Probiotic in the context of Eubacteria

Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit the air, soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Like all animals, humans carry vast numbers (approximately 10 to 10) of bacteria. Most are in the gut, though there are many on the skin. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. Bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector (biomining, bioleaching), as well as in biotechnology, and the manufacture of antibiotics and other chemicals.

Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and many are beneficial but others can cause infectious diseases. The number of these pathogenic species in humans is estimated to be fewer than a hundred. By contrast, several thousand species are considered part of the gut flora, with a few hundred species present in each individual human's digestive tract.

The body is continually exposed to many species of bacteria, including beneficial commensals, which grow on the skin and mucous membranes, and saprophytes, which grow mainly in the soil and in decaying matter. The blood and tissue fluids contain nutrients sufficient to sustain the growth of many bacteria. The body has defence mechanisms that enable it to resist microbial invasion of its tissues and give it a natural immunity or innate resistance against many microorganisms.

A living medicine is a type of biologic that consists of a living organism that is used to treat a disease. This usually takes the form of a cell (animal, bacterial, or fungal) or a virus that has been genetically engineered to possess therapeutic properties that is injected into a patient. Perhaps the oldest use of a living medicine is the use of leeches for bloodletting, though living medicines have advanced tremendously since that time.

Examples of living medicines include cellular therapeutics (including immunotherapeutics), phage therapeutics, and bacterial therapeutics, a subset of the latter being probiotics.

Yakult (ヤクルト, Yakuruto) is a Japanese sweetened probiotic milk beverage fermented with the bacteria strain Lacticaseibacillus casei Shirota. It is sold by Yakult Honsha based in Tokyo. The name "Yakult" was coined from jahurto, an Esperanto word meaning "yogurt".

Lacticaseibacillus paracasei (commonly abbreviated as Lc. paracasei) is a gram-positive, heterofermentative species of lactic acid bacteria that are commonly used in dairy product fermentation and as probiotic cultures. Lc. paracasei is a bacterium that operates by commensalism. It is commonly found in many human habitats such as human intestinal tracts and mouths as well as sewages, silages, and previously mentioned dairy products. The name includes morphology, a rod-shaped (bacillus shape) bacterium with a width of 2.0 to 4.0μm and length of 0.8 to 1.0μm.

Strains of L. paracasei have been isolated from a variety of environments including dairy products, plants or plant fermentations, and from the human and animal gastrointestinal tracts. A protracted refrigeration period before in vitro gastrointestinal transit (GIT) did not affect or influenced very weakly cell resistance.

Lactobacillus bulgaricus is the main bacterium used for the production of yogurt. It also plays a crucial role in the ripening of some cheeses, as well as in other processes involving naturally fermented products. It is defined as homofermentive lactic acid bacteria due to lactic acid being the single end product of its carbohydrate digestion. It is also considered a probiotic.

It is a gram-positive rod that may appear long and filamentous. It is non-motile and does not form spores. It is also non-pathogenic. It is regarded as aciduric or acidophilic, since it requires a low pH (around 5.4–4.6) to grow effectively. In addition, it is anaerobic. As it grows on raw dairy products, it creates and maintains the acidic environment that it needs to thrive via its production of lactic acid. In addition, it grows optimally at temperatures of 40–44 °C under anaerobic conditions. It has complex nutritional requirements which vary according to the environment. These include carbohydrates, unsaturated fatty acids, amino acids, and vitamins.

Lactobacillus is a genus of gram-positive within the Lactobacillaceae family, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus Lactobacillus comprised over 260 phylogenetically, ecologically, and metabolically diverse species; a taxonomic revision of the genus assigned lactobacilli to 25 genera (see § Taxonomy below).

Lactobacillus species constitute a significant component of the human and animal microbiota at a number of body sites, such as the digestive system and the female genital system. In women of European ancestry, Lactobacillus species are normally a major part of the vaginal microbiota. Lactobacillus forms biofilms in the vaginal and gut microbiota, allowing them to persist in harsh environmental conditions and maintain ample populations. Lactobacillus exhibits a mutualistic relationship with the human body, as it protects the host against potential invasions by pathogens, and in turn, the host provides a source of nutrients. Lactobacilli are among the most common probiotic found in food such as yogurt, and the bacteria are diverse in their application in maintaining human well-being, by helping to treat diarrhea, vaginal infections, and skin disorders such as eczema.

Bifidobacterium is a genus of gram-positive, nonmotile, often branched anaerobic bacteria. They are ubiquitous inhabitants of the gastrointestinal tract though strains have been isolated from the vagina and mouth (B. dentium) of mammals, including humans. Bifidobacteria are one of the major genera of bacteria that make up the gastrointestinal tract microbiota in mammals. Some bifidobacteria are used as probiotics.

Before the 1960s, Bifidobacterium species were collectively referred to as Lactobacillus bifidus.

Bacterial therapy is the therapeutic use of bacteria to treat diseases. Bacterial therapeutics are living medicines, and may be wild type bacteria (often in the form of probiotics) or bacteria that have been genetically engineered to possess therapeutic properties that is injected into a patient.Other examples of living medicines include cellular therapeutics (including immunotherapeutics), activators of anti-tumor immunity, or synergizing with existing tools and approaches. and phage therapeutics, or as delivery vehicles for treatment, diagnosis, or imaging, complementing or synergizing with existing tools and approaches.

The human milk microbiota, also known as human milk probiotics (HMP), encompasses the microbiota–the community of microorganisms–present within the human mammary glands and breast milk. Contrary to the traditional belief that human breast milk is sterile, advancements in both microbial culture and culture-independent methods have confirmed that human milk harbors diverse communities of bacteria. These communities are distinct in composition from other microbial populations found within the human body which constitute the human microbiome.

The microbiota in human milk serves as a potential source of commensal, mutualistic, and potentially probiotic bacteria for the infant gut microbiota. The World Health Organization (WHO) defines probiotics as "living organisms which, when administered in adequate amounts, confer a health benefit on the host."