According to the World Health Organization, probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. In most cases, probiotics are bacteria, although there is also a non-pathogenic yeast used as a probiotic – Saccharomyces boulardii. As a relatively young therapeutic option, only about a hundred years old, probiotics still represent an unexplored area in medicine and pharmacy. Probiotics were first discovered in 1905 when the presence of bacilli in Bulgarian yogurt was identified, later named Lactobacillus bulgaris. Two years later, it was found that Bulgarians who consumed yogurt lived longer, and this led to the first connections between probiotics and their positive effects on health. Another term for probiotics is “good bacteria,” although this is not entirely accurate. Bacteria cannot be simply categorized as “good” or “bad” because a “good” bacterium can become “bad” under certain conditions and locations in the body. For example, Escherichia coli is a normal part of the intestinal microbiota but is also a well-known cause of urinary tract infections. Additionally, not all probiotics are bacteria; some are yeasts.
A probiotic is characterized by several criteria that guarantee its efficacy. In other words, when choosing a probiotic, we select a specific type of microorganism of a precise strain with evidence for a particular indication at a specific dose.
When selecting a probiotic, we are actually choosing the type and strain of microorganism present in the formulation. For example, the probiotic Streptococcus salivarius K12 illustrates this classification. The term Streptococcus salivarius refers to the species of microorganism belonging to the streptococcus genus, while K12 specifies the exact strain of Streptococcus salivarius. A strain can be defined as a subtype of microbe determined by its genetic structure. Different strains vary, for example, in the types of bacteriocins they produce, which are important for the therapeutic effect of the probiotic. Therefore, when choosing a probiotic for a particular indication, it is necessary to select the strain for which efficacy research has been conducted. Today, the market offers both single-strain probiotics and multi-strain probiotics. It is difficult to decisively state which product is better – while a single-strain probiotic relies on one strain with proven efficacy, multi-strain probiotics can provide a synergistic effect (one strain may support the growth of another). The most common types of probiotics come from the genera Bifidobacterium and Lactobacillus. There are also less common probiotics from the genera:
Another important aspect of probiotics is their dosage. While the dosage of traditional chemical medications and dietary supplements is expressed in mass units, such as milligrams, the dosage of probiotics is defined differently. A crucial factor for probiotics is that, after being ingested, they must have the ability to divide and colonize the mucous membranes they are intended for. Therefore, the dosage of probiotics is defined in units known as CFU (Colony-Forming Units) – this unit defines the number of viable cells, or cells capable of forming colonies (reproducing). The commonly accepted minimum therapeutic dose of probiotics is 1×10^9 CFU, although this varies depending on the strain. Thus, when choosing an appropriate probiotic, be sure to check the CFU on the packaging.
People often like to flatter themselves by considering humans as the most intelligent, advanced, capable, powerful, and superior species on Earth. However, I would not entirely agree with this – why? Simply put, there is a group of living organisms on Earth that can stop the world and confine us to our homes for more than half a year. As much as we flatter ourselves as the masters of the planet, there are still invisible entities that can create problems. These are microorganisms. The interaction, interdependence (whatever you may call it) between humans and microorganisms has existed since the inception of the human species. Historically, humans have been in a constant battle with microorganisms – sometimes we win the battle, sometimes they do. Consider all the pandemics throughout history, from the plague to COVID-19. On the other hand, humans, through evolution and medical achievements, have achieved only temporary victories in this ongoing war – antiseptics, vaccines, antibiotics, and our strongest artillery, the immune system. However, it is important to emphasize – we have won battles, but not the war. And this war will continue as long as humans exist on Earth. While this has a militant tone, not everything is so bleak. Microorganisms, as much as they can cause problems, also represent why we are alive. They are an integral part of our body and an unavoidable factor for the normal functioning of the organism.
Once consumed “orally,” probiotics must pass through the upper part of the digestive system. A common question is how bacteria manage to survive the acidic environment of the stomach. The answer is twofold – it depends on the strain and the formulation. When determining the efficacy of probiotics, it is necessary to conduct research on the survival rate in the acidic environment of the stomach. Certain strains are more resistant, so they survive the environment and reach the intestines where they colonize. On the other hand, more sensitive strains are protected by pharmaceutical technology – a process called coating technology is used, which coats the probiotics and makes them more resistant. Once they reach the more alkaline environment of the intestines, this “coating” breaks down, and the probiotic strains are released into the intestinal lumen. One form of this technology is microencapsulation, a technological process that immobilizes beneficial bacteria within a restricted space to protect them during production and use. Microencapsulated cultures survive simulated gastrointestinal conditions better than non-microencapsulated cells. The materials used for microencapsulation in the food industry must have GRAS status, be biodegradable, and form a barrier between the encapsulated ingredient and its environment. Another option is to use time-release capsules that gradually release their contents over a longer period. Given this mode of action for probiotics, it is advisable to take them on an empty stomach – 1 hour before or 2 hours after a meal. This ensures faster passage through the stomach and arrival at the target site – the intestines. 
Probiotics have a specific mechanism of action that consists of several aspects. People often think that by using probiotics, they change their gut microbiota. However, this is not true – our microbiota is unique and is defined by factors such as pregnancy, birth, the number and type of infections in childhood, our diet, etc. Probiotics can only serve as support for this microbiota. The mechanisms of action of probiotics encompass 5 fundamental principles:
Probiotics “compete” for the surface area of the mucosa in the gut lumen – by occupying (more vividly, adhering to) the inner surface of the intestine, they prevent the colonization of pathogenic bacteria. This is one reason why they are used, for example, in the prevention of traveler’s diarrhea.
Probiotic strains produce a range of antimicrobial agents. For example, they produce bacteriocins (antimicrobial substances that inhibit the growth of other microorganisms), hydrogen peroxide, and short-chain fatty acids (SCFAs). The purpose of producing these antimicrobial agents is to prevent the colonization of pathogenic bacteria that can cause infections.
The gut is a crucial aspect of human health and must have a well-defined barrier to prevent the entry of antigens (endotoxins, toxins, undigested components, etc.). When the gut barrier is compromised, it leads to a condition known as leaky gut syndrome, which results in local and systemic immune responses (inflammation) and can further lead to a range of inflammatory diseases. Probiotics help maintain the integrity of the gut barrier.
Probiotic strains contribute to the balance between Th1 and Th2 immune responses (they act as immunomodulators). By doing so, they prevent inflammation, which is extremely beneficial for individuals suffering from inflammatory bowel diseases (such as ulcerative colitis, Crohn’s disease, etc.).
Probiotics are involved in the production of neurotransmitters like serotonin, dopamine, and GABA. Therefore, there is potential for using probiotics in the treatment of certain neurological conditions, such as migraines.
Probiotics can be used by everyone, from children and pregnant women to the elderly. The only group that should use probiotics with caution includes those who are severely immunocompromised due to the risk of infections, as well as patients on a sterile diet. Probiotics are used for both preventive and therapeutic purposes. The most common preventive use is taking probiotics before traveling to prevent traveler’s diarrhea. On the other hand, probiotics can also be used in the case of already existing diarrhea to help balance stool. The following section will outline standard and new indications for probiotic use and which strains are recommended for each case. 
Different probiotic strains are used for different indications. All effective strains are stored in culture banks, such as the Collection nationale de cultures de microorganismes (CNCM) at the Pasteur Institute. Standard Indications Include:
New Indications Include:
Which Probiotic Strain to Take Depending on the Indication
| Probiotic Strain | Indication |
| Lactobacillus acidophilus CL1285 | vaginal dysbiosis, irritable bowel syndrome |
| Lactobacillus helveticus | vaginal dysbiosis |
| Lactobacillus casei sp. paracasei | Periodontitis, atopic dermatitis |
| Saccharomyces boulardii | infectious diarrhea, antibiotic-induced diarrhea |
| Streptoccocus salivarius K12 | Otitis media |
| Lactobacillus rhamnosus GG | Immune system stimulation |
| Lactobacillus reuteri DSM 17938 | Infant colic, antibiotic-induced diarrhea |
| Bifidobacterium longum 35624 | Reduces symptoms of irritable bowel syndrome, decreases frequency of constipation |
| Bifidobacterium animalis subsp. lactis BB-12 | Constipation |
| Bacillus coagulans MTCC 5856 | Diarrhea associated with irritable bowel syndrome |
There are many food products that contain probiotics. In these products with live and active cultures, you will often find bacteria from the genera Bifidobacterium and Lactobacillus. The label should list the full name of the probiotic strain present in the product. Some foods that are rich in probiotics include:
Yogurt is made by fermenting milk with bacteria that produce lactic acid, such as Lactobacillus bulgaricus and Streptococcus thermophilus, although other strains can also be added. If yogurt is heat-treated, it loses its probiotic content.
Various fermented dairy products are referred to as buttermilk, but the only one that contains live probiotic cultures is the liquid remaining after butter is made. Many store-bought buttermilks do not actually contain live probiotics, so check the label carefully.
Since homemade cheese is not aged or heat-processed, it retains a high level of probiotics. Other cheeses that often contain probiotics include:
Tempeh is a soy product that provides numerous health benefits. Besides probiotics, tempeh offers excellent sources of fiber, protein, and iron.
Sauerkraut is essentially fermented cabbage, and fermented foods have been increasingly studied in recent years for their health benefits. Unpasteurized sauerkraut is the type that contains a lot of nutrients and healthy probiotics.
Miso is a paste made by fermenting soybeans with salt and several types of beneficial bacteria. It can be added to soups and other dishes to create a healthy soup rich in probiotics.
Prebiotics are substances that promote the growth and activity of beneficial microorganisms. The most well-known prebiotics are indigestible fibers that support the activity of bacteria in the colon. One example of a prebiotic is the café Bianca that our mothers and grandmothers used to make for us when we were young – it contains chicory, which is rich in indigestible fibers like inulin that acts as a prebiotic. In dietary supplements, prebiotics such as fructooligosaccharides (FOS) and arabinogalactan are also commonly used. Synbiotics are formulations that contain both probiotics and prebiotics, or probiotics along with certain minerals or extracts that enhance the effectiveness of the probiotics. Prebiotics are beneficial because they stimulate the growth of both the probiotic strains used in synbiotics and our own gut microbiota. A prime example of a synbiotic is kefir – not only does it contain probiotic strains, but its components also contribute to their growth (prebiotics).
