Streptococcus thermophilus – The Friendly Dairy Bacterium
- NPSelection
- Jan 1
- 4 min read
What it is
Streptococcus thermophilus (often shortened to S. thermophilus) is one of the most essential bacteria used in the food industry. It’s a small, round, heat-loving bacterium that thrives in warm milk. It belongs to the Streptococcus family, which includes both helpful and harmful species, but S. thermophilus is completely harmless and has been safely consumed for centuries.
This bacterium helps turn milk into yoghurt and cheese by converting milk sugar (lactose) into lactic acid. The process gives fermented dairy its tangy flavour, creamy texture, and natural protection against unwanted microbes.
Discovery and early studies
The bacterium was first described in 1919 by the Danish scientist Orla-Jensen, who studied lactic acid bacteria. For many years, scientists thought it was simply a variety of Streptococcus salivarius, a harmless bacterium from the human mouth. Later genetic studies in the 1980s and 1990s proved that S. thermophilus is actually its own species.
Modern DNA research shows that it probably evolved from an ancestor that lived in the mouths of humans or animals. Over time, it adapted perfectly to life in milk and lost many genes it no longer needed — a process scientists call “genome reduction.”
Where it comes from
Unlike many bacteria that live in soil or water, S. thermophilus has specialised almost entirely to dairy environments. Its natural home is milk, yoghurt, and cheese. It grows best at warm temperatures (around 40–45 °C) and prefers low-oxygen conditions.
Occasionally, scientists have found related strains on plants or in the environment, but these are rare. Most likely, S. thermophilus has lived alongside humans for thousands of years, quietly helping to preserve milk and create nutritious food.
How it’s used in food
1. Yoghurt production
S. thermophilus is one of two essential bacteria used to make traditional yoghurt. The other is Lactobacillus delbrueckii subsp. bulgaricus.
The two work together in a perfect partnership:
S. thermophilus starts fermenting quickly and produces formic acid and carbon dioxide, which help L. bulgaricus grow.
L. bulgaricus breaks down milk proteins into amino acids that S. thermophilus needs.
This teamwork gives yoghurt its smooth texture, pleasant acidity, and fresh aroma.
2. Cheese making
In many cheeses — such as Emmental, Gruyère, Mozzarella, and Parmesan — S. thermophilus is used as a starter culture. It helps acidify milk rapidly, making it curdle and develop the right texture. Some strains also produce exopolysaccharides, natural sugars that make the cheese softer and creamier.
3. Food safety
Because it produces lactic acid quickly, S. thermophilus lowers the pH of milk, which prevents harmful bacteria from multiplying. This natural acidification is one reason yoghurt and cheese can last longer without spoiling.
Health benefits
Although not all strains are officially classed as “probiotics,” S. thermophilus can still be very good for health:
Helps digest lactose – It produces lactase (β-galactosidase), an enzyme that breaks down milk sugar. This makes yoghurt easier to digest for people with mild lactose intolerance.
Supports gut balance – It can survive passage through the digestive system for a short time, helping maintain a healthy balance of gut bacteria.
Reduces antibiotic-related diarrhoea – Studies show that eating yoghurt containing S. thermophilus during antibiotic treatment can lower the risk of diarrhoea.
Strengthens the gut lining – Animal and cell studies suggest that it may protect intestinal cells and reduce inflammation.
Safe and well-tested – It is officially recognised as safe by both the European Food Safety Authority (EFSA) under the QPS list and the U.S. FDA under GRAS (Generally Recognised As Safe) status.
How does it help the food texture?
Some strains of S. thermophilus produce long sugar molecules called exopolysaccharides (EPS).These act as natural thickeners and stabilisers. They make yoghurt creamier, reduce whey separation (the watery layer on top), and improve mouthfeel. In low-fat products, this helps replace the texture typically provided by fat.
Scientific importance
S. thermophilus has also played an essential role in scientific discovery. When scientists studied how bacteria defend themselves from viruses (called bacteriophages), they discovered a natural immune system in S. thermophilus — the CRISPR system. This discovery has since become one of the most revolutionary tools in genetics, allowing scientists to edit DNA with great precision.
Fun facts
Without S. thermophilus, there would be no yoghurt as we know it today.
It’s eaten by billions of people every day — one of the most widely consumed bacteria in the world.
It’s one of the few Streptococcus species that is completely non-pathogenic (not linked to disease).
Its name comes from Greek: “streptos” (twisted chain) and “thermophilus” (heat-loving).
It doesn’t form spores and doesn’t move — it simply grows in small round chains.
Some strains can’t fully use galactose (half of milk sugar), so they release it during fermentation. Cheesemakers must balance this carefully to avoid off-flavours.
Because it’s sensitive to bacteriophages (viruses that infect bacteria), dairies often rotate starter cultures to keep production stable.
Points to remember
Every strain of S. thermophilus is slightly different so that the benefits can vary.
It doesn’t permanently stay in the human gut — it’s more like a helpful visitor than a resident.
It must be protected from high heat after fermentation, since pasteurisation would kill it.
In the EU, it’s listed as a Qualified Presumption of Safety (QPS) organism — a list reserved for microbes with an excellent safety record.
In summary
Streptococcus thermophilus is a quiet hero of everyday life. It turns fresh milk into yoghurt and cheese, helps people digest dairy, protects food from spoiling, and has even contributed to breakthroughs in modern science.
With its long history of safe use, S. thermophilus reminds us how beneficial microbes can be — not only for flavour and nutrition, but also for human health and discovery.
Sources
Orla-Jensen, S. (1919). The Lactic Acid Bacteria. Copenhagen.
Schleifer, K.H. et al. (1991). “Reclassification of Streptococcus salivarius subsp. thermophilus as Streptococcus thermophilus.” International Journal of Systematic Bacteriology.
FEMS Microbiology Reviews (2005). “Comparative genomics of Streptococcus thermophilus.” Oxford University Press.
Wikipedia – Streptococcus thermophilus
Frontiers in Microbiology (2023). “Functional and safety characterisation of S. thermophilus strains.”
Microbiology Research (2024). “Re-evaluation of S. thermophilus taxonomy.”
ASM Microbe Blog (2020). “A Short History of Yoghurt and the CRISPR connection.”
EFSA QPS list – European Food Safety Authority (2024 update).
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