The term ‘mycoplasmas’ is often used as a trivial name for all members of the bacterial class Mollicutes (lat. mollis = „soft“, cutis = „skin“). Mollicutes are characterized by the lack of a cell wall and a small genome size (0.5 –2.2 megabase pair) with low GC (guanine-cytosine) content (20–40 mol%). Due to their small genome, mycoplasmas are host-dependent and live as commensals or infectious agents in or on a variety of hosts, including humans, other vertebrates, plants, and insects. These microorganisms can multiply under aerobic or anaerobic conditions. They have a pleomorphic cell morphology, with the exception of spiroplasmas, which have a spiral shape, and some mycoplasmas of the genus Mycoplasma, which have a flask-like shape due to a terminal (tip) structure (Mycoplasma gallisepticum, Mycoplasma pneumoniae).
Depending on species, mycoplasmas can grow in liquid media, either as single cells (Mycoplasma arthritidis) or in aggregates (Acholeplasma laidlawii, Mycoplasma pneumoniae, Mycoplasma fermentans). The lack of a cell wall makes mycoplasmas resistant to cell wall-targeting antibiotics such as penicillin. Furthermore, some mycoplasmas can form biofilms on solid surfaces in liquid media, such as glass or plastic surfaces, which provides another level of resistance, namely to disinfecting agents and environmental stress conditions.