8.3Symbiotic Microorganisms in the Gastrointestinal Tract

A welter of symbionts inhabits the human body. Oral microbes cause dental caries, alveolar pyorrhea, and halitosis, whereas skin microbes give rise to body odor. They also cause opportunistic infections when the anti-infective ability of hosts is lowered. At the same time, symbionts play important roles in biodefense. Humans are believed to have a symbiotic relationship with indigenous microbes in a wide sense.
Although fetuses are in aseptic conditions, bacteria spread in their intestines shortly after their birth, ensuring a life-long companionship. Enteric bacteria account for approximately 1/3 of the volume of feces, i.e., at least 100 billion bacteria in 1 g of feces and 100 trillion in about 1 kg. In other words, more bacteria than the number of human cells proliferating daily are produced in the gut while replacing themselves almost daily.
The intestinal tract is the largest contact zone of the body with the external environment, and it has a developed immune system (see Chapter 9). It does not respond excessively to ordinary beneficial food and microorganisms (oral tolerance) while eliminating pathogens by making use of antibodies. Abnormalities in the mechanism of immunological tolerance induce food allergies. Although intestinal bacterial flora changes in accordance with age, diet, and physical condition, what consist of a stable bacterial flora varies depending on each individual.
No oxygen exists inside the intestines; thus, a majority of bacteria inhabiting the intestine are anaerobic. Although E. coli are able to grow anaerobically as well, they proliferate faster in the presence of oxygen. They therefore form large colonies when isolated from feces, but are minorities in the intestine. Although the E. coli strain K-12 used in laboratories is innocuous, there are some harmful strains such as O157. Beneficial enteric bacteria mainly include lactic acid bacteria such as bifidobacteria and lactobacilli. The name lactic acid bacteria is not based on genealogical classification, but rather a generic name for anaerobes that release lactic acid as an end product by metabolizing sugar. They can suppress the proliferation of detrimental bacteria by acidifying their surroundings. Food items containing lactic acid bacteria that stabilize the intestines and ones that support their growth are sometimes referred to as probiotics and prebiotics, respectively.
Helicobacter pylori inhabit the gastric wall that is exposed to gastric acid. They secrete urease, neutralizing the surrounding acid with ammonia generated through the hydrolysis of urea. They have been suggested to be an indirect cause of stomach ulcers; therefore, sterilizing them reduces the possibility of contracting the disease.


Food and Various Kinds of Fermentation

Fermentation and decay are both processes of decomposition and denaturation by microorganisms, i.e., processes harmful to humans are called decay, whereas beneficial to humans are called fermentation. Providing air to aerobic bacteria promotes decomposition and increases their number. There is a process to dispose industrial effluent by turning it into activated sludge—a mass of proliferating bacterial bodies. In contrast, if oxygen is not supplied, various stages of decomposition and reaction by anaerobic bacteria engender a variety of low-molecular-weight substances such as organic acids. Although unpleasant odors may be generated, scents and tastes beneficial to humans, as well as functional substances, can also be produced. As opposed to aerobic metabolism, these processes of anaerobic metabolism are sometimes referred to as "fermentation."
Sugars such as glucose are decomposed into pyruvic acid, which can then be turned into either lactic acid (lactic acid bacteria) or ethanol (yeast). Lactic acid fermentation and alcohol fermentation are extracellular processes and humans have taken advantage of these processes to make foods and liquors. The froth of beer is carbon dioxide that is trapped after being generated through fermentation. Vinegar (brewed vinegar) is made from having acetic acid bacteria oxidize alcohol, i.e., acetic acid fermentation. Processes to produce useful substances through utilization of the metabolism of microorganisms in this manner are called "so-and-so fermentation" by prefixing fermentation with the name of the substance to be produced. Kikunae Ueda discovered glutamic acid, an umami substance, from kelp about 100 years ago. Following this, glutamic acid was obtained from the hydrolysates of soybean and wheat proteins until about 50 years ago when glutamic acid fermentation utilizing bacteria was invented, originating a full-fledged fermentation industry in Japan. Nowadays, not only the food field but also the production of medicinal products such as antibiotics relies much on fermentation technology.
Liquors have developed in miscellaneous manners all over the world since long before humans perceived the existence of microorganisms. Although yeast plays a leading role in production of any type of alcohol, the raw materials vary. In the case of wine, yeast can directly utilize glucose and sucrose in grapes. On the other hand, when grains are used, different methods are employed depending on how starch is decomposed so that yeast can utilize it. In the making of beer, sprouts with high activity to decompose barley starch during the germination of barley (malt) are employed. For East Asian liquors though, molds with high activity to decompose starch, e.g., koji mold in the case of sake, are widely used. Koji mold is also used in the making of miso and soy sauce, but the yeast to be added concomitantly belongs to salt-tolerant strains. Besides koji mold and yeast, lactic acid bacteria also play a role in adding flavor. Baker's yeast is also closely related to brewer's yeast; as alcohol fermentation progresses a little within bread dough, baking the bread causes air bubbles inside to expand, making it fluffy. Pure-cultured yeasts with good performance are used for bread making. Although such yeasts are not branded as natural, it does not imply that they are artificial; they are naturally derived nonetheless.
Yoghurt, cheese, and pickles typify lactic acid bacteria food. However, cheese is not solidified because of fermentation, but it is precipitation of fermented milk in which a part of casein is severed after a protease analogous to pepsin secreted from the abomasum of a calf and chymosin are added to the milk. At present, instead of slaughtering calves, purified recombinant chymosin or a mold-derived enzyme with activity equivalent to chymosin is predominantly used as a substitute. Blue molds and mildews are planted on cheese when it is ripening to make molded cheese. Salt-tolerant yeast and lactic acid bacteria are added in pickles to prevent germs from proliferating with hyperosmotic pressure from salt and lactic acid.

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