Lactobacillus delbrueckii subsp. bulgaricus
|Subspecies:||L. d. bulgaricus|
Rogosa & Hansen 1971
Weiss et al. 1984 (subspecies status)
Lactobacillus delbrueckii subsp. bulgaricus (until 1984 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 or acidophilic, since it requires a low pH (around 5.4–4.6) to grow effectively. The bacterium has complex nutritional requirements.
Use in industry
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 also produce bacteriocins, which have been shown to kill undesired bacteria in vitro.
It has also been considered a contaminant of beer due its homofermentatative production of lactic acid, an off-flavor in many styles of beer. In other styles of beer, however, lactic acid bacteria can contribute to the overall appearance, aroma, taste, and/or mouthfeel, and generally produce an otherwise pleasing sourness. 
Ilya Metchnikoff, a professor at the Pasteur Institute in Paris, researched the relationship between the longevity of Bulgarians and their consumption of yogurt. He had the idea that aging is caused by putrefactive activity, or proteolysis, by microbes that produce toxic substances in the intestine.
Proteolytic bacteria such as clostridia, which are part of the normal intestinal flora, produce toxic substances including phenols, ammonia and indols by digestion of proteins. These compounds are responsible for what Metchnikoff called intestinal auto-intoxication, which, according to him, was the cause of the physical changes associated with old age, a concept that has no scientific basis. It was already known at that time that fermentation with lactic acid bacteria inhibits the deterioration of milk because of its low pH.
Metchnikoff's research had also noted that, in Europe, Bulgaria and the Russian steppes, some rural populations who had consumed milk fermented with lactic acid bacteria lived relatively long lives. Based on these data Metchnikoff proposed that consumption of fermented milk would seed the intestine with harmless lactic acid bacteria, which would increase intestinal acidity and thus suppress the growth of proteolytic bacteria.
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