Lactobacillus delbrueckii subsp. bulgaricus (until 2014 known as Lactobacillus bulgaricus) is one of several bacteria used for the production of yogurt. It is also found in other naturally fermented products. First identified in 1905 by the Bulgarian doctor Stamen Grigorov, the bacterium feeds on lactose to produce lactic acid, which is used to preserve milk.
It is a gram-positive rod that may appear long and filamentous. It is non-motile and does not form spores. It is regarded as aciduric oracidophilic, since it requires a low pH (around 5.4–4.6) to grow effectively. The bacterium has complex nutritional requirements.

Lactobacillus delbrueckii subsp. bulgaricus is commonly used alongside Streptococcus thermophilus as a starter for making yogurt.

The two species work in synergy, with L. d. bulgaricus producing amino acids from milk proteins, which are then used by S. thermophilus.

Both species produce lactic acid, which gives yogurt its tart flavor and acts as a preservative. The resulting decrease in pH also partially coagulates the milk proteins, such as casein, resulting in yogurt's thickness. While fermenting milk, L. d. bulgaricus produces acetaldehyde, one of the main yogurt aroma components.

Some strains of L. d. bulgaricus, such as L. bulgaricus GLB44, also produce bacteriocins, which have been shown to kill undesired bacteria in vitro.

It is often helpful to sufferers of lactose intolerance, whose digestive systems lack the enzymes to break down lactose to simpler sugars.
Lactobacillus bulgaricus is the only probiotic, named after a nation and a geographic territory. It is the first microorganism, selected by man thousands of years ago that has changed the human evolution.

Lactobacillus bulgaricus is of vegetable origin, isolated from the green plants only on the territory of Bulgaria.

In other regions of the world it mutates and stops its reproduction. In contrast to the other lactobacilli, it produces D (-) lactic acid, which determines its powerful antimicrobial, anti-carcinogenic, anti-atherosclerotic, antiallergic, antioxidative, ray-protective and other beneficial effects. Lactobacillus bulgaricus develops in symbiosis with other plant probiotic – Streptococcus thermophilus.
Their joint development leads to a mutual intensification and balancing of their favourable effects. If administered orally, the plant probiotics Lactobacillus bulgaricus and Streptococcus thermophilus quicken the development of Lactobacillus acidophilus, living in human and animal organisms. It is difficult for Lactobacillus acidophilus to develop on its own, its cells die easily and cannot reach the necessary amount. Symbiosis like this happens between the plant lactobacilli and the Bifidobacteria. Administered by food with the help of the lactobacilli they have a secondary reproduction in body, reaching the dose needed to heal the human organism.

Yoghurt consumption delays organism’s aging.
Yoghurt and dairy products are the best source of easy to assimilate Calcium which is vitally important for bones and teeth building.
Lactic acid bacteria prevent cancerogenic substances synthesis, limit mutations and formation of cancer cells.
Yoghurt has a probiotic effect – it influences extremely beneficially stomach and intestines.
Lactic acid bacteria in yoghurt help improving the immunity and for a successful struggle against infections.
Yoghurt helps prevention of cardiac diseases.
Milk and dairy products lower the risk of caries development in teeth.
Yoghurt is of an ant toxic action.
Fresh milk and dairy products are one of the most full value and balanced foods ever known to humanity.
And not in the last place – dairy products are extremely tasty and are an indispensable part of the national cuisine since centuries.

Ilya Metchnikoff, a professor at the Pasteur Institute in Paris, researched the relationship between the longevity of Bulgarians and their consumption of yogurt.
Metchnikoff's research also noted that rural populations in Southeastern Europe daily consume milk fermented with lactic acid bacteria and live relatively longer than other populations. Based on these data Metchnikoff proposed that consumption of fermented milk seeds the intestine with harmless lactic acid bacteria increasing intestinal acidity and suppressing the growth of proteolytic bacteria.