Streptococcus thermophilus is a widely recognized lactic acid bacterium (LAB) of immense commercial significance. Formally known as Streptococcus salivarius subsp. thermophilus, it is commonly referred to simply as S. thermophilus. Its widespread use in the food industry and positive effects on human health have made it one of the most studied bacterial species globally. It fundamentally shapes the production of dairy products and provides distinct benefits as a consumed probiotic.
Classification and Unique Traits
Streptococcus thermophilus is classified as a Gram-positive bacterium. Microscopically, this species has a spherical or ovoid shape (coccus) and is typically observed arranged in pairs or long chains resembling a string of beads. The organism is non-motile.
A defining characteristic is its thermophilic nature, reflected in its name, which translates to “heat-loving.” This organism thrives at high temperatures, with an optimal growth range between 40 and 45 degrees Celsius (104–113°F). This heat tolerance is a unique trait among the Streptococcus genus. This ability to flourish in warmth makes it useful in industrial processes that require high-heat incubation.
Essential Role in Dairy Fermentation
The primary application of S. thermophilus is its use as a starter culture in the dairy industry for fermented products, including yogurt and certain cheeses like mozzarella. This bacterium rapidly initiates fermentation by metabolizing lactose, the natural sugar in milk, into lactic acid. This conversion quickly lowers the pH of the milk, a process called acidification, which preserves the product and develops its characteristic tangy flavor.
As the acidity increases, the milk proteins, primarily casein, begin to coagulate and link together. This protein denaturation transforms liquid milk into the thick, gel-like texture of yogurt or the curd structure necessary for cheese production. The rapid acidification rate of S. thermophilus is highly valued because it shortens the fermentation time, making the production process more efficient for commercial applications.
Symbiotic Relationship in Yogurt
In traditional yogurt production, S. thermophilus is typically used synergistically with Lactobacillus delbrueckii subsp. bulgaricus. In this symbiotic relationship, S. thermophilus grows quickly and produces compounds that stimulate the growth of its partner bacterium. Conversely, L. bulgaricus breaks down milk proteins into peptides and amino acids, which S. thermophilus utilizes for its own growth, fostering a mutually beneficial environment. Furthermore, certain strains of S. thermophilus produce exopolysaccharides (EPS), which are complex sugars that contribute significantly to the smooth texture and increased viscosity of the final fermented product.
Benefits as a Probiotic
S. thermophilus is recognized as a beneficial probiotic, offering distinct health advantages when consumed. A primary benefit is its role in aiding the digestion of lactose, particularly for individuals who experience symptoms of lactose intolerance. The bacterium naturally produces the enzyme beta-galactosidase (lactase), which breaks down lactose into more easily digestible sugars.
When live S. thermophilus is consumed in fermented products, the organism remains active in the gastrointestinal tract and continues to produce lactase. This enzyme activity within the gut significantly reduces the amount of undigested lactose that reaches the large intestine, alleviating common symptoms such as bloating and gas. The bacterium has also demonstrated an ability to survive the harsh, acidic environment of the stomach and the presence of bile in the small intestine, allowing it to transiently colonize the gut.
Once in the lower digestive tract, S. thermophilus contributes to maintaining a balanced gut microbiota. Its presence helps support the growth of other beneficial microorganisms and may assist in regulating the overall microbial ecosystem. Some research suggests that the consumption of this bacterium may also help to reduce the duration of certain types of diarrhea, including those associated with antibiotic use.