Mycoplasmas are prokaryotes of the class Mollicutes. Their size (150–350 nm) is closer to that of viruses than to that of bacteria. Unlike viruses, however, mycoplasmas grow in cell-free culture media; in fact, they are the smallest organisms capable of independent replication.
The entire genomes of many Mycoplasma species have been sequenced and have been found to be among the smallest of all prokaryotic genomes. Sequencing information for these genomes has helped define the minimal set of genes necessary for cellular life. The absence of genes related to the synthesis of amino acids, fatty acid metabolism, and cholesterol dictates the mycoplasmas’ parasitic or saprophytic dependence on a host for exogenous nutrients and necessitates the use of complex fastidious media to culture these organisms. Mycoplasmas lack a cell wall and are bound only by a cell membrane. The absence of a cell wall explains the inactivity of β-lactam antibiotics (penicillins and cephalosporins) against infections caused by these organisms.
At least 13 Mycoplasma species, two Acholeplasma species, and two Ureaplasma species have been isolated from humans. Most of these species are thought to be normal inhabitants of oral and urogenital mucous membranes. Only four species—M. pneumoniae, M. hominis, U. urealyticum, and U. parvum—have been shown conclusively to be pathogenic in immunocompetent humans. M. pneumoniae primarily infects the respiratory tract, while M. hominis, U. urealyticum, and U. parvum are associated with a variety of genitourinary tract disorders and neonatal infections. Some data indicate that M. genitalium may be a cause of disease in humans. Other mycoplasmas may cause disease in immunocompromised persons.
M. pneumoniae is generally thought to act as an extracellular pathogen. Although the organism has been shown to exist and replicate within human cells, it is not known whether these intracellular events contribute to the pathogenesis of disease. M. pneumoniae attaches to ciliated respiratory epithelial cells by means of a complex terminal organelle at the tip of one end of the organism. Cytoadherence is mediated by interactive adhesins and accessory proteins clustered on this organelle. After extracellular attachment, M. pneumoniae causes injury to host respiratory tissue. The mechanism of injury is thought to be mediated by the production of hydrogen peroxide and of a recently identified ADP-ribosylating and vacuolating cytotoxin of M. pneumoniae that has many similarities to pertussis toxin. Because mycoplasmas lack a cell wall, they also lack cell wall–derived stimulators of the innate immune system, such as lipopolysaccharide, lipoteichoic acid, and murein (peptidoglycan) fragments. However, lipoproteins from the mycoplasmal cell membrane appear to have inflammatory properties, probably acting through Toll-like receptors (primarily TLR2) on macrophages and other cells. Lung biopsy specimens from patients with M. pneumoniae respiratory tract infection reveal an inflammatory process involving the trachea, bronchioles, and peribronchial tissue, with a monocytic infiltrate coinciding with a luminal exudate of polymorphonuclear leukocytes.