Lactobacillus bulgaricus (formally Lactobacillus delbrueckii subsp. bulgaricus) is widely recognized for its use in fermenting dairy products, particularly yogurt. The question of whether it qualifies as a probiotic depends on its ability to survive digestion and confer a health benefit after consumption. Examining the scientific criteria for a probiotic helps clarify the difference between a beneficial food culture and a therapeutic microorganism.
What Defines a Probiotic
The World Health Organization (WHO) and the Food and Agriculture Organization (FAO) define probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.” This definition requires the organism to be sufficiently characterized, safe for its intended use, and have its purported health benefits backed by positive human clinical trials.
A defining feature is that the microorganism must remain viable and present in the product at an effective dose throughout its shelf life. Crucially, the bacteria must be able to survive the journey through the hostile environment of the human gastrointestinal tract. This means passing through the acidic stomach and resisting the effects of bile salts in the small intestine to reach the colon.
The Essential Role in Fermented Foods
Lactobacillus bulgaricus is primarily known as a starter culture, not a gut colonizer. It is one of the two bacteria legally required for traditional yogurt, working alongside Streptococcus thermophilus in a synergistic symbiosis that drives the fermentation process.
L. bulgaricus breaks down milk proteins into amino acids, which in turn are used by S. thermophilus to thrive. Both organisms produce large amounts of lactic acid by fermenting the milk sugar, lactose. This rapid drop in pH causes the milk’s casein proteins to coagulate, which gives yogurt its characteristic thick texture and tart flavor. The primary function of L. bulgaricus is to transform milk into yogurt, a metabolic action that is complete before the product is consumed.
Navigating Survival in the Digestive Tract
The key factor disqualifying Lactobacillus bulgaricus as a traditional probiotic is its limited ability to survive the gastrointestinal transit, which prevents it from establishing significant colonization in the gut. Unlike true probiotic strains that are selected for their hardiness, L. bulgaricus is generally sensitive to the low pH of stomach acid. Some studies have found that it adheres poorly to intestinal cells and its viability can be significantly reduced by gastric juices.
While some research indicates that a substantial number of these bacteria can be recovered in the feces of people who regularly consume yogurt, suggesting some survival, this transient passage does not equate to colonization. The organisms that survive may offer temporary effects, but they do not typically set up a long-term presence in the intestinal microbial community. For this reason, L. bulgaricus is more accurately classified as a beneficial food culture rather than a true probiotic.
The main health benefit attributed to the active cultures in yogurt, including L. bulgaricus, is the improvement of lactose digestion in individuals who are lactose intolerant. This benefit is largely due to the bacteria carrying their lactase enzyme through the digestive tract, which continues to break down the remaining lactose in the food product itself. This beneficial effect is directly tied to the food matrix and the bacteria’s activity within it, not its ability to permanently colonize the host.