What is Lactobacillus? It is one of the most searched words on a probiotic label this week, and it is easy to see why. Turn a probiotic bottle around and most of what you read is Latin: two-word names, sometimes three, with an odd little "subsp." tucked in the middle.

Those names follow rules. Once you know the rules, the list stops looking like a wall of italics and starts reading like an address: which group, which member of the group, and sometimes which branch of that member. This piece covers the vocabulary only. Every naming fact below names the reference it came from.

First, the search data. According to Google Trends, for US searches over the past seven days related to the seed term "lactobacillus," the top related query was "probiotic lactobacillus" at 100. "Lactobacillus rhamnosus" and "what is lactobacillus" each scored 69, followed by "lactobacillus probiotics" at 53, "lactobacillus reuteri" at 46, "lactobacillus bacteria" at 40 and "lactobacillus plantarum" at 36. "Bifidobacterium lactobacillus" and "bifidobacterium" each scored 34, and "lactobacillus species" came in at 21.

A quick note on those numbers. Google Trends scores are relative within the seed term: 100 marks the most searched query in that set, and everything else is scaled against it. A score is not a count of searches. What the list shows is the shape of the curiosity, and almost every query on it is a naming question.

What is Lactobacillus? Start with the two-part name

The bacteria on a probiotic label are written in a fixed two-part format. The List of Prokaryotic names with Standing in Nomenclature (LPSN), the reference database for bacterial names, describes it in its glossary as binary nomenclature "established by Carl von Linné": a genus name plus a species epithet, always used together.

The first word is the genus, and it names the group. The LPSN glossary puts the rule plainly: each prokaryotic species "must be included in a genus," and the genus name forms part of the species name. The second word is the species epithet, and on its own it is not a name. You can see the convention in every LPSN entry: genus capitalized, epithet lowercase. "Plantarum" is not a name. "Lactobacillus plantarum" is.

So the short naming answer to "what is Lactobacillus" is this: it is a genus name, the first half of a two-part label. LPSN gives its etymology as "milk rodlet," from Latin lac (milk) and bacillus (a small rod), and credits the name to Beijerinck in 1901. The second genus you will see on many labels, Bifidobacterium, is credited by LPSN to Orla-Jensen in 1924, and its etymology there is "a cleft rodlet," from Latin bifidus, meaning cleft or divided.

Reading the species epithets

The second words usually carry their own small story, and LPSN records it. A few that show up on probiotic labels:

• plantarum: LPSN gives the meaning as "of plants."
• reuteri: LPSN gives it as "of Reuter," named for G. Reuter, a German bacteriologist.
• rhamnosus: LPSN gives it as "pertaining to rhamnose," a sugar.

Those three show the range: a plant word, a person and a sugar, each turned into Latin and attached to the genus.

What "subsp." adds to a probiotic name

Some names on a label have three parts. Bifidobacterium animalis subsp. lactis is the common example. "Subsp." is short for subspecies, and the LPSN glossary calls it the category "of the lowest rank" covered by the ICNP, the code under which LPSN records bacterial names as validly published.

That three-part name has a history of its own. LPSN records that it began as a separate species, Bifidobacterium lactis, named by Meile and colleagues in 1997. In 2004, Masco and colleagues, writing in the International Journal of Systematic and Evolutionary Microbiology, moved it under Bifidobacterium animalis as a subspecies. The old species name, lactis, survived as the subspecies name.

Subspecies can also move the other way. LPSN lists the original name of today's Lacticaseibacillus rhamnosus as Lactobacillus casei subsp. rhamnosus, from Hansen in 1968. A former subspecies became a species in its own right, and then, as the next section explains, it changed genus too.

Why Lactobacillus species were renamed in 2020

Here is the part that confuses most label readers. In April 2020, fifteen researchers, with Zheng as first author, published "A taxonomic note on the genus Lactobacillus" in the International Journal of Systematic and Evolutionary Microbiology. According to the paper's abstract, the genus then held 261 species, which the authors described as "extremely diverse at phenotypic, ecological and genotypic levels."

Working from whole genome sequences, the authors proposed splitting Lactobacillus into 25 genera: a narrower, emended Lactobacillus, the existing genus Paralactobacillus, and 23 new genera. Three of the new genus names matter for anyone reading a probiotic lactobacillus label. The International Scientific Association for Probiotics and Prebiotics (ISAPP), which published a conversion list after the change, gives them as:

• Lactobacillus plantarum became Lactiplantibacillus plantarum
• Lactobacillus reuteri became Limosilactobacillus reuteri
• Lactobacillus rhamnosus became Lacticaseibacillus rhamnosus

Notice what stayed the same. The species epithet did not change, only the genus. LPSN's glossary has a term for exactly this: a "new combination," where the specific epithet remains the same when a species moves from one genus to another. LPSN even spells out the new genus for plantarum: Lactiplantibacillus, from lactis (milk), planta (plant) and bacillus (a rod).

ISAPP also points out a convenient detail. All of the new genera begin with the letter L, so the "L." abbreviation may still be used. "L. reuteri" reads correctly under the old name and the new one.

Why the old name is still on your label

The Zheng paper itself kept a place for the older vocabulary. Its abstract says the generic term "lactobacilli" will remain useful for all organisms classified as Lactobacillaceae until 2020. In practice, plenty of labels, including ours, still print the pre-2020 genus. When you see Lactobacillus reuteri on a bottle, you are looking at the same organism the current literature calls Limosilactobacillus reuteri.

The same thing happened outside the lactobacilli. LPSN records that Bacillus coagulans, a name from Hammer in 1915, was moved to the genus Weizmannia by Gupta and colleagues in a 2020 paper in the same journal. LPSN now lists Weizmannia coagulans as a synonym, with Heyndrickxia coagulans as the correct name. Names can keep moving after a reclassification.

Our take: read the second word first

We think the 2020 renaming is the single most useful thing to know about Lactobacillus probiotics labels, because it explains most of the confusion in this week's searches. Two bottles can name the same organism with two different first words. The species epithet is the stable part, so that is the word we read first.

We also think the Latin rewards a little patience. A genus tells you the group. A species epithet narrows it. "Subsp." narrows it again. None of those words, on their own, is a promise about anything. They are an address, and an address is useful precisely because it is specific.

A probiotic label rarely stops at bacteria, either. Ours also names Saccharomyces boulardii, a phage blend and a fiber, and we have written about each: the -biotic words and Saccharomyces boulardii in our guide to prebiotic, probiotic and postbiotic, the phage in what a bacteriophage is, and the fiber in our acacia fiber explainer. Questions about whether any of it fits your own situation are for your clinician.

Why we carry it at PRSPR

This topic comes straight off our own label. FLORÉ Probiotic is a once-daily capsule, listed as FLORÉ Daily Synbiotic + Saffron. Its ingredient list names seven organisms: Saccharomyces boulardii, Bifidobacterium longum, Bifidobacterium animalis subsp. lactis, Lactobacillus plantarum, Lactobacillus reuteri, Weizmannia coagulans and Lactobacillus rhamnosus, the last one marked heat-treated/postbiotic. The listing also names a PreforPro® bacteriophage blend, acacia fiber and Affron® saffron stigma extract.

Read with this article in hand, that list holds two Bifidobacterium names, one of them a subspecies, three species that carry the pre-2020 Lactobacillus genus, and one name from the 2020 Bacillus split. Like everything we carry, it is made in a GMP-certified facility and third-party batch tested, so what's on the label is what's in the bottle. It is listed at $61.36 and sits in our Vitamins & Supplements collection.

If you want to read the Latin for yourself, see FLORÉ Probiotic at PRSPR.

FAQ

What is Lactobacillus?

As a word, it is a genus name. LPSN gives its meaning as "milk rodlet" and credits it to Beijerinck in 1901; in a probiotic name it is the capitalized first word, followed by a species epithet.

Is Lactobacillus rhamnosus the same as Lacticaseibacillus rhamnosus?

Yes, as names. ISAPP lists Lacticaseibacillus rhamnosus as the post-2020 name for Lactobacillus rhamnosus, following the reclassification Zheng and colleagues published in the International Journal of Systematic and Evolutionary Microbiology.

What is the new name for Lactobacillus reuteri and Lactobacillus plantarum?

Per ISAPP, Lactobacillus reuteri is now Limosilactobacillus reuteri and Lactobacillus plantarum is now Lactiplantibacillus plantarum. In both cases only the genus changed; the species epithet stayed the same.

What is the difference between Bifidobacterium and Lactobacillus?

On a label, they are two different genus names. LPSN credits Bifidobacterium to Orla-Jensen in 1924, with the meaning "a cleft rodlet," and Lactobacillus to Beijerinck in 1901, with the meaning "milk rodlet."

How many Lactobacillus species are there?

According to Zheng and colleagues, the genus held 261 species as of March 2020, before their paper split it into 25 genera.