Yeast are single-cell fungi with ovoid or spherical shapes, which are grouped according to the cell division process into budding yeast (e.g., the species and strains of Saccharomyces cerevisiae and Blastomyces dermatitidis ), or fission yeast (e.g., Schizosaccharomyces ) species.
Yeast species are present in virtually all natural environments such as fresh and marine water, soil, plants, animals, and in houses, hospitals, schools, etc. Some species are symbiotic, while others are parasitic. Parasitic species may be pathogenic (i.e., cause disease) either because of the toxins they release in the host organism or due to the direct destruction of living tissues such as skin, internal mucosa of the mouth, lungs, gastrointestinal, genital and urinary tracts of animals, along with plant flowers, fruits, seeds, and leaves. They are also involved in the deterioration and contamination of stored grains and processed foods.
Yeast and other fungal infections may be superficial (skin, hair, nails); subcutaneous (dermis and surrounding structures); systemic (affecting several internal organs, blood, and internal epithelia); or opportunistic (infecting neutropenic patients, such as cancer patients, transplant patients, and other immunocompromised patients). Opportunistic infections acquired by patients inside hospitals, or due to medical procedures such as catheters are termed nosocomial infections , and they are a major concern in public health , because they increase both mortality and the period of hospitalization. An epidemiological study, with data collected between 1997 and 2001 in 72 different hospitals in the United States, showed that 7–8% of the nosocomial blood-stream infections were due to a Candida species of yeast, especially Candida albicans. About 80% of Candida infections are nosocomial in theUnited States, and approximately 50% of them are acquired in intensive care units. A national epidemiology of mycoses survey in the early 1990s showed that in neonatal ICUs C. albicans was the cause of about 75% of infections and Candida parapsilosis accounted for the remaining 25%. Candida albicans frequently infects infants during birth, due to its presence in the mother's vaginal mucosa, whereas C. parapsilosis was found in the hands of healthcare professionals of the neonatal ICUs. In surgical ICUs, C. albicans was implicated in 50% of infections while Candida glabrata responded for another 25% of the cases. The most frequently community-acquired yeast infections are the superficial mycoses, and among other pathogenic fungi, Candida albicans is the cause of mouth thrush , and vaginitis. Gastrointestinal yeast infections are also transmitted by contaminated saliva and foods.
Although immunocompetent individuals may host Candida species and remain asymptomatic for many years, the eventual occurrence of a debilitating condition may trigger a systemic candidiasis . Systemic candidiasis is a chronic infection that usually starts in the gastrointestinal tract and gradually spreads to other organs and tissues, and the Candida species commonly involved is C. albicans. They release about 79 different toxins in the hosts' organism, and the lesions they cause in the intestinal membranes compromise nutrient absorption by reducing it to about 50% of the normal capacity. C. albicans intestinal colonization and lesions expose internal tissues and capillary vessels to contamination by bacteria present in fecal material. The elderly, cancer patients, and infants are especially susceptible to Candida infections, as are AIDS patients. In the long run, systemic candidiasis may lead to a variety of symptoms, such as chronic fatigue, allergies , cystitis, endometriosis, diarrhea, colitis, respiratory disorders, dry mouth, halitosis (bad breath), emotional disorders, etc.
The indiscriminate prescription and intake of antibiotics usually kills bacteria that are essential for normal digestion and favors the opportunistic spread of Candida species on the walls of the digestive tract, which can be worsened when associated with a diet rich in sugars and carbohydrates. Once yeast species colonize the intestinal walls, treatment becomes difficult and is usually followed by recurrence. Another challenge when yeast systemic infection is involved is that they are not detected by standard blood tests. However, laboratorial analysis of collected samples of mucus and affected tissue may detect yeast infection and identify the implicated species.
Another yeast infection, known as blastomycosis, is caused by the species Blastomyces dermatitidis, a spherical budding yeast. The main targets of this pathogen are the lung alveoli (60%). Pulmonary blastomycosis is not easily diagnosed because its symptoms are also present in other lung infections, such as cough, chest pain, hemoptysis, and weight loss. Pulmonary lesions may include nodules, cavities, and infiltration, with the severe cases presenting pleuritis. Blastomycosis may also be disseminated to other organs, such as liver, central nervous system, adrenal glands, pancreas, bones, lymph nodes, and gastrointestinal and genitourinary tracts. Osteomyelitis (bone infection) and arthritis may also be caused by this yeast, and about 33% of the patients were diagnosed with skeletal blastomycosis as well. Although the cutaneous chronic infection is curable, the systemic form of the disease has a poor prognosis.