One thing scientists can agree on is that the Lactobacillus genus is too heterogeneous. It does not conform to taxonomic conventions. The genome sizes of included species range from 1.23Mb (L. sanfranciscensis) to 4.91 Mb (L. parakefiri). The G+C content of included species ranges from 31.93 – 57.02 %. A phylogenetic tree reveals that the genus is ‘polyphyletic’, meaning that many other genera – Pediococcus, Leuconostoc, Convivina, Fructobacillus, , Weissella, and Oenococcus – are contained within. This heterogeneity has been known for decades, but now methodologies are able to establish robust groupings of existing species. An important paper describing the phylogentic clustering is Salvetti et al. 2018.
Another thing that scientists can agree on is that this situation must be rectified. “The status quo is not an option,” stated Paul O’Toole at a recent workshop, organized by LABIP.
What changes can we expect? The Lactobacillus genus will be split into new genera, although the exact number is not known. From 13 – 23 genera will likely emerge. Species names will not change. But many traditional ‘probiotic’ species will no longer be called ‘Lactobacillus’: L. casei/L. paracasei, L. plantarum, L. salivarius, L. reuteri, and L. rhamnosus will no longer be ‘Lactobacillus’. Because the first species of Lactobacillus that was describes was L. bulgaricus, this species will retain the Lactobacillus genus name. Other species closely related to it, such as L. acidophilus, will as well.
This means that yogurts will still be able to declare ‘Lactobacillus bulgaricus’ as the starter bacterium. The standard of identity for yogurt will not have to be amended to reflect this new nomenclature. But other fermented dairy products, such as kefir or yogurts containing added probiotics, will likely be impacted.
The new genus names take effect as soon as the paper proposing the new names is published in International Journal of Systematic and Evolutionary Microbiology. Researchers suggest that this will occur sometime in 2019. The researchers are tasked with deciding on how many new genera will be formed and what their names will be. I don’t envy them the task.