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Definition: Probiotics from Black's Medical Dictionary, 43rd Edition

Living BACTERIA, which, when given in sufficient dose to colonise the intestine, may produce a beneficial effect for the host. Many probiotics are LACTIC ACID bacteria – for example, LACTOBACILLUS or bifidobacterium. They may help some cases of travellers’ DIARRHOEA, reducing the period of acute symptoms – particularly if the infection is caused by one of the ROTAVIRUSES and may be of use in eczema in young children. Many other claims are made for their benefit but these are not backed up by research evidence.

Summary Article: Probiotics
From The SAGE Encyclopedia of Intellectual and Developmental Disorders

Probiotics are defined by the Food and Agricultural Organization and the World Health Organization as live microorganisms, which when administered in adequate amounts, may bring about a beneficial health effect on the host. The term probiotics is the antonym of antibiotics and was introduced by Daniel Lilly and Rosalie Stillwell in 1965 referring to the substances produced by microorganisms that encourage the growth of other microorganisms. The word probiotic, meaning “for life” in Latin, has been used in reference to bacteria associated with useful effects on humans and animals. The bacteria in yogurt and fermented milk products constitute the most common source of probiotics for humans. The popularity of probiotics in recent years has generated interest in studying their underlying role in various gastrointestinal and autoimmune conditions. The belief in the beneficial effects of probiotics is based on the knowledge that the intestinal flora can protect humans against infection and that disturbance of this flora can increase susceptibility to infection.

The term microbiota refers to the various microorganisms that inhabit the gastrointestinal tract of mammals, and the term microbiome refers to the specific environment that these microorganisms inhabit, in this case, the gut. Probiotics manipulate the gut microbiome by increasing microbiota diversity and beneficial bacteria compositions. The skin and gut are the two main physical interfaces between the environment and our bodies. However, unlike the skin, the gut has a complex nervous system, the enteric nervous system, which allows bidirectional information flow between the microbiome of the gut and the brain. This close interaction suggests that aspects of brain development, function, mood, and cognition may be influenced by our gastrointestinal contents.

Studies have shown that physical and psychological stressors can affect the composition and metabolic activity of the gut microbiota, and contrarily, changes to the gut microbiome can affect emotional behavior and related brain systems. These findings have resulted in speculation that alterations in the gut microbiome may play a pathophysiological role in human brain diseases, including autism spectrum disorder, anxiety, depression, and chronic pain. In recent years, there has been an increased interest in the role of probiotics in restoring the gut microbiota for maintaining healthy minds and behaviors via optimizing neurologic functioning. This entry introduces probiotics with a focus on their discovery, function, and effects.

Probiotics in History

Human have a long history of consuming probiotics. In 76 BCE, the Roman naturalist and philosopher Plinius recommended the ingestion of fermented milk products to a patient who had gastroenteritis. Various cultures and civilizations have consumed fermented milk and yogurt for its medicinal values. However, its use didn't become widespread until the late 1800s. In 1899, Henry Tissier at the Pasteur Institute in Paris discovered bifidobacterium in the intestine of breastfed infants. He reported that infants with bifidobacterium in their intestines had fewer diarrheal episodes. In 1907, Russian scientist Élie Metchnikoff first proposed the idea of using probiotics for health benefits. He noticed health effects stemming from the alteration of the intestinal microbial balance, and he proposed that the consumption of fermented milk products containing Lactobacillus would result in a decrease in toxin-producing bacteria in the gut and an increase in the longevity of the host. He had observed that certain rural populations in Eastern Europe, who survived largely on milk fermented by lactic-acid bacteria, lived for an exceptionally long time. In 1917, a strain of Escherichia coli (E. coli Nissle 1917) was isolated by German scientist Alfred Nissle and was used to treat patients suffering from shigellosis outbreak. Scientists in the 1960s and 1970s observed a reduction in serum cholesterol after consumption of milk fermented with Lactobacillus or Bifidobacterium. Research dedicated to determining optimal doses and strains of probiotic bacteria for various medical conditions has grown since the later part of the 20th century and into the 21st century. Studies using probiotics to change central nervous system functions, particularly psychiatric and memory disorders, have increased during the last several years.

Mechanism of Action

In humans, by far the most commonly used probiotics are bacteria from the genus Lactobacillus or Bifidobacterium, which are gram-positive rods and obligated facultative anaerobes. Overall, the beneficial effects of probiotics result from a change in the composition of gut flora and modification of immune response. When ingested orally, probiotics pass through the stomach and attach to the intestinal mucosa, preventing epithelial attachment of pathogenic bacteria. Probiotics also activate mucosal immunity, stimulate cytokine production and immunoglobulin A (IgA) secretion, and increase the production of hydrogen peroxide and bacteriocins, which both are inhibitory to pathogens. They also compete for nutrients with pathogenic bacteria and inhibit attachment and action of microbial toxin.

Probiotic Use for Medical Conditions

Probiotics are reported to decrease the number of potentially pathogenic gastrointestinal microorganisms and pathogens, reduce gastrointestinal discomfort, flatulence, and bloating, and improve bowel regularity. Probiotics can also enhance the immune system, improve the skin's function, enhance resistance against allergens, and decrease body pathogens. At the cellular level, probiotics protect deoxyribonucleic acid (DNA), proteins, and lipids from oxidative damage and degradation. Animal and human studies have been conducted on numerous medical conditions (Table 1).

Gut Bacteria and the Brain

The gut's evolving capacity to adapt and maintain normal microbiota, which begins at birth and continues throughout one's life, is necessary to support the metabolic activities of the brain. The correlation between impaired intellectual development and a prolonged state of malnutrition in infants and young children is inevitable. Gut bacteria directly stimulate afferent neurons of the enteric nervous system to send signals to the brain via the vagus nerve, as well as shape the architecture of sleep and stress reactivity of the hypothalamic-pituitary-adrenal axis. In addition, gut microbes influence memory, mood, and cognition and are clinically and therapeutically relevant to a range of neuropsychiatric disorders, including schizophrenia, alcoholism, fibromyalgia, and restless legs syndrome.

Probiotics may directly alter central nervous system biochemistry by affecting levels of brain-derived neutrophic factor, gamma-aminobutyric acid (GABA), serotonin, and dopamine, thus influencing mind and behavior. Studies have shown that some of the typical behavioral and physiological abnormalities associated with neurodevelopmental disorders can be modulated by reconfiguring the gut microbiome composition by consuming probiotics. As such, there appears to be great potential in using probiotics to treat various neurologic and psychiatric conditions

See also Naturalistic Observation; Nutrition: Importance in First Years of Life; Nutrition, Prenatal

Further Readings
  • Banan-Mwine Daliri, E.; Lee, B. H. (2015). New perspectives on probiotics in health and disease. Food Science and Human Wellness, 4(2), 56-65.
  • Galland, L. (2014). The gut microbiome and the brain. Journal of Medicinal Food, 17(12), 1261-1272. doi:10.1089/jmf.2014.7000.
  • Tillisch, K. (2014). The effects of gut microbiota on CNS function in humans. Gut Microbes, 5(3), 404-410. doi:10.4161/gmic.29232.
  • Zebunnissa Memon
    Elena Pezzino
    Osman Farooq
    Copyright © 2018 by SAGE Publications, Inc.

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