Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus

What it is

These are the two lactic acid bacteria that turn milk into yogurt. They grow at high temperature, are sensitive to bile, and do not survive well in the human gut.

What the research says

The best-supported effect is on lactose. The enzyme these live cultures carry helps people who cannot digest lactose to digest the lactose in yogurt, and FAO, WHO and EFSA recognise this. Beyond that, the evidence is weak. In a large UK trial in older hospital patients, a drink containing these cultures plus L. casei did not reduce antibiotic-associated diarrhoea. Only a few bloodstream infections have been reported, in people with serious bowel disease.

Evidence grade: Well established.

What goes wrong

A regular yogurt eater with transient bowel ischaemia developed S. thermophilus bacteraemia, reported as the first case linking the bacterium from food to bloodstream infection. (Source 1)

• Case report, Very low certainty.
• Size: 1 patient.
• Who: Adult with polycythaemia and transient bowel ischaemia.
• How long: n/a.
• Result: S. thermophilus isolated from blood cultures.
• Funding: not stated.

Here we present a case of a regular consumer of Activia yoghurt who developed S. thermophilus bacteraemia, probably due to transient bowel ischaemia secondary to polycythaemia.

What the evidence supports

In people who cannot digest lactose, all live yogurts tested improved lactose digestion (measured by breath hydrogen), and L. bulgaricus milk gave nearly complete digestion. (Source 2)

• Randomized trial, Moderate certainty.
• Size: not given in the abstract.
• Who: Healthy adults with lactose maldigestion.
• How long: Single-meal tests.
• Result: All yogurts improved lactose digestion similarly, whatever their beta-galactosidase activity.
• Funding: not stated on the fetched page.

All yogurts dramatically and similarly improved lactose digestion, regardless of their total or specific β-gal activity.

A review states that the lactose effect is shared by all yogurt starter cultures and is recognised by FAO, WHO and EFSA; the review was produced with Danone-funded support. (Source 3)

• Systematic review, Moderate certainty.
• Size: Narrative review.
• Who: General population.
• How long: n/a.
• Result: Species-level trait; needs live cells at ingestion, not survival in the gut.
• Funding: industry-linked (editorial support funded by Danone Institute International)

This effect is shared by all yogurt starter cultures and results from the presence of the lactose-hydrolyzing enzyme in all strains of the used species of lactic acid bacteria.

What the evidence does not support

A drink of L. casei DN114001 with these two yogurt cultures did not prevent antibiotic-associated diarrhoea in about 1,126 older UK hospital patients (the paper reports 1,127 randomised). (Source 4)

• Randomized trial, Moderate certainty.
• Size: 1,127 patients (group sizes 549 + 577 = 1,126)
• Who: UK hospital patients over 55 receiving antibiotics.
• How long: 2 weeks follow-up.
• Result: AAD 19.3% vs 17.9%; OR 1.10, 95% CI 0.82-1.49, P = 0.53.
• Funding: not stated on the fetched page.

Limit of this finding: The paper says 1,127 patients were randomised, but its two groups (549 + 577) add up to 1,126, so one patient is unaccounted for in the abstract. This does not change the result (19.3% vs 17.9%, P = 0.53), but readers should treat the total as approximately 1,126 to 1,127. The authors also say changes in antibiotic stewardship and falling C. difficile rates mean the trial cannot rule out a role for the probiotic.

No significant evidence was found of a beneficial effect of the specific probiotic formulation in preventing AAD in this elderly population

Only about 1% of these bacteria reach the duodenum, so claims that they colonise the gut are not well supported. (Source 5)

• Systematic review, Low certainty.
• Size: Narrative review.
• Who: Healthy people eating yogurt.
• How long: n/a.
• Result: About 1% survive to the duodenum.
• Funding: industry-linked (Danone)

survival in the upper part of the gastrointestinal tract is low (ie, only 1% of the bacteria are able to reach the duodenum)

How much

• Reference intake: No recommended intake or upper limit exists: live bacteria are not nutrients, and no body has set an RDA, AI or UL for them. The EU (QPS list) and US (GRAS) treat this group as presumed safe in foods. (Source 3)
• Upper limit: No tolerable upper intake level has been set for yogurt cultures. (Source 6)
• Studied: Yogurts and single-species fermented milks given as test meals to lactose maldigesters. (Source 2)
• Studied: A daily drink of L. casei DN114001 plus S. thermophilus and L. bulgaricus during antibiotic treatment. (Source 4)

A common belief, and what the research shows

The belief: Yogurt cultures settle in and recolonise your gut.

What the research shows: These species are 'not ideally suited for survival into the human gut', and the lactose benefit 'does not require survival and reproduction of the bacterial cells during intestinal transit'.

Questions and answers

What is it?

These are the two bacteria used to ferment milk into yogurt. They are heat-loving lactic acid bacteria that are sensitive to bile. (Source 6)

What does it do in the body?

Their main known effect is to help break down lactose in yogurt through an enzyme they carry. This helps people who digest lactose poorly, and it works even if the bacteria do not survive in the gut. (Source 3)

Is it good or bad for you?

For people who digest lactose poorly, live yogurt improved lactose digestion. They are generally regarded as low-risk, but one bloodstream infection has been reported in a yogurt eater with bowel ischaemia. (Source 1)

How do you get more of it?

In studies, people ate yogurt or fermented milk containing live cultures. The lactose benefit needs the cells to be alive when the yogurt is eaten. (Source 3)

If it is harmful, what reduces it?

Does not apply in healthy people. The case report advises that finding S. thermophilus in blood may point to underlying bowel disease rather than contamination. (Source 1)

Why might someone be low in it or missing it?

These are not established gut residents. Few survive transit, so a person who does not eat live yogurt would not be expected to carry them. (Source 5)

Which whole foods contain it or feed it?

Yogurt is made with these two cultures by definition, and they are used across the dairy industry. (Source 1)

What happens if you do not have it?

Nothing is known to go wrong without them, because they are not normal gut residents. People who digest lactose poorly may simply lose the lactose-digesting help that live yogurt gives. (Source 2)

How can you test for it?

No validated consumer test for these cultures exists. Their survival is low, which leaves few fecal studies, and consumer stool tests disagree widely with each other. (Source 7)

References

1. JMM Case Reports. Streptococcus thermophilus bacteraemia in a patient with transient bowel ischaemia secondary to polycythaemia. 2015. DOI 10.1099/jmmcr.0.000060. Read the source
2. The American Journal of Clinical Nutrition. Strains and species of lactic acid bacteria in fermented milks (yogurts): effect on in vivo lactose digestion. 1991. DOI 10.1093/ajcn/54.6.1041. Read the source
3. The American Journal of Clinical Nutrition. Yogurt, living cultures, and gut health. 2014. DOI 10.3945/ajcn.113.073072. Read the source
4. Journal of Hospital Infection. Do probiotics prevent antibiotic-associated diarrhoea? Results of a multicentre randomized placebo-controlled trial. 2020. DOI 10.1016/j.jhin.2020.01.018. Read the source
5. The American Journal of Clinical Nutrition. Yogurt, living cultures, and gut health. 2014. DOI 10.3945/ajcn.113.073072. Read the source
6. The American Journal of Clinical Nutrition. Yogurt, living cultures, and gut health. 2014. DOI 10.3945/ajcn.113.073072. Read the source
7. Communications Biology. Evaluating the analytical performance of direct-to-consumer gut microbiome testing services. 2026. DOI 10.1038/s42003-025-09301-3. Read the source