Warm a pan of milk to about 110°F and add two bacteria. Under the international Codex standard for fermented milks, those two are what make the result yogurt: Streptococcus thermophilus and the subject of this page. Each one grows better in the other's company than alone. Neither one makes yogurt by itself.
Lactobacillus bulgaricus is therefore one of the best-known organisms in fermented food, and one half of a partnership, which is not how it is usually presented. It is a long, rod-shaped lactic acid bacterium that prefers heat, growing best at temperatures warmer than most of its relatives tolerate, and it turns the sugar in milk almost entirely into lactic acid.
It is also an organism that has been shedding parts of itself for a long time. That turns out to be the most interesting thing about it.
Where the name came from
The name records a country, not a property. The organism was first isolated in 1905 by the Bulgarian physician Stamen Grigorov, from Bulgarian soured milk, and it is named for where that milk came from.
Its formal names have changed several times since. Sigurd Orla-Jensen called it Thermobacterium bulgaricum in 1919. It was moved into the genus Lactobacillus in 1971, and in 1983 Weiss, Schillinger and Kandler made it a subspecies of Lactobacillus delbrueckii. Its full current name is Lactobacillus delbrueckii subsp. bulgaricus.
When the old genus Lactobacillus was split into 25 genera in 2020, most of the organisms you will find on a label were given new genus names, including Lactobacillus plantarum and Lactobacillus reuteri. This one kept its name. It belongs to the delbrueckii group that the narrowed genus Lactobacillus was built around (Zheng et al., International Journal of Systematic and Evolutionary Microbiology, 2020).
Why the Lactobacillus bulgaricus genome is shrinking
Most bacteria are generalists by necessity. They live in changeable places, so they carry the machinery to handle changeable food.
This one has been living in milk. When its genome was sequenced in 2006, the researchers found an organism in the middle of what they called extensive and ongoing reductive evolution: a large number of broken genes, incomplete metabolic pathways and relatively little regulatory machinery (van de Guchte et al., PNAS, 2006). It has been losing functions it no longer needs.
The authors read the genome as the record of a move. They concluded that L. bulgaricus adapted from a life associated with plants to the stable, protein-rich and lactose-rich environment of milk, by dropping what it no longer used and by cooperating with S. thermophilus. Milk supplies much of what the missing pathways used to make, so the organism breaks down milk protein and takes what it needs.
It got better at milk by getting worse at almost everything else. That is specialization by subtraction, and it is the reason anybody has ever heard of it.
Where Lactobacillus bulgaricus finally turned up in the wild
The genome pointed to plants. A separate study published the following year found it there (Michaylova et al., FEMS Microbiology Letters, 2007).
They started from a traditional Bulgarian way of making a yogurt starter: put a branch of a particular plant into boiled sheep's milk, keep it warm, and wait for it to set. Following that method, they collected hundreds of plant samples from four regions of Bulgaria, incubated them in sterile milk, and recovered both L. bulgaricus and S. thermophilus, at low frequencies. Yogurt made with the plant strains was not significantly different from yogurt made with commercial starters. The authors reported it as the first isolation of either organism from plants.
That makes the traditional method something more than folklore. It was a way of collecting the culture from the place it came from.
Does Lactobacillus bulgaricus live in the gut?
It is best understood as a food organism. It is known for what it does in milk, and it is not counted among the long-term residents of the human gut microbiome the way some of the bacteria listed beside it on a label are. That is not a criticism of it. It is a description of where it lives and what it is for.
As with every species on this blog, the published research attaches to named strains rather than to the species, and a finding about one strain is not a finding about another.
Questions people actually ask
What is L. bulgaricus?
Lactobacillus bulgaricus is a heat-loving lactic acid bacterium, first isolated in 1905 from Bulgarian soured milk. Its full current name is Lactobacillus delbrueckii subsp. bulgaricus. With Streptococcus thermophilus it is one of the two organisms that define yogurt under the Codex standard.
Why is it called bulgaricus?
Because the milk it was first isolated from was Bulgarian. The name records a place, not a property of the organism.
Is it the same as Lactobacillus delbrueckii?
It is a subspecies of it, and has been since 1983. Labels generally print the short form.
Did its name change in 2020?
No. Unlike most organisms formerly called Lactobacillus, it stayed in the genus.
What do Streptococcus thermophilus and Lactobacillus bulgaricus do together?
They ferment milk into yogurt. Each grows better alongside the other, and the genome study of L. bulgaricus describes the relationship as cooperation.
For the vocabulary around all of this, see prebiotics, probiotics and postbiotics. More of this kind of reading is on our science pages.
