Lactobacillus are one of many species of beneficial (also known as friendly or commensal) bacteria that can be found in the human digestive tract. They were first noted by Nobel Laureate Elie Metchnikoff in the early 1900s when he associated the longevity of Bulgarians with their regular consumption of fermented milk products containing Lactobacillus.
As their name might suggest, they feed on lactose and glucose, turning them into lactic acid. For this, they are known as lactic acid bacteria (LAB). They are frequently used in the fermentation of yoghurt – the lactic acid gives the typical sourness of live yoghurt – cheese, sauerkraut and other fermented foods.
Unlike some other bacteria, they can survive both aerobically (requiring oxygen for their growth and survival) and anaerobically (without oxygen). As a result, they can inhabit both the small intestine and the large intestine, although they reside preferentially in the small intestine which has a greater amount of oxygen. In addition, they can be found in the mouth and urogenital tract, where they account for 90-100% of the vaginal microbiome.
Lactobacillus are a gram-positive Firmicutes bacteria, rod-shaped, non-spore-forming and in general non-motile, meaning that they do not move from whichever site in the body they are inhabiting. Small numbers of them are autochthonous (indigenous), while the majority are consumed through foods and supplements. They will populate the digestive tract for a period before dying off. It is for this reason that regular consumption of foods rich in Lactobacillus or probiotic supplements can help to bolster their numbers.
Types of Lactobacillus
Often found in probiotic supplements, L. acidophilus (which can be translated as ‘acid-loving milk bacteria’) is possibly the most well-known species of Lactobacillus. You can read more about it here.
A gram-positive bacteria, rod-shaped with rounded ends, it occurs singly, in pairs or in short chains and is commonly found in fermented foods. In the human body, it is present in the mucosa (mucous lining) of the mouth, gastrointestinal (GI) tract and vagina.
So-called because of the high number found in saliva, L. salivarius can help maintain the health of the mouth. Both here and in the other mucosal sites of the body, it has been shown to multiply quickly, which assists in crowding out less beneficial bacteria.
L. paracasei is another gram-positive, rod-shaped bacterium, often with square ends. It has played an important role in dairy fermentation historically, used to ripen cheeses, and can be found in the digestive tract as well as naturally occurring in fermented foods.
Genetically similar to L. acidophilus, L. gasseri was identified through genome sequencing in the 1980s. It has been shown to colonise the digestive tracts of infants from birth and is a key species in the female urogenitary tract.
Where are Lactobacilli found?
As mentioned above, Lactobacillus spp. are found in the digestive and urogenital tracts, mouth, skin and breast milk in humans. Able to live both aerobically and anaerobically, they have been found in greater numbers in the duodenum of the small intestine – 6% of the total bacteria – compared to just 1% in the colon – representing the fact that the small intestine is dominated by facultative bacteria species that include Lactobacilli and the colon is dominated by anaerobic bacteria, such as Bacteroidetes. The 1% also represents a far greater absolute number than the 6%, as there are many times more bacteria present in the large intestine than the small intestine.
Within the digestive tract, they attach to the gut wall and produce layers of mucous in which to reside, helping to crowd out pathogenic bacteria that might otherwise take up any available space.
Lactobacillus have been shown to be one of the first bacteria to colonise the digestive tract of a baby after birth, thought to be via transmission from the mother’s birth canal during a vaginal delivery.
Ways in which Lactobacillus benefit us
Help control pathogenic bacteria
Within the digestive tract, Lactobacillus help to control the quantity of pathogenic bacteria by lowering the overall pH through the fermentation of the sugars (carbohydrates) lactose and glucose into lactic acid.
Secrete antimicrobial compounds
Some Lactobacillus strains produce hydrogen peroxide and other antimicrobial compounds which can further reduce numbers of pathogenic bacteria and help prevent dysbiosis.
Produce short chain fatty acids (SCFAs)
SCFAs are a vital energy source for the cells of the gut wall, as well as contributing to the overall health of the digestive tract. The different strains of Lactobacillus each produce different SCFAs; propionate, butyrate and acetate.
Support cholesterol levels
The lactic acid produced by some Lactobacillus strains has been demonstrated to positively affect cholesterol levels.
Contribute to overall health
Studies, both in vitro and in vivo, have shown a number of ways in which different strains of Lactobacillus can support our overall health. At the same time, a decreased presence of Lactobacillus has been linked to various conditions such as diarrhoea-dominant IBS (IBS-D), IBD and Type 2 diabetes.
Play a key role in traditional methods of fermented foods
Their role in the fermentation of products such as sourdough bread, sauerkraut, kimchi and yoghurt gives both the typical sour taste, and probiotic benefits, of these gut-friendly foods.
Lactobacillus in supplements
Lactobacillus have a long history of safe use in fermented foods, and as such have GRAS status (Generally Recognised as Safe). Lactobacillus strains are often found in friendly bacteria supplements, either on their own or in combination with other strains (often Bifidobacteria and Acidophilus).
These supplements may be used to restore good bacteria in the gut that may have been affected by gastrointestinal infections (diarrhoea and vomiting) or medications, such as antibiotics.
The gut microbiome
Each of us has around 100,000 billion live bacteria living in our intestines.
Probiotics & Antibiotics
Research has shown that taking friendly bacteria (probiotics) alongside antibiotics can help to maintain a healthy balance of the bacteria in the gut.
Should I take probiotics with food?
If live bacteria supplements are not taken alongside food, 90-95% of the bacteria will die in the stomach acid.