294 OLIVE OILS AND HEALTH MICROBIOME (See MICROBIOTA) MICROBIOTA Set of microorganisms (mostly bacteria, but also viruses, fungi, and protozoa) that colonise different parts of the human body (particularly the gastrointestinal tract -including the mouth-, the skin, and the genital tract). In all these sites, the microbiota establishes a symbiotic relationship (a mutually beneficial association) with the host and helps to maintain physiological homeostasis. These microorganisms begin to inhabit the body at birth and continue to evolve and change throughout life. The main factors regulating the composition and function of the human microbiota are lifestyle, age, and diet. The digestive tract alone normally harbours around 100 trillion microorganisms, which together account for approximately 200 grams of body weight. Although these bacteria are acquired after birth, the microbiota has been proposed as an additional organ of the human body. A healthy and balanced microbiota (eubiosis) protects against colonisation by potentially harmful microorganisms (microbial antagonism) and against toxic or carcinogenic substances. It also contributes to the proper functioning of the immune system, aids in the metabolism of food and the digestion of certain components that the body cannot break down on its own (e.g. short-chain saturated fatty acids), produces vitamins such as K and B12, and is involved in the regulation of intestinal neurotransmitter secretion, insulin, and peptides essential to vital physiological processes. An imbalance or alteration of the microbiota is referred to as dysbiosis. (See other aspects of the microbiota in Chapter 12) - bacteroidetes and firmicutes Bacteroidetes and Firmicutes are two bacterial families present in the human gut microbiota. Both are types of bacteria widely distributed in the environment, including soil, sediments, water, and the digestive tract of animals. In humans, Bacteroidetes have been associated with a lower risk of obesity. People with obesity often have a relatively low proportion of Bacteroidetes and a high proportion of Firmicutes. A high presence of Firmicutes in the gut microbiota has been linked to an increased risk of obesity, diabetes, and hypercholesterolaemia, and therefore a higher risk of cardiovascular disease. Firmicutes are extremely efficient at breaking down food, enabling the body to absorb most of the energy, which over time can lead to weight gain if more food is consumed than strictly needed. In contrast, when Bacteroidetes predominate, less fat and consequently fewer calories are absorbed. Bacteroidetes tend to be more abundant in individuals with a low body mass index or in those following diets rich in plant-based foods. - bifidobacterium Bifidobacteria are one of the most important genera of bacteria in the human gut microbiota. Their name comes from their characteristic Y-shaped (bifid) structure. They belong to a group of bacteria known as lactic acid bacteria and are naturally found in fermented foods such as yoghurt and cheese. Bifidobacteria can be cultivated and used in probiotic products (foods containing live microorganisms that are beneficial to health and commonly found in the normal microbiota).
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