Intestinal probiotics, also known as lactic ferments, represent a heterogeneous population of bacterial species residing in our digestive system. They are essential for human life thanks to some functions performed, such as synthesis of vitamins and modulation of the immune system. Main characteristics, functions and properties of probiotics will be collected in this article.
Bacteria, which normally also reside in our intestines, were the first inhabitants of the earth, when there was still no oxygen in the atmosphere, subsequently they adapted to the optimal environment of the interior of the intestine of the most evolved organisms in which they found hot, humid, lack of oxygen, darkness and food.
The bacterial population residing in our digestive tract is made up of more than 100,000 billion units, a figure equal to at least 10 times the total number of cells that make up an adult man, for a total weight of about 1.5 kg.
Considering such a large number of units, the entire intestinal bacterial population can be understood as a real organ, or rather a meta organ, able to exert an important influence on the daily and physiological activities of man. Within this vast microbial population, about 500 different bacterial species coexist. Germ free laboratory animals, ie animals artificially deprived of intestinal bacteria from birth, grown in a glass hood, are small and thin.
Colonization of the intestine by bacteria
The composition of the bacterial flora varies considerably from the mouth to the colon: the particular physiology of the various segments of the digestive system, bathed by different secretions and characterized by different movements, favors the development of heterogeneous microbial populations.
There are numerous factors that influence the composition of the intestinal bacterial flora at birth: the type of birth, nutrition, the quality of hospital services and treatments with antibiotic drugs. The digestive canal of man is sterile until conception, then, from the first moments of life, during the passage through the birth canal, it is colonized by an enormous variety of microbial species. In children born by caesarean section, colonization of the gastrointestinal tract occurs late, around the third-fourth day of life, by the microorganisms present in the hospital environment, so their microflora will differ from that of the mother.
At birth, when the space and nutrients available to microorganisms are abundant, in the gastrointestinal tract the bacteria that have a high speed of reproduction dominate; with the passage of time, however, the number of microorganisms grows and the nourishment and space available begin to run out, so only the most highly specialized microbial species survive.
Regarding the type of feeding, in breastfed babies, the intestinal microflora is almost entirely made up of Bifidobacteria is Lactobacilli, but after weaning the appearance changes: i Clostridia and numerous are formed Bacteroids, the Eubacteria and of Peptococci. This is how the innumerable bacterial types that lodge in the gastrointestinal tract of an adult individual begin to be defined.
Although consolidated already after the first years of life, the bacterial population (the microbiota) may later be modulated by lifestyle, stress, smoking, physical activity, drugs, ingested food and other factors, some of which are yet to be defined.
Some bacterial strains are common to the majority of humanity; 80% of bacteria causes the fermentation(decomposition of carbohydrates for energy), such as Lactobacillus is Bifidobacteria, and the remaining 20% causes the rot (decomposition of proteins by anaerobic bacteria), such as Escherichia, Bacteroides,Eubacteria, Clostridium.
Many of these bacteria are useful or harmless as constituents of the microbiota, while others could become pathogenic if allowed to proliferate without the control of the immune system and other bacterial populations.
Among the components of the human microbiota, some of the most representative species, generally harmless and useful for humans, are listed below:
Acinetobacter calcoaceticus, Alcaligenes faecalis, Anaerobiospirillum, Bifidobacteria breve, Bifidobacteria infantis,Bifidobacterium lactis, Bifidobacteria longum, Enterococcus fecalis, Lactobacillus acidophilus, Lactobacillus casei,Lactobacillus delbrueckii, Lactobacillus plantarum, Lactobacillus rhamnosus, Staphylococcus faecium, Streptococcus salivarius thermophilus.
On the other hand, some bacterial species can become dangerous for the life of the host if they invade other organs or replicate in an uncontrolled manner. Their proliferative activity is controlled by the immune system and by competition with other intestinal microorganisms for nourishment and space. Among the main ones are listed the following species:
Bacteroides fragilis, Candida albicans, Clostridium, Enterococcus faecium, Eubacterium nodatum, Escherichia coli Nissle,Fusobacterium, Peptococcus, Peptostreptococcus, Plesiomonas shigelloides, Porphyromonas gingivalis, Ruminococcus.
Function of the microbiota
Microbes are not simple diners, but have precise metabolic and immunological functions. They are fundamental for our health: many studies confirm, for example, that obese people have a different composition of the gut microbiota from thin people, whether the altered composition of the microbiota is the cause of obesity or its consequence is still subject to study, but in any case it has been seen that transplanting the microbiota of a fat animal into a lean one causes the latter to become fat and vice versa, all this means that it probably also controls man's weight gain. Furthermore, by modifying the microbiota of a diabetic, blood sugar and insulin resistance can be controlled. More generally, the microbiota sends a whole series of signals in a system that connects it to adipose tissue, pancreas, liver, brain, organs that are continuously in contact with each other. All this translates into the possibility of communicating to the brain what to eat and what to absorb and perhaps it also ends up influencing our way of thinking and our behavior.
The development of the microbiota goes hand in hand with the GALT (Gut Associated Lymphoid Tissue), that is the intestinal immunological system, which will be educated by it and that microbiota will then be tolerated by that immune system. This is not a negligible aspect if you think that the 70% of the cells that make up the human immune system is located in the intestine.
This biosystem, with which our body is confronted every day, is not static, but is characterized by a dynamic balance between the different bacterial species, maintaining a fairly constant composition in the same individual even in the presence of variable environmental and dietary conditions.
The alteration of homeostasis, i.e. the balance of bacterial species of the microbiota, is known as dysbiosis and can lead to more or less serious consequences such as colonization by the Clostridium difficile (a species of bacteria extremely resistant to antibiotics that causes diarrhea and intestinal malabsorption); dysbiosis can also be the cause of disorders and diseases that affect the digestive system such as dyspepsia (digestive disorders), irritable bowel syndrome and chronic intestinal diseases.
Any change in the balance of the intestinal bacterial population significantly affects the course of many diseases, including obesity, the allergic states, the inflammatory bowel diseases and metabolic pathologies.
The mapping of the intestinal bacterial population can be requested by general practitioners or by clinical reference centers for pathologies associated with alterations in the intestinal bacterial flora. The test draws a complete genetic map of the species of bacteria that make up the individual microbiota, together with a biochemical map, which is an overall picture of how these bacteria interact with each other and how they change in relation to the state of health or disease. This information is useful for diagnostic purposes to perform interventions aimed at rebalancing the population of intestinal microbes through the optimization of the diet, the administration of probiotics, up to the microbiota transplant.
Balance of intestinal bacteria and main strains
The microbiota is an open biosystem; it is populated by resident bacterial species, called autochthonous, and by a variegated population of transient species, defined as allochthonous, which include both microorganisms whose natural habitat is located elsewhere in the digestive tract and the considerable quantities of bacteria that are ingested with food. The microbiota maintains continuous relationships with the outside world and is subjected to incessant movements, both in and out (expulsion of bacteria through the faeces). Despite this, the composition of the bacterial flora tends to remain constant over time. The main factors that allow the achievement and maintenance of this balance, albeit with large individual variations are:
- Gastric secretion: the acid juice produced by the stomach is a powerful bactericide, capable of controlling the reproduction of microorganisms present in food and saliva. THE proton pump inhibitor drugs and antacid drugs can in fact cause dysbiosis since, by altering the gastric pH, facilitating the passage of pathogenic bacterial strains in the intestine, which, in conditions of physiological gastric acidity, would not survive.
- Oxygen contained in the intestine: regulates the distribution and activity of aerobic microorganisms that need oxygen to live, removing it from the environment, facilitating the growth of anaerobic strains.
- Peristaltic activity: helps to keep the composition of the bacterial flora constant, continuously exposing the gastrointestinal tract to a great variety of bacteria from the external environment, to food (whose mixing promotes a continuous elimination of microorganisms) and to gastric, pancreatic and biliary secretions.
- Interactions between microorganisms: the balance of the bacterial flora also depends a lot on the relationships between the different microbial species that populate it and which have found, over time, the conditions to coexist in a balanced way, sharing spaces, food and helping each other, as for example in case of aerobic and anaerobic bacteria, or preventing pathogenic microorganisms from colonizing their territory.
One way to ensure, modulate and positively influence the balance of the intestinal bacterial flora is to introduce live microorganisms into this ecosystem, belonging to species considered beneficial for health: these organisms are called probiotics from the Greek pro + bios, or "for life", "for life". A food can therefore be defined as a probiotic when, thanks to the presence, in high quantities, of live probiotic microorganisms, it is able to modify the balance of the intestinal bacterial flora and strengthen the immune defenses, thus contributing to the improvement of health.
Among the microorganisms used in probiotic foods, i lactic bacteria; these are gram-positive non-pathogenic microorganisms that are capable of producing lactic acid as an end product of the primary metabolism.
Lactic acid bacteria include several bacterial species such as i Lactobacilli, i Lactococci he Streptococci. THE Lactobacillithey have extremely variable characteristics: they may in fact have a different genetic makeup and different ability to adhere to the intestinal epithelium and colonize it; therefore any particular type of Lactobacillus it must be studied individually to identify its probiotic properties and beneficial effects on human health.
In order for a food or probiotic supplement to be used in the prevention and treatment of diseases or disorders in the gastrointestinal tract, it must be safe and therefore belonging to the population of microorganisms that normally live in the intestine. Furthermore, an extremely important factor to consider in determining the efficacy of a probiotic microorganism is its ability to survive the acidic environment and the proteolytic action of digestive enzymes in the gastrointestinal tract; many microorganisms in fact do not resist the acid pH of the stomach or the biliary secretions of the duodenum.
Characteristics required of probiotic microorganisms taken with the diet or food supplement:
- General requirements: no risk for immunosuppressed subjects, intestinal origin, resistance to both the acidic and neutral environment and to the proteolytic action of digestive enzymes and to the bile produced by the liver;
- Technological requirements: ability to survive in the period of marketing (shelf-life), not to be responsible for the production of acid compounds;
- Functional requirements: inhibition action on pathogenic bacteria, ability to adhere to the intestinal epithelium with a barrier function, intestinal colonization, inhibition of pro-carcinogenic enzymatic activity in the intestine.
As for the use of prebiotics, these are substances that cannot be digested by the human digestive system, capable of selectively stimulating the proliferation and activity of one or more beneficial bacteria, providing them with nourishment; particularly useful is theinulin from chicory or from agave.
Longevity is a complex trait in which genetics, the environment and chance play a key role. Influencing multiple aspects of human physiology, such as the proper functioning of the immune system and energy metabolism; the intestinal microbiota can represent an important element in defining how and how much a human being can age while maintaining good health.
However, recent studies have shown that the cumulative abundance of bacterial species making up the intestinal microbiota decreases with advancing age, favoring the progressive proliferation of sub-dominant and opportunistic species with pro-inflammatory action, present in reduced concentrations in the intestine of young adults.