支原体（**称霉浆菌）是一种能够自我复制的最小的原核微生物，属厚壁菌门柔膜菌纲，但和其他细菌不同的是它们没有细胞壁，所以细胞形态不固定，可以有多种变化，但可以在培养基上形成极小的菌落。由于不具细胞壁，许多常见的抗生素，如盘尼西林或β－内酰胺类抗生素对支原体是无效的。支原体是寄生生物，但同时也具有腐生功能。许多种类的支原体可导致疾病，如肺炎支原体，是某些非典型肺炎与其他呼吸疾病的病原体。生殖道支原体则会引起骨盆腔发炎。

支原体少数可以自由生活在静水中，但多数存在于人类与动物的消化道、呼吸道和泌尿生殖道中，可导致疾病。有的支原体可导致植物病害。

Mycoplasma is a genus of bacteria that lack a cell wall around their cell membrane. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of atypical pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. Mycoplasma species are the smallest bacterial cells yet discovered, can survive without oxygen, and come in various shapes. For example, M. genitalium is flask-shaped (about 300 x 600 nm), while M. pneumoniae is more elongated (about 100 x 1000 nm). Hundreds of mycoplasmae infect animals.