Molecular Biology & Biotech Notes

PROBIOTICS, Features and their Mode of Action

Written by MicroDok

According to the World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO), the term “probiotics” refers to live microorganisms (especially bacteria and fungi) that when administered in adequate amounts confer health benefits to the host taking it. The intestinal microbiota is important for the maintenance of good health in both man and animals. These normal microfloras that colonize the gastrointestinal tract (GIT) provide energy, nutrients and protection against invading pathogenic microorganisms that may colonize the gut and cause infection or disease. Despite the fact that the normal microbiota of the gut/GIT is relatively stable throughout the life of an individual (especially at adulthood), some physiological changes in the individual host such as stress, changes in diet, age and changes in the host immune system may adversely affect the population of the normal microflora that prevent pathogenic bacteria from colonizing the gut. These changes may lead to increased putrefaction in the colon and a greater susceptibility to infectious disease. Since resident microorganisms in the gut (inclusive of bacteria and fungi) are considered to represent a crucial line of defense against colonization by potentially pathogenic microorganisms, it is therefore vital to use dietary supplements containing probiotics (live microorganisms) to remedy the situation by replenishing or restoring the normal microflora of the GIT to its normal working condition.

Probiotics stimulate the growth of other beneficial microorganisms in the gut unlike antibiotics – which are known to either kill or inhibit the growth of other microbes. Most probiotics are comprised of bacterial species especially Lactobacillus, Bifidobacterium, Streptococcus and Enterococcus. Escherichia and Bacillus are other bacterial genera that have been studied and used as probiotics for either human or animal consumption. Though most probiotics are often bacteria, some fungi especially yeast such as Saccharomyces cerevisiae have also been studied and used as probiotics for human and animal consumption. Nevertheless, Bifidobacterium and Lactobacillus are the most commonly used probiotic bacteria. The main health benefit of probiotics aside its plethora of benefits is that probiotics are used to promote the digestive health of humans and animals.  Probiotics which also means “for live” is used to describe those groups of live microorganisms that that confer beneficial effects to humans and animals especially as it regards to the improvement of their health status. The primary sources of these live microorganisms use for the production of probiotics are from the gastrointestinal tracts (GIT) of humans and animals; and these microorganisms are usually isolated, purified and found to confer health potential benefits to humans and/or animals prior to their usage.

Probiotics are Generally Regarded As Safe (GRAS) products for both human and animal consumption by WHO/FAO due to the health benefits that they confer on their host. Currently, health practitioners recommend probiotic foods to their patients especially those presenting with gastrointestinal infections; and the usage of probiotic foods in both human and animal populations is steadily on the increase worldwide due to their beneficial effects on the nutrition and health status of the consumers. Probiotics play several roles in the nutritional, respiratory, immunological and digestive functions of humans and animals. There are diverse probiotic products in the market for human consumption and many of these products come as foods or drinks. One of the most common probiotic-product in the market is yoghurt. Other available probiotic-products in the market include but not limited to sauerkraut, fermented milk and unfermented milks, infant formulas, cheese, cereal-based foods and some juices (Figure 1).

Figure 1: Illustration of some probiotic products.

The use of probiotics or probiotic food products offers humanity the choice of using natural means to treat or manage some microbial infections while maintaining good health and proper wellbeing in the process. These food products usually serve as food-delivery channels via which probiotics are delivered to the end users. Most dairy products in the market aside yoghurt also contain different amount of probiotics which confer health benefits when consumed. Probiotics can also come in the form of drugs and food supplements. The use of live microorganisms or probiotics for the treatment and/or management of some human/animal infectious is largely attributed to the research of the notable scientist, Elie Metchnikoff – who postulated that lactic acid bacteria (LAB) conveys some health benefits that are capable of promoting longevity in consumers.

Lactic acid bacteria (LAB) comprises of a functional classification of nonpathogenic, nontoxigenic and fermentative Gram positive bacteria that are usually associated with the production of lactic acid from carbohydrates; and LAB are very beneficial for the fermentation of food. LAB has been used over the years in the food industry to produce a wide variety of goods because they are able to convert sugars and other carbohydrates into lactic acid. They help to provide the characteristic sour taste associated with these fermented dairy foods such as yogurt. And the activities of LAB in the fermentation of food help in lowering the pH of the food – thus creating fewer or no opportunities for spoilage/pathogenic microorganisms to grow in the food, and cause food spoilage and foodborne diseases. Typical examples of LAB include Lactobacillus, Lactococcus, Thermophilus and Streptococcus. Bifidobacterium and other bacteria that are used as probiotics are not regarded as LABs because they do not carry out food fermentation.

Fermentation is defined as the anaerobic degradation of glucose/carbohydrates and other organic materials to obtain energy that is conserved as adenosine triphosphate (ATP). It is the process in which microorganism’s transforms food and other materials into other useful products especially through the production of metabolic end-products including but not limited to ethanol, acetic acid and lactic acid. Aside their notable fermentative ability, LABs imparts significant health benefits in vivo when used by human and animal hosts. Elie Metchnikoff hypothesized in 1907 that: “the dependence of the intestinal microbes on the food that we eat makes it possible to adopt measures geared towards modifying the gut/GIT flora, and thus replace the pathogenic microbes with beneficial microorganisms”. Though Elie Metchnikoff did not propose or coin the phrase “probiotics” as at the time; his remarkable suggestion about the beneficial effects of some good bacteria in the GIT gave impetus to the use of live microorganisms today to confer several health benefits to both humans and animals. The term “probiotics” was actually coined in 1965 by Lilly and Stillwell. Elie Metchnikoff proposed that the acid producing bacteria in fermented milk products could prevent “fouling” in the large intestine and if consumed regularly, it could lead to a longer and healthier life in the individual.

Several factors including the intake of antibiotics and imbalances in the immune system of a host deplete and destabilizes the normal gut flora (i.e. the good bacteria); and this phenomenon allows pathogenic microorganisms (i.e. the bad bacteria) to thrive and cause infectious disease. Probiotics have been commonly used when the normal flora of the GIT has been disrupted by antibiotic usage or when chronic bacterial infections (such as that caused by Clostridium difficile) are thriving in the GIT. Apart from killing or inhibiting the growth of pathogenic bacteria in the gut or GIT, antibiotic usage could sometimes lead to the disruption of the normal flora of the GIT, and this predisposes the affected individual to several gut infections. The depletion of intestinal bacteria in such a condition is usually restored by the administration of live microorganisms or probiotics – which help to improve the intestinal flora and thus restore it to its normal working condition devoid of any disease state or any disease-causing microbe. Other factors that could affect the normal flora of the GIT include but not limited to prolonged stress, chronic diarrhea, intake of food low in fiber content, microbial infection and the use of chemotherapy and radiation. The mechanisms of action of probiotics have only been substantiated in animal models – with little or no clinical trials in human host. However, some of the notable modes of action of probiotics (as experimented in animal models) include competition for binding sites to the intestinal walls; competition for essential growth nutrients in vivo, production of antimicrobial substances, improvement of the gut transit, stabilization of the intestinal barrier against infection and immunomodulation.


Not all microorganisms can be used as probiotics. Before a microorganism can be certified and used as a probiotic, it must meet certain criteria. For probiotics to function effectively irrespective of their target site in vivo, they must meet and overcome certain physiological features that may impede their beneficial activities upon usage. These features are highlighted in this section.

  • Microorganisms used as probiotics must be non-pathogenic organisms. They must not produce any pathogenic or virulence factor, and must not be found to be resistant to antimicrobial agents.
  • They must be non-toxic in nature and significantly free from any untoward effects on the host. However, most side effects associated with the use of probiotics are only mild gastrointestinal side effects such as gas.
  • They must survive the acid nature of the stomach; and thus reach the colon (large intestine) where their beneficial effects are needed.
  • Probiotics must also be resistant to pancreatic secretions such as bile and digestive enzymes.
  • They must be able to attach to the intestinal epithelia and colonize it.
  • They must be able to maintain good viability in vivo; and thus utilize or harness the available nutrients and substrates in a normal diet.
  • Probiotics must be able to confer the intended health benefits to the host when used.
  • They must be stable upon usage until their health benefits have been effectively delivered.


Probiotics have been reported to confer several health benefits to both animal and human hosts upon usage. Some of the health benefits that these live microorganisms confer are highlighted in this section.

  • Probiotics enhance the immune system of their host. Due to their ability to adhere strictly to the intestinal epithelial cells in their bid to modulate and stabilize the gut flora, probiotics help to regulate the immunity of the intestinal environment. And they also stimulate the production of immune system mediators such as cytokines and dendritic cells (DCs). Thus, probiotics play immunomodulatory and immune system roles in the GIT of their hosts.
  • Probiotics produce vitamins especially Vitamin B complex – which help to maintain healthy skin and healthy nervous system.
  • They also synthesize other beneficial nutrients such as folic acid and niacin.
  • Probiotics have also been used to treat atopic eczema and some allergic diseases.
  • They prevent pathogenic bacteria from attaching to the intestinal wall of the GIT; and thus prevent several GIT infections from occurring.
  • Probiotics have also been reported to help lactose maldigestion. Lactose maldigestion occurs in individuals that fail to produce lactase enzyme – for the digestion or breakdown of lactose. Lactose is broken down to galactose and glucose by lactase enzyme. And people who fail to produce lactase enzyme suffer from several GIT problems including abdominal cramp, diarrhea, bloating and flatulence. People experiencing lactose maldigestion develop these aforementioned symptoms because they do not produce lactase. However, the consumption of dairy products such as yoghurt containing sufficient amounts of starter bacterial cultures like Lactobacillus and Streptococcus can improve lactose digestion in persons with lactose maldigestion.
  • Probiotics are generally used to remedy gastrointestinal infections. Some of the GIT problems in which probiotics have been used to manage or treat include irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), antibiotic-associated diarrhea (AAD), traveler’s diarrhea, and infectious diarrhea.


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