M. genitalium
M. hominis
M. pneumoniae
etc.
Mycoplasma is a genus of bacteria that lack cell walls. Because they lack cell walls, they are unaffected by antibiotics such as penicillin that target cell wall synthesis. They can be parasitic or saprophytic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. They may cause or contribute to some cancers.
The genus Mycoplasma is one of several genera within the class Mollicutes. Mollicutes are bacteria which have small genomes, lack a cell wall and have low GC-content (18-40 mol%). There are over 100 recognized species of the genus Mycoplasma. Their genome size ranges from 0.6 - 1.35 megabase-pairs. Mollicutes are parasites or commensals of humans, other animals including insects, and plants; the genus Mycoplasma is by definition restricted to vertebrate hosts. Cholesterol is required for the growth of species of the genus Mycoplasma as well as certain other genera of mollicutes. Their optimum growth temperature is often the temperature of their host if warmbodied (e.g. 37 degrees Celsius in humans) or ambient temperature if the host is unable to regulate its own internal temperature. Analysis of 16S ribosomal RNA sequences as well as gene content strongly suggest that the mollicutes, including the mycoplasmas, are closely related to either the Lactobacillus or the Clostridium branch of the phylogenetic tree (Firmicutes sensu stricto).
Mycoplasmas are often found in research laboratories as contaminants in cell culture and come from careless handling; due to their small size, they are difficult to detect under a microscope and their presence can skew experimental results.