Leuconostoc mesenteroides is a widely recognized species of bacteria belonging to the lactic acid bacteria (LAB) group, important in both food science and industrial biotechnology. It is defined by its ability to convert sugars into organic acids. This organism is a powerful natural fermenter, playing a role in food production, often determining the flavor, texture, and shelf-stability of many common foods.
Identifying the Bacteria
This microorganism is classified as a non-spore forming, Gram-positive, facultative anaerobic coccus. Structurally, L. mesenteroides appears as spherical cells, often forming pairs or short chains, and is typically non-motile. It has complex nutritional requirements, needing various amino acids and vitamins to grow efficiently.
The natural habitat of L. mesenteroides is primarily the surface of plants, making it an epiphyte that relies on other organisms for support. It is particularly prevalent on the skin of fruits and vegetables, such as cabbage and cucumbers, which explains its common involvement in vegetable fermentations. Its psychrotrophic nature, meaning it can grow at refrigeration temperatures, contributes to its survival and dominance in cold environments. The organism is also heterofermentative, a distinction that describes its complex sugar metabolism.
The Primary Function in Food Fermentation
The primary benefit of L. mesenteroides in food production stems from its heterofermentative metabolism. This process involves converting hexose sugars, like glucose, into a mixture of end products rather than just lactic acid. The end products include roughly equimolar amounts of lactic acid, ethanol or acetic acid, and carbon dioxide gas.
This specific metabolic pathway is responsible for the characteristic qualities of many fermented foods. The organic acids, particularly lactic acid, lower the pH of the food matrix, which preserves the product by inhibiting the growth of spoilage and pathogenic organisms. Carbon dioxide gas provides the desirable texture and leavening in products like sourdough bread and the characteristic fizz in fermented beverages.
Furthermore, the mixture of ethanol and acetic acid, alongside other compounds like diacetyl, contributes to the complex flavor profile of fermented items. L. mesenteroides initiates the fermentation of vegetables like cabbage into sauerkraut and cucumbers into pickles. It is considered a starter culture in these processes because its rapid growth and acid production quickly create an environment that favors the subsequent growth of other, more acid-tolerant lactic acid bacteria. In traditional foods like idli batter, the carbon dioxide it produces is essential for the leavening that gives the final product its light texture.
Commercial Production of Dextran
Beyond its role in food preservation, L. mesenteroides is used in industrial settings for its ability to produce the exopolysaccharide dextran. Dextran is a complex sugar polymer synthesized by the bacterium when grown in a sucrose-rich medium. The organism uses the enzyme dextransucrase to cleave the sucrose molecule and build the long, branched chain of the dextran polymer.
This microbial product is isolated and purified for applications in the pharmaceutical and chemical industries. A recognized medical use is as a blood plasma volume expander, helping maintain circulatory volume following severe blood loss. Specific, hydrolyzed fractions of dextran are also used to create materials like Sephadex, which are cross-linked gels employed for separating and purifying proteins.
Dextran is also used as a stabilizer, emulsifier, and carrier molecule in various pharmaceutical preparations and cosmetic products. The molecular weight of the dextran produced can be highly controlled, with some strains producing polymers up to 2.0 million Daltons. In the food industry, this compound acts as a thickener, preventing sugar crystallization and improving moisture retention in products like jams and ice cream.
Negative Impacts: Spoilage and Health Concerns
Despite its beneficial applications, L. mesenteroides can act as a food spoilage organism, particularly in sugar-rich or refrigerated environments. When the bacterium metabolizes sucrose in uncontrolled conditions, it produces excessive amounts of dextran polymer. This overproduction results in a slimy, viscous texture on the food surface, commonly known as “ropiness,” which is undesirable in products like sugar syrups, fruit juices, and chilled meats.
Its ability to grow at low temperatures makes it a frequent problem in refrigerated, modified atmosphere packaged meats and vegetable products, where it can cause discoloration and off-odors. The gas production that is desirable in fermentation can also cause spoilage in packaged foods, leading to the undesirable “blowing” or swelling of the packaging. Research has shown that high levels of carbon dioxide, often found in modified atmosphere packaging, can actually trigger the bacterium to upregulate its dextran synthesis genes, exacerbating the spoilage issue.
While L. mesenteroides is generally considered safe for human consumption and has a “Generally Recognized as Safe” (GRAS) status, it can pose a health risk in rare circumstances. Infections are almost exclusively limited to immunocompromised individuals or those with underlying health issues. In these vulnerable populations, the bacterium can act as an opportunistic pathogen, leading to infections such as bacteremia. These rare cases are often associated with the use of medical devices like central venous catheters, rather than the consumption of fermented food.