Megasphaera is a genus of bacteria composed of species that are anaerobic, meaning they thrive in environments without oxygen. Their name is derived from their characteristically large, spherical cell shape. These bacteria are found in a wide variety of locations, from the digestive tracts of animals and humans to industrial settings like breweries.
Key Metabolic Functions
The defining metabolic feature of Megasphaera is its role as a major consumer of lactic acid. It efficiently converts lactate, a byproduct of other microbial fermentation, into several different short-chain fatty acids (SCFAs). The primary SCFA produced is butyrate, but it can also generate propionate, valerate, and caproate depending on the species and conditions. This process serves to regulate the local environment.
By consuming lactate, Megasphaera prevents its accumulation, which can lower an environment’s pH to harmful levels. This utilization is facilitated by enzymes that allow the bacteria to process it through specific biochemical routes. When lactate is abundant, it is converted mainly into acetate and propionate. In environments where lactate is limited, the metabolic pathway shifts to produce more butyrate and acetate.
The conversion process involves complex pathways. One route transforms lactate into propionate through a series of steps. Another pathway converts lactate into pyruvate, which then enters the pathways that produce acetate and butyrate. This ability to switch between metabolic outputs demonstrates the bacterium’s flexibility.
Role in Animal and Human Microbiomes
In the digestive systems of ruminant animals like cattle, Megasphaera is a resident of the rumen. High-grain diets cause rapid fermentation that produces large amounts of lactic acid. The species Megasphaera elsdenii consumes this excess lactate, preventing a sharp drop in rumen pH. This action helps avert a condition known as ruminal acidosis.
Within the human large intestine, Megasphaera species contribute to gut health by producing SCFAs. Butyrate is a preferred energy source for the cells lining the colon, known as colonocytes, helping maintain the gut barrier’s integrity and supporting cellular health. This function is often part of a process called cross-feeding, where Megasphaera utilizes lactate produced by other gut bacteria, like Bifidobacterium, and converts it into butyrate.
The presence of Megasphaera can influence the gut microbiota’s overall composition and function. While its butyrate production is beneficial, some studies associate M. elsdenii with increased gas production during certain food fermentation. The balance of its contributions against effects like gas depends on the host’s diet, the specific strain present, and other microbes in the gut community.
Industrial Impact in Brewing
In brewing, Megasphaera is a spoilage microorganism. It is suited to survive in beer, an environment inhospitable to many bacteria due to its low pH, high carbon dioxide, ethanol, and limited nutrients. Improvements in filling technology that create more anaerobic conditions have seen a rise in contamination events involving anaerobic bacteria like Megasphaera.
When Megasphaera contaminates beer, its metabolic activity produces undesirable compounds. While it can cause turbidity, the primary impact is on flavor. The bacteria generate large amounts of butyric acid, imparting a rancid aroma and taste, and other fatty acids also contribute to off-flavors.
Species such as Megasphaera cerevisiae, Megasphaera paucivorans, and Megasphaera sueciensis, are associated with beer spoilage. Its metabolism can also produce diacetyl, a compound that creates a buttery flavor considered a defect in most beer styles. The presence of these bacteria can lead to financial losses for breweries due to product recalls and damage to brand reputation.
Methods for Identification
Identifying Megasphaera can begin with microscopy. Under a microscope, the bacteria exhibit a large, spherical shape, which provides an initial clue to its presence. This allows technicians to visually distinguish it from other potential contaminants or gut residents.
For more precise identification, molecular techniques are employed. Polymerase Chain Reaction (PCR) is a common method where primers, which are short nucleic acid sequences, target and amplify unique regions of Megasphaera’s 16S rRNA gene. Detecting the amplified product confirms the bacterium’s presence.
Gene sequencing provides the highest level of detail. By sequencing the 16S rRNA gene amplified by PCR, researchers can identify the bacteria down to the species level. This technique is useful for distinguishing between different Megasphaera species, such as identifying M. cerevisiae in a spoiled beer sample or M. elsdenii in a gut microbiome analysis.