formally recognized as a distinct species in 1989, following studies on vancomycin-resistant cocci.
Defining the Bacterium Leuconostoc citreum
Leuconostoc citreum is classified as a Gram-positive, non-spore-forming bacterium belonging to the family Lactobacillaceae. Its cells are spherical (coccus-shaped) and typically arrange themselves in pairs or short chains. This organism is facultatively anaerobic and catalase-negative.
A primary characteristic of L. citreum is its obligate heterofermentative metabolism. This means it ferments sugars into a mix of products, including lactic acid, carbon dioxide, and ethanol or acetate, distinguishing it from homofermentative bacteria. The bacterium is naturally associated with plant materials, roots, and greenery.
Its natural habitats include raw milk, cold food goods, brines, and various fermented foods. It is frequently isolated from traditional products like kimchi, sauerkraut, and sourdough environments. Unlike closely related species such as L. mesenteroides, L. citreum is unable to metabolize lactose.
L. citreum is intrinsically resistant to the antibiotic vancomycin, a trait shared with other Leuconostoc species. This resistance stems from the unique structure of its cell wall peptidoglycan. While this feature is a point of interest in clinical microbiology, the bacterium is primarily recognized for its beneficial roles in food systems.
Genetic Structure and Capabilities
Genomic sequencing reveals that L. citreum has a relatively small genome, typically around 1.71 to 1.80 megabases (Mb) in length. This compact size reflects a streamlined genetic architecture focused on its specific lifestyle.
The bacterium’s genetic material frequently includes multiple circular plasmids, which are small, non-chromosomal DNA molecules. These plasmids often carry specialized genes that confer selective advantages, such as those related to antibiotic resistance or the production of antimicrobial compounds.
The core genome confirms the necessary gene set for heterolactic fermentation. Beyond fermentation, the genome contains an extensive collection of genes encoding carbohydrate hydrolases and transporters. This genetic machinery suggests a strong adaptation to environments rich in complex plant-derived materials, allowing it to efficiently utilize a wide spectrum of sugars.
Specific gene clusters are dedicated to the synthesis of exopolysaccharides (EPS), which are high molecular weight carbohydrates secreted outside the cell. These genes enable the organism to produce glucans and fructans, like dextran and levan, when sucrose is available. The organism also possesses the complete genetic inventory required for the biosynthesis of certain vitamins, including riboflavin (Vitamin B2).
These genetic capabilities also extend to biopreservation mechanisms, as many strains possess genes for the production of bacteriocins. Bacteriocins are proteinaceous toxins that inhibit the growth of competing bacteria, including some foodborne pathogens. This genomic potential allows L. citreum to thrive in competitive microbial ecosystems and contributes to its utility in food processing.
Metabolic Processes and Fermented Products
The core of L. citreum’s function lies in its heterofermentative metabolism. This process breaks down glucose to yield lactic acid, carbon dioxide (CO2), and either ethanol or acetate. The production of CO2 contributes to the characteristic gas and “open” texture found in many fermented foods like sourdough and cheese.
The bacterium is valued for its ability to metabolize citrate, a common organic acid in milk and plant substrates. Citrate is transported into the cell and converted into pyruvate. This metabolic shunt results in the formation of flavor-active compounds such as diacetyl and acetoin.
Diacetyl is a volatile compound that imparts the distinct buttery aroma and flavor to fermented dairy products like buttermilk and certain cheeses. The production of these aromatic compounds, alongside organic acids, significantly shapes the final sensory profile of the food product.
L. citreum is also a prolific producer of Exopolysaccharides (EPS), such as dextran and other glucans and fructans, when grown on sucrose. These EPS molecules are secreted into the surrounding medium and are responsible for increasing the viscosity and improving the texture and mouthfeel of fermented foods. Dextran contributes a desirable slimy or ropy texture in certain traditional fermented beverages and foods.
A commercially relevant metabolic product is the sugar alcohol mannitol, which L. citreum efficiently produces from fructose. Mannitol is a low-calorie sugar substitute used in the food industry. Strains of L. citreum are utilized to reduce the overall sugar content in products like baked goods while maintaining sweetness and improving texture.
Role in Food Production and Microbial Ecology
Leuconostoc citreum is widely used in food production, particularly as a starter culture to ensure consistent fermentation. It is a dominant species during the early and middle stages of fermentation in traditional vegetable products, most notably kimchi, where its presence is linked to rapid acidification and flavor development.
In the dairy industry, its ability to produce diacetyl makes it a valued component in starter cultures for artisanal cheeses and fermented creams. The CO2 it generates also assists in forming the characteristic small holes, or “eyes,” in certain cheese varieties. Its role extends to the baking industry, where it is used in sourdough fermentation, contributing to flavor, texture, and nutritional enhancement, including in gluten-free products.
The bacterium plays a significant role in biopreservation by extending a product’s shelf life and inhibiting undesirable microorganisms. This protective function is accomplished through the production of organic acids, which lower the pH, and the secretion of bacteriocins, which act as a targeted antimicrobial defense against pathogens like Listeria monocytogenes.
In the broader microbial ecosystem, L. citreum contributes to diversity and can influence the growth of other beneficial microbes. Research indicates that its presence can synergistically enhance the colonization and function of other probiotic bacteria, such as Lactiplantibacillus plantarum, suggesting potential for use in multi-strain probiotic products.
Despite its beneficial uses, L. citreum has a dual ecological role and can occasionally be involved in food spoilage, particularly in vacuum-packed, cooked meat products where its EPS production causes an undesirable slimy texture. However, the organism is largely recognized as safe, holding a Generally Recognized As Safe (GRAS) status, affirming its safety for consumption and widespread use in the food industry.