Mycoplasma

Mycoplasma is a genus of bacteria. They do not have a cell wall around their cell membrane. 
They are gram-negative, and need sterols such as cholesterol for growth. Many common antibiotics such as penicillin that target cell walls do not affect mycoplasma. "Mycoplasma" are the smallest bacterial cells yet discovered. They are typically about 0.1  µm in diameter.
Many species are parasitic, pathogenic or saprophytic, and can survive without oxygen. In humans, "M. genitalium" causes urethritis, and "M. pneumoniae" is a common cause of pneumonia. Some live as saprophytes but the majority are parasites of plants and animals. The parasitic nature is due to the inability of mycoplasmal bacteria to synthesise the required growth factor.