Yeasts are eukaryotic single-celled organisms. Currently, about 60 different genera of yeast fungi with 500 species are known.
What are yeast fungi?
Yeast fungi are unicellular fungi. Because they have a nucleus, they are eukaryotes. Since yeasts reproduce by fission or sprouting, they are also titled as sprouting fungi. Most sprout fungi belong to the tubular fungi (Ascomycota). However, various developmental stages of other fungi are also called yeasts. Unlike bacteria, yeast fungi have the complex cell structures of eukaryotes. They have complex membrane structures, possess chromosomes, and have cell organelles such as mitochondria and endoplasmic reticulum. Most yeast fungi are facultative anaerobes. They prefer to live when oxygen is available, but can also exist without oxygen. In the presence of oxygen, yeast use oxidative energy metabolism. They can produce carbon dioxide and water from various sugars. In the absence of oxygen, yeast also metabolize sugars, but they produce only alcohols and carbon dioxide in the process.
Occurrence, distribution, and characteristics
Yeast fungi are widespread in the environment, so that humans are actually permanently confronted with them. With a normal diet, it is almost impossible to prevent yeast fungi from entering the intestine. In particular, plant components of the diet are naturally contaminated with yeast fungi. The yeast Geotrichum candidum is often found on the skin of pome fruit. Grapes and soft fruit also carry a wide variety of fungi on their surface. Fresh raw vegetable salads are often particularly contaminated. According to the recommendations of the German Society for Hygiene and Microbiology (DGHM), ready-to-eat salads, such as those offered at salad counters, may contain up to 5000000 colony-forming units per gram. A raw salad weighing 200 grams may thus contain several billion yeasts. The yeasts have a high resistance to gastric acid, so it is to be expected that a large proportion of the fungi will also reach the intestinal tract. Normally, killing by digestive enzymes takes place in the intestine. Some specimens of the yeast fungi survive this as well. Usually, however, yeast fungi do not manage to settle permanently in the intestine if colonization resistance is intact. To this day, researchers and scientists argue whether yeasts and molds belong to the normal intestinal flora or not. Up to now, they have tended to be classified as transient flora. This means that they pass through the intestinal tract but do not remain permanent residents. Nevertheless, in a certain percentage of the population, yeasts are always found in the stool. As a rule, however, the bacterial counts do not exceed 10² colonization-forming units per gram of stool. Useful functions of yeast fungi are not yet known.
Diseases and ailments
Conversely, however, fungi in the body also do not necessarily lead to disease. Thus, yeast fungi do not belong to the obligate pathogens. They only acquire pathogenetic relevance when the body is weakened. This weakness may be due to childbirth, old age, immunosuppression, diabetes mellitus, surgery or stress. If the infection is limited to certain areas or organs, it is called a local infection or organ mycosis. In contrast, spread via the bloodstream is called systemic mycosis. Most fungal infections are found in the intestine. In stool examinations, the yeasts Candida albicans, Candida tropicalis, Candida glabrata, Candida krusei and Geotrichum spp. are most frequently detected. Less commonly found are the species Candida stellatoidea, Candida parapsilosis, Candida guilliermondii, and Candida lusitaniae. The dominant species is Candida albicans. If the intestinal mucosa and colonizing colonial resistance allows, yeasts attach to the intestinal mucosa. Yeast fungi are extremely adaptable. Depending on pH, oxygen content and nutrient supply, they change their appearance. Due to this antigen variability, they often escape the body’s immune defenses. Particularly feared is the transformation into a filamentous form. The so-called pseudohyphae not only adhere particularly well, they can also grow into the mucosa. As the yeast fungi multiply in the intestine, there is an increased accumulation of dead cells.These cells deteriorate and release antigens in the process. The antigens enter the bloodstream via the damaged intestinal mucosa. In the presence of an allergic disposition, they can cause allergies here. In addition, under certain conditions the yeast fungi produce ethanol and fusel oils as well as isoamyl alcohol or isobutanol during the utilization of carbohydrates. Particularly in the case of prolonged fungal infestation, the liver is massively stressed by the fusel alcohols produced. Recent studies indicate that the yeast Candida albicans produces not only alcohols but also toxins. Animal studies have shown that these toxins damage lymphocytes, enterocytes and glial cells. However, candidiasis can occur not only in the intestine. Various Candida species also live in the throat or esophagus. In the mouth, the oral mucosa under dentures is particularly affected. In the case of a so-called thrush (candidiasis ), the tongue is covered with a white fungal layer. An infection of the vagina with yeast fungi is also called vaginal mycosis. Colloquially, the disease is simply called vaginal mycosis. As a rule, Candida albicans is the causative agent here as well. Vaginal mycosis is manifested by a white discharge with itching. White, non-wipeable coatings become visible on the vaginal mucosa. The skin lesions may extend to the inner thighs, where they cause severe itching. Fungal vaginal infections often occur in combination with bacterial vaginal infections.
