Limosilactobacillus reuteri is a lactic acid bacterium found in a variety of natural environments, including the gastrointestinal tract of humans and other animals. It does not appear to be pathogenic and may have health effects.
Discovery
At the turn of the 20th century, L. reuteri was recorded in scientific classifications of lactic acid bacteria, though at this time it was mistakenly grouped as a member of Lactobacillus fermentum. In the 1960s, further work by microbiologist Gerhard Reuter, for whom the species eventually was named, reclassified the species as L. fermentum biotype II.
Significant differences were found between biotype II and other biotypes of L. fermentum, to the point that in 1980 it was identified as a distinct species and the formal species identity, L. reuteri, was proposed. In April 2020, L. reuteri was reassigned to the genus Limosilactobacillus.
Prevalence
Limosilactobacillus reuteri is found in a variety of natural environments. It has been isolated from many foods, especially meats and dairy products. It appears to be essentially ubiquitous in the animal kingdom, having been found in the gastrointestinal tracts and feces of healthy humans, sheep, chickens, pigs, and rodents. It is the only species to constitute a "major component" of the Lactobacillus species present in the gut of each of the tested host animals, and each host seems to harbor its own specific strain of L. reuteri. It is possible that L. reuteri contributes to the health of its host organism in some manner.
Limosilactobacillus reuteri is present as a dominant member of fermenting organisms in type II sourdoughs; several metabolic traits of L. reuteri, including exopolysaccharide formation and conversion of glutamine to glutamate, improve bread quality.
Effects
Antimicrobial
Limosilactobacillus reuteri is known to produce reuterin, reutericin 6 and reutericyclin.
Reuterin
In the late 1980s, Walter Dobrogosz, Ivan Casas and colleagues discovered that L. reuteri produced a novel broad-spectrum antibiotic substance via the organism's fermentation of glycerol. They named this substance reuterin, after Reuter. Reuterin is a multiple-compound dynamic equilibrium (HPA system, HPA) consisting of 3-hydroxypropionaldehyde, its hydrate, and its dimer. At concentrations above 1.4 M, the HPA dimer was predominant. However, at concentrations relevant for biological systems, HPA hydrate was the most abundant, followed by the aldehyde form.
Reuterin inhibits the growth of some harmful Gram-negative and Gram-positive bacteria, along with yeasts, fungi and protozoa. Researchers found that L. reuteri can secrete sufficient amounts of reuterin to achieve the desired antimicrobial effects. Furthermore, since about four to five times the amount of reuterin is needed to kill "good" gut bacteria (i.e. L. reuteri and other Lactobacillus species) as "bad", this would allow L. reuteri to remove gut invaders without harming other gut microbiota.
Some studies questioned whether reuterin production is essential for L. reuteris health-promoting activity. The discovery that it produces an antibiotic substance led to a great deal of further research. In early 2008, L. reuteri was confirmed to be capable of producing reuterin in the gastrointestinal tract, improving its ability to inhibit the growth of E. coli.
The gene cluster controlling the biosynthesis of reuterin and cobalamin in the L. reuteri genome is a genomic island acquired from an anomalous source.
Clinical results in humans
Although L. reuteri occurs naturally in humans, it is not found in all individuals. Dietary supplementation can sustain high levels of it in those with deficiencies. Oral intake of L. reuteri has been shown to effectively colonize the intestines of healthy individuals. Colonization begins within days of ingestion, although levels drop months later if intake is stopped. L. reuteri is found in breast milk. Oral intake on the mother's part increases the amount of L. reuteri present in her milk, and the likelihood that it will be transferred to the child.
Safety
Manipulation of gut microbiota is a complex process that may cause bacteria-host interactions. Although probiotics in general are considered safe, concerns exist about their use in certain cases. Some people, such as those with compromised immune systems, short bowel syndrome, central venous catheters, heart valve disease, and premature infants, may be at higher risk for adverse events. Rarely, consumption of probiotics may cause bacteremia, fungemia and sepsis, potentially fatal infections, in children with compromised immune systems or who are already critically ill.