Álvaro García
Ensiling, the process of preserving forage crops, relies heavily on bacterial communities to ferment plant sugars into organic acids, primarily lactic acid. This fermentation process lowers the pH and preserves the silage. Among the many bacteria involved, Enterococcus faecium plays an early but crucial role. Though not commonly recognized as a main silage inoculant, E. faecium is essential for “jumpstarting” fermentation, preparing the environment for more dominant lactic acid bacteria (LAB) like Lactobacillus and Pediococcus to take over.
Characteristics of Enterococcus faecium
Enterococcus faecium is a facultative anaerobic bacterium, meaning it can grow in both oxygen-rich (aerobic) and oxygen-depleted (anaerobic) environments. This adaptability is particularly valuable in the initial stages of silage fermentation when oxygen is still present in the forage mass. It is metabolically versatile, utilizing sugars to produce acetic acid, ethanol, carbon dioxide, and other fermentation byproducts.
Unlike LAB, which focuses mainly on lactic acid production, E. faecium produces less lactic acid but plays a critical role in slightly lowering the pH during this early phase. This initial pH drop is essential for creating favorable conditions for LAB to flourish, particularly when the initial LAB population in the forage is low.
E. faecium proliferates best at temperatures between 25-37°C (77-99°F), making it particularly active during the early stages of fermentation when the temperature remains relatively stable. As fermentation progresses, LAB activity begins to lower the silage temperature, which also influences the microbial community shift from E. faecium to LAB.
Early fermentation role of Enterococcus faecium
In the first few days of fermentation, E. faecium consumes available oxygen and sugars, producing carbon dioxide and some acetic acid. This activity helps reduce the silage pH from approximately 6.5 to around 5.5. While this reduction is not enough to fully preserve the silage, it is a necessary step in priming the environment for lactic acid bacteria (LAB) such as Lactobacillus and Pediococcus. By consuming oxygen and initiating this slight pH drop, E. faecium helps create the anaerobic conditions required for the more efficient LAB to thrive.
As oxygen is depleted and the pH continues to fall, E. faecium becomes less competitive. LAB, which are more acid-tolerant and highly efficient at producing lactic acid, begin to dominate the fermentation process. At this point, Lactobacillus and Pediococcus take over, rapidly lowering the pH to levels below 4.0, which are crucial for long-term silage preservation. As the environment becomes more acidic, E. faecium is outcompeted and its population gradually declines.
LAB dominance and silage preservation
The efficiency of LAB in silage fermentation is largely due to their ability to produce high quantities of lactic acid, the primary acid responsible for the rapid pH drop necessary for effective preservation. Lactobacillus plantarum, for example, is known for its ability to quickly convert sugars into lactic acid, driving the pH down to around 4.0 within just a few days. Pediococcus pentosaceus works alongside Lactobacillus, particularly in multi-strain inoculants, by contributing to faster acidification and helping stabilize the silage.
LAB are homofermentative, meaning they produce primarily lactic acid, which results in a more rapid and efficient pH drop compared to E. faecium. As the pH falls below 4.0, LAB thrive while E. faecium is unable to maintain its population in the increasingly anaerobic and acidic conditions.
Although Enterococcus faecium is eventually outcompeted by LAB, its early action is particularly critical in forage crops that have low initial LAB populations. In these cases, E. faecium serves as a bridge, facilitating the transition to a LAB-dominated fermentation by slightly lowering the pH and reducing oxygen levels. This early contribution ensures that LAB can quickly take over, leading to faster and more complete fermentation, which ultimately improves the quality and preservation of the silage.
While Enterococcus faecium may not be the primary bacterium responsible for long-term silage preservation, its role in the early stages of fermentation is vital. By helping create anaerobic conditions and slightly lowering the pH, E. faecium sets the stage for more efficient fermenters like Lactobacillus and Pediococcus. As LAB outcompete E. faecium, the fermentation process stabilizes, resulting in high-quality, well-preserved silage. Understanding the role of Enterococcus faecium highlights the importance of the complex bacterial succession that occurs during ensiling, with each microorganism contributing to different stages of the process.
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