Fungi are eukaryotes, meaning they have a cell nucleus containing their DNA. Some multicellular fungi, such as mushrooms, resemble plants, but they are actually quite different and are more closely related to animals. Fungi are not photosynthetic, and their cell walls are usually made out of chitin rather than cellulose. Unicellular fungi, yeasts, are included within the study of microbiology. There are more than 1000 known species and yeasts were probably the first organisms to be domesticated. Yeasts are found in many different environments, including soil, water, plants, animals, and insects. Some yeasts have beneficial uses, such as causing bread to rise and beverages to ferment; but yeasts can also cause food to spoil. Some even cause diseases, such as vaginal yeast infections and oral thrush.