What are Probiotics? Example, Criteria and Common Mechanism

Etymologically the word probiotic was derived from the Latin preposition pro (“for” or “in support”) and the Greek adjective (biotic) from the noun bios (“life”) meaning ‘for life’ or ‘in support of life’ [1].

In 1907, the concept behind probiotics was introduced by the Russian-born Nobel laureate Elie Metchnikoff, known as the “father of probiotics, who suggested that “The dependence of the intestinal microbes on the food makes it possible to adopt measures to modify the flora in our bodies and to replace the harmful microbes by useful microbes” [2]. After that, in 1954, Vergin first introduced the term “probiotics”, when he compared in his paper “Anti- and Probiotika”, the detrimental effects of antibiotics and other antimicrobial substances on the gut microbial population with factors “probiotika” favorable to the gut microflora [3].

Originally the word probiotic was used to describe substances produced by one protozoan which stimulated another. But chronologically the concept of probiotic is changed and nowadays probiotics are defined as live microorganisms which, when administered in appropriate amounts, may confer health benefits to their host generally by improving or restoring the gut flora. However, the word probiotic has been used in a number of different ways over the years.

A Chronicle of the Evolution of the definition of Probiotics

1965Products that contain live organisms shown in reproducible human studies to confer a health benefit can actually claim to be probiotics [18].
1971Tissue extracts that stimulate microbial growth [5].
1974Organisms and substances that have a beneficial effect on the host animal by contributing to its intestinal microbial balance [6].
1989A live microbial feed supplement that beneficially affects the host animal by improving its intestinal microbial balance [7].
1992A viable mono- or mixed culture of microorganisms that, applied to animals or humans, beneficially affects the host by improving the properties of the indigenous microflora [8].
1996A live microbial culture of cultured dairy product that beneficially influences the health and nutrition of the host [9].
Viable bacteria, in a single or mixed culture, that has a beneficial effect on the health of the host [10].
1998Living microorganisms that on ingestion in certain numbers exert health benefits beyond inherent basic nutrition [11].
1999A microbial dietary adjuvant that beneficially affects the host physiology by modulating mucosal and systemic immunity, as well as improving nutritional and microbial balance in the intestinal tract [12].
2001A preparation of or a product containing viable, defined microorganisms in sufficient numbers that alter the microflora (by implantation or colonization) in a compartment of the host and by that exert beneficial health effects in this host [13].
2002Specific live or inactivated microbial cultures that have documented targets in reducing the risk of human disease or in their nutritional management [14].
Live microorganisms which, when ingested in appropriate amounts, may confer health benefits to their host. [15].
2002Specific live or inactivated microbial cultures that have documented targets in reducing the risk of human disease or in their nutritional management [14].
Live microorganisms which, when ingested in appropriate amounts, may confer health benefits to their host. [15].
2004Preparation of viable microorganisms that is consumed by humans or other animals with the aim of inducing beneficial effects by qualitatively or quantitatively influencing their gut microflora and/or modifying their immune status [16].
2006Probiotics are human-associated microorganisms which are consumed either with food or as a supplement with the purpose of improving the health of the host [17].
2010Products that contain live organisms shown in reproducible human studies to confer a health benefit can actually claim to be probiotic [18].
2013Probiotics are live microorganisms that may exert beneficial effects regarding the sufficiency of consumption via their impact on the microbial balance of the gut [19].
2018Probiotics are live microorganisms that are intended to provide health benefits when consumed, generally by improving or restoring the gut flora. Products sold as probiotics include foods (such as yogurt), dietary supplements, and products that aren’t used orally, such as skin creams [20].

Criteria of Good Probiotics

There are a number of criteria that must meet during the selection of a probiotic bacterial strain with the utmost importance placed on safety issues. Strains of the Lactobacillus and Bifidobacterium genera are usually regarded as safe from the basis of long-term human use.

In 1989, Fuller [7] listed the following as criteria of a good probiotic:

  • Probiotics should be a strain, which is capable of exerting a beneficial effect on the host, for example, increased growth or resistance to disease.
  • It should be non-pathogenic and non-toxic.
  • They should be present as viable cells, preferably in large numbers.
  • It should be capable of surviving and metabolizing in the gut environment, for example, it should be resistant to low pH and organic acids.
  • They should be stable and capable of remaining viable for periods under storage and field conditions.
  • A probiotic must be alive when administered [21].
  • It must have undergone controlled evaluation to document health benefits in the target host [22].
  • Probiotics should adhere to the human intestinal cells.
  • It should colonize of the human intestinal tract.
  • They should antagonize against pathogenic bacteria.
  • It must be safe for the human which means the bacteria may generally recognize as safe (GRAS) and the safety of the potential probiotic should assess by the determination of antibiotic resistance patterns and certain metabolic activities.

How do Probiotics work?

The exact mechanisms by which probiotics accomplish their beneficial actions are unknown. Consumption of probiotics may associate with immune system stimulation, decreased cholesterol blood levels, protection against respiratory and intestinal diseases, reduction of inflammatory responses, and antitumorigenic effects. These alleged health claims apparently stem from the ability of probiotics to secrete antimicrobial substances, compete with other pathogens, strengthen the intestinal barrier and modulate the immune system [23]. Four different mechanisms under research by which probiotics may defend against pathogens in the intestine [24].

Competition for nutrients

Probiotics may compete against pathogens for the same essential nutrients, leaving less available for the pathogen to utilize (Fig 1).


Figure 1: Competition for nutrients

Blocking of Adhesion Sites

They may bind to adhesion sites, preventing pathogen attachment by reducing the surface area available for pathogen colonization (Fig 2).


Figure 2: Blocking of Adhesion Sites

Immune Stimulation

Specific strains of probiotics may influence the innate and the acquired immune system, thus playing an important role in human diseases. Probiotic bacteria may affect the epithelial cells, the dendritic cells, the monocytes or macrophages and the several types of lymphocytes (Natural killer cells, T-cells and T-cell redistribution) directly or secondarily. Signaling of immune cells by probiotics may result in the secretion of cytokines, targeting the pathogen for destruction (Fig 3).


Figure 3: Immune Stimulation

Direct Antagonisms

Probiotics may attack pathogenic organisms by releasing antimicrobial agents such as bacteriocins and killing them directly (Fig 4).


Figure 4: Direct Antagonisms

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Examples of common probiotic microorganisms

There are many different microorganisms currently used as probiotics (Table 1) including yeast [25], bacilli [26], Escherichia coli [27, 37], enterococci [28], and the more commonly used bifidobacteria and lactic acid bacteria, such as lactobacilli, lactococci and streptococci [29]. Lactobacillus and Bifidobacterium are two common inhabitants of the human intestinal microbiota [30]. They are known to have no harmful effects, which is in contrast to other gut bacteria [31].

Table 1: Common probiotic microorganisms [32-38]

LactobacillusL. acidophilus, L. delbrueckii ssp. bulgaricus, L. fermentum             L. crispatus, , L. gallinarum, L. gasseri, L. johnsonii, L. brevis,  L. plantarum, L. reuteri, L. rhamnosus, L. salivarius, L. casei,   L. cellobiosus, L. curvatus, L. amylovorus, L. helveticus
EnterococcusEnterococcus faecium, Enterococcus faecalis
LactococcusLactococcus lactis
LeuconostocLe. citreum, Le. mesenteroides subsp. cremoris
PediococcusPd. acidilactici, Pd. pentosaceus
SporolactobacillusSporolactobacillus inulinus
StreptococcusS. thermophiles, S. faecalis S. faecium, S. salivarius
BifidobacteriumBf. adolescentis, Bf. animalis, Bf. bifidum, Bf. breve, Bf. Infantis, Bf. longum
BacillusBc. cereus, Bc. coagulans, Bc. clausii, Bc. pumilus, Bc. subtilis,
EscherichiaEscherichia coli Nissle 1917
PropionibacteriumPr. acidipropionici, Pr. freudenreichii subsp. shermanii,          Pr. jensenii,
SaccharomycesSaccharomyces cerevisiae, Saccharomyces boulardii


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1 thought on “What are Probiotics? Example, Criteria and Common Mechanism”

  1. Great article about probiotics, recently I read an article and it says: Deregulation of the gut microbiota results in various pathological disorders such as diabetes, inflammation, cancer, dyslipidemia, etc. Modulation of intestinal microbiota by probiotics may facilitate the management of a number of clinical conditions of diabetes.
    Link: Sohag, M.S.U., Paul, M., Al-Bari, M.A.A., Wahed, M.I.I. and Khan, M.R.I. (2019) Potential Antidiabetic Activities of Probiotic Strains, L. acidophilus and L. bulgaricus against Fructose-Fed Hyperglycemic Rats. Food and Nutrition Sciences, 10, 1419-1432. https://doi.org/10.4236/fns.2019.1012101
    You may read to enrich your article. Thank you.

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