Leuconostoc mesenteroides: Metabolism, Habitat, Electrogenics

Leuconostoc mesenteroides is a lactic acid bacterium known for its role in food fermentation and potential biotechnology applications. It contributes to the production of fermented foods like sauerkraut and dairy products while exhibiting metabolic traits relevant to industrial and scientific research.

Beyond fermentation, this microorganism has diverse metabolic pathways and has been studied for its ability to generate electrical activity under specific conditions.

Classification And Morphology

Leuconostoc mesenteroides belongs to the phylum Bacillota, class Bacilli, order Lactobacillales, and family Lactobacillaceae. Historically classified under Leuconostocaceae, phylogenetic analyses based on 16S rRNA sequencing led to its reassignment. It is a Gram-positive, facultatively anaerobic bacterium, meaning it thrives in low-oxygen environments but can tolerate some oxygen exposure. Its ability to ferment carbohydrates into lactic acid places it within the lactic acid bacteria (LAB) group.

Morphologically, it appears as irregular, ovoid cocci that form short chains or pairs. Unlike rod-shaped lactobacilli, its coccoid structure provides a distinct identification marker under a microscope. Its thick peptidoglycan cell wall, characteristic of Gram-positive bacteria, enhances resistance to osmotic stress, an advantage in environments with fluctuating solute concentrations. The bacterium is non-motile, lacks endospores, and reproduces through vegetative cell division.

A key trait is its production of extracellular polysaccharides, particularly dextrans and levans, through sucrose metabolism. These exopolysaccharides contribute to biofilm formation and viscosity in food products, influencing texture and stability. Dextran synthesis, catalyzed by dextransucrase enzymes, has been extensively studied for food and pharmaceutical applications. This polysaccharide production aids in surface adherence and environmental resistance.

Common Habitats

Leuconostoc mesenteroides thrives in carbohydrate-rich environments, particularly plant material and fermented foods. It is frequently isolated from raw vegetables such as cabbage, beets, and carrots, where it initiates natural fermentation. These plant-based habitats provide abundant simple sugars for metabolism, supporting bacterial growth. The outer surfaces of these vegetables harbor diverse microbial communities, with Leuconostoc mesenteroides contributing to spontaneous fermentation in traditional food preservation.

Fermented foods are a major ecological niche for this bacterium. It plays a dominant role in sauerkraut, kimchi, and pickles, converting sugars into organic acids that enhance flavor and inhibit spoilage microorganisms. In dairy products like certain cheeses and cultured butter, its metabolic activity influences texture and viscosity.

Beyond food-related environments, Leuconostoc mesenteroides is found in soil and plant rhizospheres, where it interacts with plant roots and microbial populations. Carbohydrate-rich root exudates support its growth, while its biofilm-forming ability aids colonization. It has also been identified in water sources associated with decaying plant material, highlighting its ecological adaptability.

Metabolic Pathways

Leuconostoc mesenteroides employs heterolactic fermentation, distinguishing it from homofermentative lactic acid bacteria. Instead of exclusively producing lactic acid, it metabolizes glucose via the phosphoketolase pathway, yielding lactic acid, ethanol or acetic acid, and carbon dioxide. This flexibility allows it to adapt to varying carbohydrate availability, adjusting fermentation products accordingly. The carbon dioxide produced contributes to the texture and aeration of fermented foods.

Its ability to synthesize exopolysaccharides like dextran and levan adds metabolic complexity. When exposed to sucrose, it utilizes glucansucrase enzymes to produce dextran, a high-molecular-weight polysaccharide with industrial applications in food and pharmaceuticals. Dextran also enhances biofilm formation, aiding surface adherence and environmental resilience.

Leuconostoc mesenteroides adapts to oxygen variations by shifting its metabolic profile. Under microaerophilic conditions, it increases acetic acid production over ethanol, altering the acid balance in its environment. This flexibility influences microbial interactions in fermentation ecosystems. The bacterium also possesses nitrate reductases, allowing it to use alternative electron acceptors when oxygen is scarce, further enhancing survival in fluctuating conditions.

Electrogenic Observations

Recent studies suggest Leuconostoc mesenteroides may exhibit electrogenic capabilities, particularly in microbial fuel cell (MFC) research. While primarily studied for fermentation, certain strains appear capable of extracellular electron transfer (EET) under specific conditions. This trait, usually associated with electroactive bacteria like Geobacter and Shewanella, indicates potential bioelectrochemical applications.

Research has identified redox-active metabolites, such as quinones and flavins, that may facilitate electron transfer between bacterial cells and external electrodes. These compounds enable indirect electron transfer, allowing the bacterium to generate bioelectric currents despite lacking conductive nanowires or cytochromes typical of electroactive microbes. Experiments using glucose-based media have recorded measurable current outputs, though efficiency remains lower than established electroactive species. While not highly specialized for electrogenesis, Leuconostoc mesenteroides may still contribute to electron flow in mixed microbial communities, particularly in environments where metabolic byproducts interact with conductive surfaces.