Genus, families and strains
Since the 2000s, scientific research has become increasingly interested in the wonderful, tiny world of the microbiota and lactic ferments.
The term 'probiotic' is only recognised in some countries to identify Live lactic ferments, but as early as 2001 the FAO/WHO Expert Consultation on Evaluation of Health and Nutritional Properties of Probiotics in Food published the first definition of probiotics, referring to 'live, viable microorganisms which, when consumed in adequate quantities as part of a food or supplement, have beneficial effects on the health of the host'.
The "probiotic" bacterial species considered safe for edible use are included in a list which is periodically updated by EFSA (European Food Safety Agency), based first on reliable taxonomic identification of the micro-organism, secondly on the existence of a long tradition of safe use in food, and finally after observing existing scientific evidence.
There is now broad scientific consensus that correct taxonomic characterisation is essential to ensure the safety and reliability of lactic acid bacterium.
A bacterial strain used as lactic ferment is always identified by genus, species, subspecies (if possible) and an alphanumeric code that characterises the specific strain. It is also possible to add a commercial 'nickname', also used for advertising purposes, which however has no taxonomic or health value. Often, commercial bacterial strains are also identified by a code indicating that they belong to an international strain library (see FAQ).
Taxonomic characterisation is not always definitive. In 2020, for instance, changes to the taxonomy of Lactobacillus were published: only 35 of the 250 previous Lactobacillus species remained, including Lactobacillus acidophilus. All the others were reclassified and placed in 23 new genera, such as Limosilactobacillus (e.g., Limosilactobacillus reuteri) or Lacticaseibacillus (such as Lacticaseibacillus paracasei).
Of course, each genus, such as Lactobacillus or Bifidobacterium, belongs to a family, which in turn belongs to an order, a class, a phylum, and a domain. For instance, Lactobacillus plantarum belongs to the family Lactobacillaceae, the order Lactobacillaces, the class Bacillus, the phylum Firmicutes and of course the bacterial domain.
Each bacterium useful as a lactic ferment can be described on the basis of its microbiological characteristics. For example, within the Lactobacillaceae family, Lactobacillus plantarum is a gram-positive, non-sporulating, facultative anaerobic and heterofermentative bacterium. Gram-positive means that the bacterium treated with a specific dye will turn purple under the microscope, and this depends on the characteristics and composition of its cell membrane, which in turn influence parameters such as lipopolysaccharide release or adherence to the intestinal epithelium. Non-sporulating indicates that the bacterium does not produce spores, which might have harmful effects on health. Facultative anaerobic indicates that it can live either in the presence or absence of oxygen. Heterofermentative means that the bacterium is able to produce different metabolic compounds by 'eating' different substrates normally present in the diet, such as sugars and pyruvate, thus playing an important metabolic role.
Lactobacillus gasseri is similar to Lactobacillus plantarum as they perform similar functions. However, L. gasseri, unlike plantarum, being an anaerobic bacterium, is much more fragile as it does not survive in the presence of oxygen and far fewer scientific studies have been performed on it than on L. plantarum. In fact, L. gasseri is mentioned in just over 30,000 studies, whereas L. plantarum is mentioned in almost 200,000 scientific studies!
With the same good sugar metabolism functions, L. plantarum tends to be preferable to L. gasseri as it is more versatile and adaptable and has a broader scientific "pedigree". Of course, each subspecies of Lactobacillus (e.g., Lactobacillus plantarum LP 115) shows slight differences from other subspecies of the same species in terms of the production of vitamins, enzymes, bacteriocins etc. which may make it more or less suitable for specific 'tasks'.
Lactobacillus acidophilus is very different from Lactobacillus plantarum. This strain is ubiquitous throughout the human body and not very ‘fussy’ about its choice of food, as it feeds on different substrates such as glucose, galactose, cellobiose, maltose etc. However, the metabolic product is always the same: lactic acid. L. acidophilus is in fact a homofermentative bacterium and since it produces large quantities of lactic acid, it can be considered a specialised ferment, whereas L. plantarum is less specialised but more versatile.
The choice of the best bacteria to include in a quality food supplement is to be based on these considerations. The choice of bacteria as well as of a mixture must always respect the different microbiological inclinations of each bacterium to ensure the best possible 'working environment' and thus the most correct and appropriate supplementation.