Most bacteria are named for what they look like or where they were found. This one is named for a person. Lactobacillus reuteri honors Gerhard Reuter, the German bacteriologist whose surveys of gut bacteria in the 1960s also gave us the name Bifidobacterium longum.
The short answer to what it is: a lactic acid bacterium that lives in the guts of many animals, including people. When researchers first isolated a compound from it in 1988, they described the species as a prominent member of the gut community in humans, poultry, pigs and other animals (Talarico et al., Antimicrobial Agents and Chemotherapy, 1988).
The more interesting answer is that "one species" undersells it. Over a very long time, L. reuteri has split into lineages that each specialize in a different host.
One species, many hosts
In 2011 a team led by Jens Walter compared 57 strains of Lactobacillus reuteri taken from six different vertebrate hosts (Frese et al., PLoS Genetics, 2011). The strains did not sort randomly. They fell into lineages that tracked the animal they came from, and the strains from rodents carried a set of genes that was rare or absent in strains from other hosts.
The researchers then tested whether those genes mattered. When they switched off rodent-specific genes in a rodent strain, seven out of eight of those changes left the strain worse at living in the mouse gut. The genes were not decoration. They were adaptations to one particular host.
The same study found the human and rodent populations evolving in different directions. The rodent strains kept a large, flexible pool of genes. The human strains showed signs of what geneticists call reductive evolution: a gradual shedding of genes, which is a pattern often seen in organisms that have settled into one host for a long time.
So a strain of L. reuteri from a mouse and a strain from a person share a species name, and a long, separate history. That is a useful thing to know before assuming any two strains are interchangeable.
Why Lactobacillus reuteri is now Limosilactobacillus reuteri
In 2020 a whole-genome review split the old genus Lactobacillus, which had grown to 261 species, into 25 genera (Zheng et al., International Journal of Systematic and Evolutionary Microbiology, 2020). This organism went to a new genus, Limosilactobacillus. Its current name is Limosilactobacillus reuteri.
The new genus name has a meaning of its own. Limosus is Latin for slimy. Most members of the genus produce exopolysaccharides from sucrose, the long sugar chains that give some cultures a slightly ropey texture, so the name translates roughly as "the slimy lactobacillus."
Many labels still print Lactobacillus reuteri. That is out of date rather than wrong. Both names refer to the same species.
Reuterin, the compound named after the organism
The 1988 paper isolated a compound from L. reuteri and named it reuterin. The organism makes it when it ferments glycerol, and in the laboratory it inhibits the growth of a wide range of other microorganisms, which is how it was first noticed.
It is a good example of the kind of chemistry a single species can carry. It is also a laboratory finding about a fermentation, and nothing more than that should be read into it.
The species is not the strain
There is a large published literature on L. reuteri, and almost all of it attaches to named strains. Given how far apart the lineages have drifted, that matters even more here than for most species. A finding about one strain is not a finding about the next, and a label that names the species without a strain designation is giving you half the identifier.
Questions people actually ask
What is L. reuteri?
A lactic acid bacterium that lives in the guts of many vertebrates, including people. It is named after the German bacteriologist Gerhard Reuter, and its current official name is Limosilactobacillus reuteri.
Is Limosilactobacillus reuteri the same as Lactobacillus reuteri?
Yes. It is the same species under its current name. The genus Lactobacillus was divided into 25 genera in 2020, and this organism was placed in Limosilactobacillus.
Why is it called reuteri?
After Gerhard Reuter. The species was formally named in the early 1980s by Kandler and colleagues, who chose to honor him.
What is reuterin?
A compound L. reuteri produces when it ferments glycerol. It was isolated and named in 1988.
For the vocabulary around all of this, see prebiotics, probiotics and postbiotics and the gut microbiome. The same 2020 name change is covered in our post on Lactobacillus plantarum, and more of this kind of reading is on our science pages.
