Our intestinal microflora is almost exclusively made up of bacteria—nearly 100 trillion in total—groupable in a fair number of genera, with each occupying specific ecological niche in our gastrointestinal system. These bacteria are classified as either dominant, subdominant or floating.
Despite being bacteria, lactic acid bacteria have never been considered “bad”. People have made use of fermented milk since biblical times. In fact, the term lactic fermenter refers to a bacterium endowed with enzymatic fermentation properties in milk, in which it uses the milk’s sugar component (lactose) to feed, grow and reproduce.
We can assume that some bacterial strains like the lactic acid bacteria (in particular those of the genera Lactobacillus and Bifidobacterium) are our collaborators and friends.
However, important distinctions must be made, as only some lactic acid bacteria are "true, faithful friends for life", in that they live within our gastrointestinal system where they tend to form stable colonies within a “permanent, physiological microbiota” that has well-defined, specific characteristics and is distinctive to the human species (the bacterial flora of cows, for example, is different from ours and is distinctive to the bovine species).
It must be reiterated that the "faithful friends for life" bacteria are very selective in their choice of the host with which they associate. These bacteria are called "species-specific". Each milk-producing species (mammals) acquires and accepts their own, specific lactic acid bacteria.
Almost all of the lactic acid bacteria products currently on the market consist of live, Bovinae-origin bacteria that have been cold-dried into a state of lethargy or freeze-dried into a state of forced hibernation. While these bacteria are useful to calves, they are not to humans (except as an excellent source of food) because they are specific to another species and can never be a human’s "friend for life" with all the benefits of symbiotic exchange.
There are, however, also some native human, freeze-dried lactic acid bacteria products on the market. These bacteria do have the capacity to form permanent colonies in our gastrointestinal system by adhering to the walls of the intestine and dividing into various ecological, congenial niches along the entire length, from the stomach to the rectum.
Lactic acid bacteria are known as “species-specific", or in other words, they have unique features that have been “imprinted” in them by the animal species that host them—features that distinguish them as being from a specific host.
A human acidophilic lactobacillus, for example, is not the same as a camel's acidophilic lactobacillus, nor that of a cow. They are similar, but not equal, and each has different effects on humans.