Every species of bacterium is anchored to one physical strain kept in a culture collection, called the type strain. If you want to know whether an organism is really a given species, you compare it to that strain. For most species this is a formality. For Lactobacillus casei it set off an argument that ran for almost twenty years.
The short answer to what it is: a rod-shaped lactic acid bacterium, named for cheese. Casei is Latin for "of cheese." Its current official name is Lacticaseibacillus casei, and it is the species the whole genus Lacticaseibacillus is built around.
The longer answer is about which organisms actually deserve the name, because for years some of the strains called L. casei arguably belonged to a different species.
Three names, one tangled group
The organism was first described by Sigurd Orla-Jensen in 1916, and it was given the name Lactobacillus casei in 1971. In 1989 a closely related species, Lactobacillus paracasei, was described alongside it. A third relative, Lactobacillus zeae, sits in the same cluster. These are close enough that telling them apart takes careful genetic work, and for a long time the labels did not keep up.
The type strain that did not fit
The type strain of Lactobacillus casei is a culture called ATCC 393. In 2002 three researchers formally asked the international committee that rules on bacterial names to change that (Dellaglio et al., International Journal of Systematic and Evolutionary Microbiology, 2002). Their argument was that ATCC 393 did not properly represent the species as most scientists were using the name. They proposed moving ATCC 393 into L. zeae, dropping the name L. paracasei entirely, and making a different strain, ATCC 334, the new reference for L. casei.
In 2008 the committee said no (Judicial Commission, International Journal of Systematic and Evolutionary Microbiology, 2008). Its ruling, Opinion 82, held that ATCC 393 remains the type strain of L. casei, that ATCC 334 belongs to a different taxon, and that the name L. paracasei stands.
That ruling had a quiet consequence. A strain that had long been filed under L. casei was, by the committee's own reasoning, not L. casei. It belonged with the organisms called L. paracasei. Anyone reading older work that names "L. casei" without a strain designation cannot be sure which of the two organisms is being described.
And then L. zeae came back
The argument did not end in 2008. In 2020 another team re-examined the group using whole genomes (Huang et al., International Journal of Systematic and Evolutionary Microbiology, 2020). Comparing the type strain of L. casei with the type strain of L. zeae, they found a digital DNA-DNA hybridization value of 57.3 percent, well under the 70 percent threshold usually used to separate species. Together with several other lines of evidence, that justified recognizing L. zeae as its own species.
The same year, the wider reorganization of the old genus Lactobacillus into 25 genera moved this group into the new genus Lacticaseibacillus (Zheng et al., International Journal of Systematic and Evolutionary Microbiology, 2020). So the full set of current names is Lacticaseibacillus casei, Lacticaseibacillus paracasei and Lacticaseibacillus zeae. We covered the same 2020 split in our posts on Lactobacillus plantarum and Lactobacillus reuteri.
Why Lactobacillus casei is a good reason to read the strain, not the species
Every species post on this blog makes the same point: published work attaches to strains, not to species. L. casei is the clearest case of why. Here the problem is not only that strains differ. It is that the name itself was applied for years to organisms the naming authority later placed in a different species.
A label that names the species and gives no strain designation is giving you half the identifier. For this species, it may not even be giving you the right half.
Questions people actually ask
What is Lactobacillus casei?
A rod-shaped lactic acid bacterium first described in 1916, named for cheese. Its current name is Lacticaseibacillus casei, and its type strain is ATCC 393.
Is Lacticaseibacillus casei the same as Lactobacillus casei?
Yes. It is the same species under its current name, after the genus Lactobacillus was divided in 2020.
What is the difference between L. casei and L. paracasei?
They are separate, closely related species. In 2008 the international committee on bacterial names confirmed both names and ruled that ATCC 334, a strain once filed as L. casei, belongs to the other taxon. Telling the two apart reliably usually takes genetic methods.
Why is it called casei?
Because it is associated with cheese. Casei is Latin for "of cheese."
For the vocabulary around all of this, see prebiotics, probiotics and postbiotics and the gut microbiome. More of this kind of reading is on our science pages.
