1. Introduction
Probiotics have been used for more than 10,000 years by humans
[1]. The use of these microorganisms, unknown for the great majority of this period, occurred in many cultures for the production of fermented beverages
[2]. Nakazawa and Hosono
[3] reported that early food manufacturers used bacteria and yeasts without knowledge of their existence to produce fermented dairy products, some of which being produced as early as 3500 BCE, and they are still common in the Middle East
[4].
The agents of fermentation were first reported by Louis Pasteur in the decade of 1850 when he established yeasts as living beings and described the lactic acid-producing bacteria (LAB). Henry Tissier, a paediatrician at the Pasteur Institute, was the first to relate the use of bacteria to the treatment of intestinal diseases in 1906
[5]. A year later, the Russian microbiologist Elie Metchnikoff, in agreement with the Bulgarian physician Stamen Grigorov, introduced probiotics as agents of longevity of life in his book “
The Prolongation of Life: Optimistic Studies”. In that book, Metchnikoff suggested that the ingestion of some specific bacteria may beneficially influence the gastrointestinal tract (GIT) of humans. This hypothesis was based on the observation of the relationship between the long and healthy life of Bulgarian peasants and the consumption of large amounts of fermented dairy products
[6]. Since then, probiotic microorganisms have been isolated from the most varied substrates and currently applied for a myriad of purposes beyond infectious diseases of the GIT, at a moment in history that Ozen and Dinleyici
[1] called this period as the “Age of Probiotics” (
Figure 1).
Figure 1. Journey of probiotic products from their production in the industry to their arrival in the intestine, where they will perform their beneficial functions for consumers. In this path, the main challenges and activities of the cells are highlighted.
2. The Origin: Evolution of Probiotic Meaning
The term “probiotic” was first coined to designate substances produced by bacteria or yeast that promoted the growth of other microorganisms
[7]. Since then, this concept is constantly under revision and is updated to cope with the evolution in food technology and microbiology with new definitions being reported over the decades (
Table 1). Along the concept evolution, the improvement of intestinal health was pivotal to any probiotic definition. Although, the main change over the years was due to the increasing attention to the organism instead its metabolites.
Table 1. Evolution of the lato sensu meaning of probiotic.
Currently, the most widely used definition is the one proposed by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO), that was updated by the International Scientific Association for Probiotics and Prebiotics
[13][16]. In this definition, a probiotic is a live microorganism that improves the host’s health when administrated in adequate amounts. However, recently Zendeboodi et al.
[15] proposed a new design for probiotic products that deviates from previous concepts. In this new formulation, non-viable or even disrupted probiotic cells can have beneficial effects on the host
[15]. Additionally, several new terms emerged in the literature to handle the innovations in the production, formulation, effectiveness and safety of those products (
Table 2).
Despite the current innovation in the field, new applications and research are in development that could ensure future reconceptualization. For instance, research in recent decades have expanded the application of the probiotic beyond health promotion but also including health restoration in ill hosts. In this scenario, medical formulation of those microorganisms can be employed for therapeutic purposes to treat gastrointestinal tract (GIT) distresses from infectious or non-infectious cause
[17][18]. Since there is no restriction on probiotic consumption, this could be a safe way to treat new diseases without a proper pharmacological approach, such as in the beginning of the COVID-19 pandemic
[18]. Additionally, any propriety of a probiotic can be enhanced by gene engineering to solve economic issues or to adjust to new purposes
[19].
Table 2. Glossary containing the main terminologies associated with probiotics in the specific literature.
3. Screening and Selection
The search for new probiotic strains has been stimulated in recent decades due the interest in healthier and functional foods
[23][24]. Additionally, the concern of synthetic drugs in allopathic medicine and the rise of bacterial resistance, has increased the pursuit for non-pharmacological treatments such as the administration of probiotics
[25][26][27][28][29]. Currently, the use of those microorganisms is considered a suitable alternative for the treatment of various infectious and non-infectious diseases and even metabolic disorders
[17][18][22]. Despite any application, a given strain must attend some functional and safety criteria to be considered a probiotic. Based on these, it is important that new strains be sought and tested to ensure its safety, which is usually stated by the status of “Generally Recognized as Safe” (GRAS). However, the GRAS status alone is not sufficient to be a probiotic. It is also required that the strain colonizes the GIT and promotes some health improvement to the host. To fulfil this requirement, those strains have been isolated from animal or human GIT samples and stool to ensure effectiveness in colonization. Probiotic strains have been isolated even from human milk, which was associated with a good health condition of infants
[30]. Although, there is no restriction of isolation source if the strain is not virulent or pathogenic
[6]. Genomic analysis has revealed interesting insights on the discrimination between probiotic and pathogenic bacteria based on the genes involved in cell mobility and mobile elements
[31] and in the synthesis of O-antigens
[32]. After isolation and identification, the microorganism must proceed to functional tests in vitro and, later, in vivo, according to FAO/WHO guidelines
[13].
The main in vitro tests used for screening of probiotic are the following: (1) resistance to gastric acidity; (2) resistance to bile acids; (3) adherence to mucus and/or human epithelial cells and cell lines; (4) antimicrobial activity against potentially pathogenic bacteria; (5) ability to reduce the adhesion of pathogens to surfaces; (6) bile acids hydrolase activity
[13]. In addition, some probiotic applications may require strains with specific features, such as the resistance to spermicides for probiotics for vaginal use
[33]. Other proprieties include cholesterol-lowering capacity, antioxidant activity, cytotoxic effect against cancer cells, bioavailability of vitamins and minerals, modulation of immune response, intestinal motility, anticaries activity and others
[34][35][36][37]. However, the heterogeneity of diet, age and microbiome in the human, imposes a challenge to the identification of new probiotic that exerts its beneficial effect in all hosts
[38].
Most of the known probiotic species belong to the Lactic Acid Bacteria group (LAB), such as the lactobacilli group which had its classification recently revised
[39]. Others probiotic strains that hold economic interests includes species from the genus
Bifidobacterium [40]. The main bacterial species of the genera Lactobacillus and Bifidobacterium used as probiotics are
L. acidophilus, L. casei, L. crispatus, L. delbrueckii subsp. bulgaricus, L. fermentum, L. gasseri, L. helveticus, L. johnsonii, L. lactis, L. paracasei, L. plantarum, L. reuteri, L. rhamnosus, L. sporogenes,
L. acidophilus, L. casei, L. crispatus, L. delbrueckii subsp. bulgaricus, L. fermentum, L. gasseri, L. helveticus, L. johnsonii, L. lactis, L. paracasei, L. plantarum, L. reuteri, L. rhamnosus, L. sporogenes [41][42]. Noteworthy, the probiotic status can also be assigned to yeast species, such as those belonging to the
Saccharomyces senso strictu group, such as
S. cerevisiae subsp.
boulardii, although most of the probiotic strain available in the market are bacteria
[43].
4. Applications in Contemporary Times
The search for food that not only provides nutrients but also benefits to the health of the consumer, even ameliorate disease conditions, have brought the concept of nutraceuticals. Nutraceutical products include the use of these fermented foods as anti-pathogenic, anti-diabetic, anti-inflammatory, anti-cancer, anti-allergic, angiogenic, and urogenital, brain and central nervous system (CNS) health care activities
[44]. Administration of
L. fermentum in high-fat-fed rats alleviated some metabolic disorders that are induced by this type of diet
[45]. In addition, the oral administration of
L. rhamnosus LPR strain promoted accelerated skin wound closure in mice, which opens therapeutical approaches for skin wound treatment
[46]. In a very recent article, Li et al.
[47] reported the positive effects of
Lactobacillus reuteri in protecting elderly women from osteoporosis. Other nutraceutical proprieties of the probiotics include reduction of cholesterol and triglycerides and the prevention and treatment of diabetes and obesity
[48][49][50][51]. These therapeutic aspects are also of interest to the pharmaceutical and food industry, as they can be used as a marketing or branding strategy that has increasingly attracted the attention of the consumer public
[52]. Nascimento et al.
[53] proposed the formulation of a nutraceutical containing
L. fermentum 296, quercetin and resveratrol which maintained high bacterial activity over 90 days of storage. Despite the various metabolic benefits that probiotics have demonstrated for the health of consumers, this is an issue that must be evaluated very carefully. For that matter, the reader can consult the recent review by Kim et al.
[54] that specifically reported little evidence of the benefits of probiotics, but also prebiotics and synbiotics on the incidence of colorectal adenomas and cancer. In the same line of reasoning, Thangaleela et al.
[55] argue that there is even evidence of the benefits of probiotics in the improvement of patients with neurological disorders, such as Alzheimer’s and Parkinson’s diseases. However, the authors alert to the fact that these benefits depend on several variables, from the type of probiotic strain to the patient’s physiology. This opens up a very promising field of research regarding the search for personalized treatments.
In addition to consumption by humans and their benefits, the use of probiotics in the supplementation of livestock feed, such as poultry, swine and ruminants, has also been gaining importance. Several studies have shown that much of the benefits attributed to probiotics in humans are also found in these animals, and this has made it possible to raise healthier animals, with less use of drugs or equivalents and with a decrease in fattening time
[6][56][57][58]. The production and marketing processes of probiotics bear much similarity for human and animal use, with cells facing very similar metabolic challenges.
Another very interesting application of probiotics is in the formation of a protective film for the preservation of fruits and vegetables from the antagonistic activity of these bacteria against spoiling microorganisms, such as fungi and bacteria
[59]. The application of this technology based on the preparation of edible films in which the bacteria are fixed can constitute an important step towards the reduction of chemical products used for the preservation of these foods.