Yeasts are a type of very small fungi that can only be seen using a microscope. They multiply through budding and consist of a single oval cell. There are also varieties that resemble molds due to the presence of pseudohyphae, which are long filaments. About 1% of all fungi are classified as yeasts, with about 1500 species described. Yeasts are mostly known for their ability to process sugars into alcohol and carbon dioxide.

Skins of berries and fruits and any other locations with a high content of sugars are very likely to contain yeasts. These include common fruits such as apples, peaches or grapes but also cacti, plant saps and other plant fluids. Many yeasts live in soil or on insects. However, species like Candida albicans, Rhodotorula rubra, Trichosporon cutaneum or Torulopsis are a part of the human skin flora and live between our toes. The gut flora of a number of mammals and insects also includes yeasts naturally. They are also very common in deep sea environments, where more complex organisms can't survive.

Some of the most common fungal infections in women are caused by yeast but many men are also affected by them, even if the risk is lower. Most people tend to ignore yeast infections and wait for them to heal on their own, which can result in very dangerous cases. The most common yeasts infections are candidiasis, triggered by the Candida variety. This infection is not very severe from a medical point of view but can be extremely embarrassing and annoying. Yeasts infections have common general symptoms such as itching, mouth corner sores, rashes in moist locations like armpits or in the groin, toes or fingernails tissue swelling and tenderness or discharge resembling cottage cheese that can emerge from vaginal infections.

Yeasts consist of a single cell and are so small that can only be seen using a microscope. They are related to mushrooms and molds, all of these organisms being classified as fungi.

Various yeast species are abundant in nature. They live everywhere: in water, on plants or in the soil. Many types of yeast inhabit the bodies of animals, as well as humans.

Many species are harmless but the ones part of the Candida genus can cause diseases and are considered to be pathogens. Candidiasis is the name of Candida infection, which can develop on the genitals, inside the throat or mouth, on the skin or even in the blood flow.

Candidiasis is usually caused by a single yeast species, Candida albicans. This strain is naturally found in the human body without causing any issues. However, it can expand quickly and cause infections, due to various causes. These include weak immune systems, antibiotic use, hormonal imbalances or stress.

Candida yeast infections become serious if they reach the bloodstream, which carries them to all parts of the body. It can lead to chills or fever, while severe cases are even lethal. Clinical data shows that candidemia is the fourth most-common blood infection if the USA. The condition is also known as invasive candidiasis.

Some yeasts are used in the production of drinks and the food industry, while others provide health benefits. Saccharomyces cerevisiae is used in bakeries as a leavening agent, which makes products softer and lightens their texture. They ferment the sugars found in the dough, releasing carbon dioxide. This gas is trapped in pockets that form inside the dough, making it expand. The alcohol produced through fermentation is destroyed along with the yeast by the high temperatures inside the oven.

Saccharomyces cerevisiae and other types of yeast are also used in the fermentation of alcoholic drinks. For example, cereals are mashed into a sweet liquid that is fermented into beer. Fruit juices ferment into wine or stronger beverages, depending on the sugar content.

Yeast is also used in the modern technique named bioremediation, which employs micro organisms to process garbage, as well as in the biofuels industry.

In order to develop, yeast needs humidity and a good amount of sugars, amino acids or any other nutrients with a simple chemical structure. Ideal environments are the surfaces of numerous varieties of food, as well as fruits or leaves. Unlike hyphen fungi, yeasts can't process starch, cellulose or other types of polymers. Only a few strains are capable of decomposing them.

Bread producers use the yeast Saccharomyces cerevisiae in order to make bread rise, due to the generation of carbon dioxide that forms pockets of gas inside the dough. There are many other strains that have a similar effect and play a key role in the production of beer, wine and other alcoholic drinks. They transform sugars of fruit origin into alcohol, a process known as fermentation. Yeats multiply by budding: the small buds grow from the mother cell until eventually a cross wall known as a septum separates them permanently.

Another yeast that buds is Cryptococcus albidus , which can be researched through negative staining and by phase-contrast microscopy. Placed in a suspension of India ink, the cells become easily visible. The Cryptococcus genus has a distinctive rigid polysaccharide capsule that protects the cells. These capsules resemble haloes when examined under a microscope.

Cryptococcus yeasts grow on the surface of leaves, where large colonies can be found. Most of them are not dangerous, but Cryptococcus neoformans can start an infection known as cryptococcosis, which is a real threat for people who have a weak immune system. AIDS patients are especially vulnerable and between 7-8% of all cases in the US and 3-6% of the ones in Europe suffer from it. C. neoformans uses the capsules typical of the species to escape the immune system, since the yeast is not properly identified as a pathogen and white blood cells don't attack it. It is also the only Cryptococcus strain that releases an enzyme named phenoloxidase. Combined with phenolic compounds, it turns into melanin, which is believed to protect the yeast from the oxidant compounds that act as antiseptic agents in human tissues.

Old bird droppings that are common in cities usually host a large number of C. neoformans. However, it is rare in fresh droppings, where a lot of bacteria are present. The infection starts in the lungs. Depending on the strength of the immune system, it starts a pneumonia that can last a very long time. Many people come in contact with this yeast strain without being infected. This was revealed by random tests conducted on people from the US, UK and Australia, whose skin reactions to C. neoformans antigens have been studied. Small number of people can be killed slowly by the yeast that colonizes their central nervous system.

Yeasts from dried bird droppings were believed for a long time to cause lung infections, since they are inhaled accidentally in airborne powder. However, scientists now know that mushroom fungi part of the Basidiomycota group release airborne spores as part of their sexual reproduction. These basidiospores are extremely small, with a diameter of no more than 3 micrometers, which made them almost invisible. Due to their tiny size, these spores are able to easily penetrate the lungs. While we know that such spores cause infections, their actual origin was not found. They might be released by yeasts that grow on plants.

The common yeast Candida albicans needs temperatures around 37 degrees Celsius and is classified as a dimorphic fungus. It lives on the mucous membranes of both humans and animals and is usually harmless. About half of all humans host Candida albicans, sometimes on the skin but commonly in wet membranes inside the mouth, the gut or the vagina.

The normally harmless C. albicans can take advantage of some situations and start a dangerous infection when it becomes a pathogen and attacks the mucous membrane that hosts it. The yeast culture can start multiplying due to various causes, whenever the bacteria population that normally keeps it in check decreases in level and can no longer compete with it. The rapid expansion of yeast is known as a hyphal outgrowth and eventually penetrates the mucus; attacking the other tissues and causing sever irritation.

The most common condition of this type is named thrush. It resembles the markings on the bird's chest, with white speckles located in the back of the throat and the tongue. Newborns are often affected, probably due to exposure in the birth canal and an immature immune system. People with a weak immune function, such as AIDS patients, commonly suffer from thrush, as well as patients with a lower bacteria population due to antibiotic treatments.

Vaginitis is another condition started by C. albicans. It is common in women who take the pill and pregnant women in their third semester, whose vaginal mucosa is invaded by the yeast, causing inflammation. It seems to be facilitated by the modification of the cell types in the lining epithelium of the vagina, due to hormonal changes.

People who wear dentures can suffer from a very similar issue inside the mouth, known as stomatitis. It happens because candida yeast is able to populate the resin surface of dentures. They are also capable of producing a mannoprotein adhesive on their cell surface, in the sugar-rich environment inside the mouth. If the yeast reaches the blood stream and starts to multiply, the result is a very dangerous disease named Systemic candidosis. The pathogen usually enters the body during surgical procedures, for example when catheters are inserted inside the veins. The yeast can bind on them and then reproduce by budding and eventually spread to other parts of the body.

In general, C. albicans is an opportunistic germ. It is normally under control and not dangerous at all but in the right conditions it can expand fast and become a threat. In lab tests, yeast cells are heated to 37 degrees, placed in tubes of horse serum and incubated for 3-5 hours. The only Candida strains that sprout hyphae in such conditions are Candida albicans and a few related pathogens, which make clinical identification easy. However, after a period of hyphal growth that doesn't last long, the fungus usually enters the yeast phase again.

Yeasts are very common on leaf surfaces, which make it easy to create lab cultures. Pressing a fresh leaf for several hours against the surface of an agar plate will result in a culture of yeast after incubation at normal room temperatures. A common strain on leaf prints is a dimorphic fungus similar to Candida, which tends to form large colonies. The center of the colony is made up of yeast cells with a yellow color, from which hyphae extend across the agar towards the edges of the colony. More diverse colonies can be found on leaves that are older or fallen. Strains of Mucor and varieties with dark pigmentation are often found on them. These include dimorphic "black yeasts" such as Aureobasidium pullulans, as well as Cladosporium species.