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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 microflora 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, irrational use of antibiotics 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 of the host 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 comprised 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 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 used for the production of probiotics are the gastrointestinal tracts (GIT) of humans and animals. These microorganisms are usually isolated, purified and found to confer health potential health 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. 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 products 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. Photo courtesy:

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 infections 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. Elie Metchnikoff (1845-1916) was also notable for his famous work on the discovery of phagocytes (white blood cells that eat or engulf bacteria) that carryout phagocytosis in living organisms.

Lactic acid bacteria (LAB) comprise a functional classification of nonpathogenic, non-toxigenic and fermentative Gram positive bacteria such as lactobacillusthat are usually associated with the production of lactic acid from carbohydrates. LABs are very beneficial for the fermentation of food. They are generally known as good bacteria. 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. 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, Leuconostoc and some strains of 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 microorganisms transform 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, LAB 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 probioticsas 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 destabilize the normal gut flora (i.e. the good bacteria). 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.    

Further reading

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Das H.K (2010). Textbook of Biotechnology. Fourth edition. Wiley edition. Wiley India Pvt, Ltd, New Delhi, India.

Davis J.M (2002). Basic Cell Culture, A Practical Approach. Oxford University Press, Oxford, UK. 

Mather J and Barnes D (1998). Animal cell culture methods, Methods in cell biology. 2rd eds, Academic press, San Diego.

Noguchi P (2003).  Risks and benefits of gene therapy.  N  Engl J Med, 348:193-194.

Sambrook, J., Russell, D.W. (2001). Molecular Cloning: a Laboratory Manual, 3rd edn. Cold Spring Harbor Laboratory Press, New York.

Tamarin Robert H (2002). Principles of Genetics. Seventh edition. Tata McGraw-Hill Publishing Co Ltd, Delhi.     

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