<unk>  is a genus of bacteria which lack a cell wall. 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
Mycobacterium
Brucella
Candida 
Corynebacterium
Cryptococcus neoformans
Legionella
Toxoplasma gondii