Microbiology Terms Starting With Y
Microbiology Glossary: Y
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Yeast
/ YEEST / · Old English gist; related to Greek zein, to boil
Yeast is an informal term for unicellular fungi that reproduce asexually by budding or binary fission, carry out fermentation, and include both model research organisms and opportunistic human pathogens.
Baker’s yeast (Saccharomyces cerevisiae) is the most thoroughly studied eukaryote, and research on its cell cycle, protein trafficking, and gene regulation has directly informed understanding of human cell biology. Candida albicans, a yeast that colonizes human mucosal surfaces, causes the most common fungal infections in immunocompromised patients by switching from a budding yeast form to invasive filamentous hyphae. Cryptococcus neoformans, another pathogenic yeast, produces a polysaccharide capsule that shields it from immune cells and can cause life-threatening meningitis in people with AIDS.
Unlike filamentous fungi, yeasts grow as single cells or short chains, a form that suits dispersal in liquid environments such as nectar, fruit juice, and blood. The roughly 1,500 described yeast species span multiple fungal phyla, meaning “yeast” describes a growth form rather than a single evolutionary lineage.
The yeast Rhodotorula mucilaginosa has been isolated from samples collected in the stratosphere at altitudes above 20 kilometers, demonstrating that airborne yeast cells can survive ultraviolet radiation and near-vacuum pressures far above the Earth's surface.
Yeast →All yeasts are used for baking or brewing. Many yeast species cause disease, spoil food, or live as neutral members of soil and animal microbiomes without any connection to food production.
Candida auris, a yeast first identified in Japan in 2009, spreads readily in hospital settings and resists multiple antifungal drugs simultaneously. Outbreaks in intensive care units have reported mortality rates between 30 and 60 percent among infected patients, illustrating how dangerous a single-celled fungus can be outside a fermentation vat.
Yeast Infection
/ YEEST in-FEK-shun / · From Old English gist, meaning yeast, and Latin inficere, meaning to taint.
Yeast Infection is a fungal infection caused primarily by Candida species, most commonly Candida albicans, occurring when these organisms overgrow on mucosal surfaces or, in severe cases, invade internal tissues.
Candida species normally inhabit the skin, mouth, gut, and vagina at low levels, kept in check by competing bacteria and immune defenses. Disruption of this balance by antibiotic use, corticosteroid therapy, diabetes, pregnancy, or HIV infection can trigger overgrowth. Vaginal yeast infections affect approximately 75 percent of women at least once in their lives, producing itching, inflammation, and a thick white discharge.
Systemic candidiasis, in which Candida enters the bloodstream, carries mortality rates of 40 to 60 percent in hospitalized patients despite antifungal treatment. Recurrent vulvovaginal candidiasis, defined as four or more episodes per year, affects roughly 5 percent of women and often requires prolonged azole therapy or newer antifungals such as ibrexafungerp.
Candida auris, first identified in 2009, differs from typical yeast infection agents because it spreads between patients on hospital surfaces, resists three or more classes of antifungals simultaneously, and has caused outbreaks with case fatality rates exceeding 30 percent in intensive care units.
Yeast infections and bacterial vaginosis are the same condition. Yeast infections are fungal, typically produce thick white discharge without odor, and require antifungal rather than antibiotic treatment.
Candida albicans colonizes the intestinal tract of roughly 50 percent of healthy adults without causing symptoms. After a broad-spectrum antibiotic course eliminates competing Lactobacillus bacteria in the vagina, C. albicans populations can expand within 48 to 72 hours, producing the itching and discharge characteristic of a vaginal yeast infection.
Yersinia
/ yer-SIN-ee-uh / · Named after Alexandre Yersin, the Swiss-French bacteriologist who discovered Yersinia pestis in 1894.
Yersinia is a genus of gram-negative, rod-shaped bacteria in the family Yersiniaceae, containing several species that infect mammals, three of which cause significant human disease.
Yersinia pestis causes plague, Yersinia enterocolitica causes gastroenteritis, and Yersinia pseudotuberculosis causes mesenteric lymphadenitis, an inflammation of abdominal lymph nodes that mimics appendicitis. Y. pestis was responsible for the Black Death, which killed an estimated 30 to 60 percent of Europe’s population during the 14th century, roughly 50 million people.
All three pathogenic species carry a virulence plasmid encoding a type III secretion system that injects effector proteins directly into host immune cells, disabling phagocytosis and inflammatory signaling. Rodents and their fleas maintain Y. pestis in nature, with human infections occurring when flea bites, direct animal contact, or inhalation of respiratory droplets from pneumonic plague patients bridge the animal-human barrier.
Yersinia pestis evolved from Yersinia pseudotuberculosis, a relatively mild intestinal pathogen, within the past 1,500 to 20,000 years. This recent divergence makes Y. pestis one of the youngest major human pathogens, yet it has caused more documented deaths than almost any other bacterium in history.
Plague no longer exists in wild animal populations. Yersinia pestis remains endemic in rodent colonies on every inhabited continent except Australia, and sporadic human cases occur each year when people contact infected animals or their fleas.
In the western United States, Yersinia pestis circulates in prairie dog (Cynomys ludovicianus) colonies across the Great Plains and Rocky Mountain states. Infected colonies can lose 90 to 100 percent of their members within weeks, and fleas abandoning dead rodents transmit the bacterium to humans who enter affected areas, producing bubonic plague with characteristic swollen lymph nodes called buboes.