The class Mollicutes contains 4 orders, 5 families, 8 genera, and more than 150 species (Table 1). A fifth order consists of MLOs, which have been implicated in ocular disease but are uncultivated as yet. The mollicutes detected in humans belong to approximately 16 of the more than 120 named species; they are detected in the genera Mycoplasma, Ureaplasma, and Acholeplasma. Unfortunately, the term Mycoplasma is occasionally used interchangeably in the literature for any organisms in the Mollicutes class. Table 2 lists the 13 Mycoplasma species, 2 Acholeplasma species, and 1 Ureaplasma species that have been isolated from humans. Table 3 lists the diseases attributed to Mycoplasma or Ureaplasma and the strength of the association.

In 1967, Japanese investigators reported that a chronic plant disease long thought to be viral in nature was caused by intracellular mollicutes.³ Cells parasitized by these mollicutes displayed dysfunction, destruction, or proliferation.^3,4 Despite many years of effort, these mollicutes have defied cultivation. Their bacterial nature and pathogenicity have been confirmed by transmission studies and their response to antibiotics.³ They have been designated MLOs. Using molecular biologic techniques, the phylogeny of these mollicutes has been elucidated and several subtypes have been identified;⁵ they have been related phylogenetically to Acholeplasma.⁶ The term “phytoplasma” has been recommended for the MLOs by the Subcommittee on the Taxonomy of Mollicutes of the International Organization for Mycoplasmology, based on phylogenetic analysis.^7,8 rRNA and tRNA sequence analyses reveal that mollicutes are not at the root of the bacterial phylogenetic tree, but rather developed by degenerate evolution from gram-positive bacteria with a low molecular percentage of guanine plus cytosine content in their DNA. This loss of coding capacity is tolerated because of the parasitic lifestyle of the mollicutes. They have never been found as freely living organisms but, rather, depend on a host cell. The lack of a cell wall probably facilitates the close contact of mollicutes and their host cells and guarantees the exchange of essential components, which support the growth of the bacterium. As a consequence of this bacterial surface parasitism, the host cell is severely damaged. No specific toxic compounds have been detected.

Their small genome size (500 to 1000 megadaltons or 750 to 1500 kilobase pairs), small number of rRNA operons and tRNA genes, lack of a cell wall, fastidious growth, and limited metabolic activities are seen as the result of their evolution.⁹ Other properties, such as the anaerobiosis of their earliest evolving members (Anaeroplasmas and Asteroleplasmas), the high adenine-plus-thymidine content of their DNA, their lack of sensitivity to rifampin, and the regulatory signals for the transcription of their DNA have been inherited from their eubacterial ancestors. They have some unique properties. High adenine-thymine pressure has resulted in a particular codon usage, where, for instance, UGA is read as tryptophan and not as a stop.¹⁰ They have developed peculiar systems for pathogenicity, cell adhesion, antigenic variation, and (in the case of the spiroplasmas) helical morphology and motility.⁹

Mollicutes (“soft skin”) lack cell walls but have distinctive sterol-containing plasma membranes. They are evolutionary descendants of the low guanine-plus-cytosine-containing gram-positive bacteria (clostridia, enterococci, lactobacilli, staphylococci, and streptococci) and, through chromosome and gene reduction, represent the smallest selfreplicating life forms. Their streamlined genome size, which illustrates extreme biologic gene economy, imposes complex nutritional requirements and dependence on external supplies of biosynthetic precursors, including amino acids, nucleotides, fatty acids, and sterols. The limited coding capacity dictates a parasitic way of life and explains why pathogenic mollicutes are among the most difficult microorganisms to grow from clinical specimens and remain frequent contaminants of primary and continuous eukaryotic cell lines and tissue cultures.¹¹ In some instances, mollicute contamination is obvious because infected eukaryotic cells exhibit aberrant growth, metabolism, and morphology.

Mollicutes are pleomorphic, varying in size from 0.2 to 0.3 μm, thus being about the size of large viruses. They are contained only by a triple-layered “unit-membrane” that contains sterol and are incapable of synthesis of peptidoglycan and its precursors. Hence, they do not take up Gram stain and are resistant to antimicrobials that affect a cell wall. They are sensitive to osmotic shock, detergents, alcohols, and specific antibody plus complement. Individual organisms can assume various shapes, from round to filamentous, depending on the mollicute species and the constituents of the broth medium. Budding forms and chains of beads, as well as classic binary fission, may be observed by dark-field and phase-contrast microscopy. Usually nonmotile, some species show gliding motility on liquid-covered surfaces.

On agar media, the mollicutes multiply to form colonies that vary in diameter from 15 to 300 μm; the largest colonies can be seen by the naked eye and often have a typical “fried-egg” appearance because of the contrast between central growth in the depth of the agar and peripheral shallow growth on the surface. The smaller colonies of ureaplasmas require low-power magnification to be seen. Generation times vary from 1 to 6 hours. Colonies are detectable in 2 to 20 or more days. Yield of organisms in broth is small. Because of their small genomes, mollicutes are fastidious in their growth requirements. They need preformed nucleic acid precursors and enriched media containing cholesterol. Most mollicutes are categorized by their abilities to ferment glucose, utilize arginine, or hydrolyze urea.

Mollicutes as infectious agents was reported in the 1930s and 1940s (see Table 2). It was not until the 1960s that Mycoplasma pneumoniae was established as the singular cause of cold agglutinin-associated primary atypical pneumonia. They are also called pleuropneumonia-like organisms. M. pneumoniae is now also reported in association with tracheobronchitis and pharyngitis^12,13 and has been associated with hematopoietic, dermatologic, rheumatic, central nervous system, gastrointestinal, and cardiovascular syndromes.¹⁴ Mycoplasma hominis and Ureaplasma urealyticum are associated with adult urogenital tract diseases, surgical infections,¹⁵ neonatal respiratory infections, and a range of other diseases, usually in immunocompromised patients.¹⁶ Mycoplasma fermentans stains were isolated from the lower genital tract of both adult men and women in the early 1950s, but their role in classic lower genital tract disease has still not been firmly established.¹⁷ M. fermentans has been incriminated in human respiratory and joint disease.

The association between immunodeficiency and mollicute infections was first reported in the mid-1970s in patients with primary hypogammaglobulinemia and infections with U. urealyticum, M. pneumoniae, Mycoplasma salivarium, and M. hominis that localized in joint tissues, frequently with destructive arthritis.¹⁸ Mollicute infections after organ transplantation and immunosuppressive chemotherapy were observed in the early 1980s, with both M. hominis and U. urealyticum reported most often. Patients with antibody defects or those receiving immunosuppressive drugs appear to be the most susceptible to infections with mollicutes. They are present in healthy tissues, and emerging evidence indicates that contact with other mollicutes in the environment may be an important hazard.¹

A virus-like agent that arose through transfection of NIH3T3 cells with DNA from Kaposi sarcoma tissues of AIDS patients was later shown to be M. fermentans.¹ Independent laboratories have implicated M. fermentans as a cause of systemic infections and organ failure in AIDS patients.¹⁹ M. fermentans has been isolated from blood and urine samples of HIV-infected persons, detected by polymerase chain reaction (PCR) and immunochemistry in multiple tissue sites at various stages of AIDS, and is implicated as a cofactor in AIDS by virtue of its ability to stimulate lymphocytes and other immunomodulatory activities. M. fermentans acts synergistically with HIV to enhance cytopathic effects on human CD4+ lymphocytes. Mycoplasma penetrans has emerged as a potential cofactor in AIDS progression.²⁰ It has been isolated almost exclusively from the urine of HIV-infected patients and not in HIV-seronegative persons. It has the capacity to invade target cells and activate the immune system of HIV-infected patients at various stages of disease. The ability of mollicutes to establish persistent infections with concomitant activation of the immune system, their potential mitogenicity stimulation of cytokine production, and induction of oxidative stress correlate with increased HIV replication and disease production.¹ The proposed role of mollicutes as infectious agents and cofactors in AIDS-related disorders remains a stimulating hypothesis.

Mollicute-infected cell lines have been associated with chromosomal aberrations, altered morphologies, and cell transformation.²¹ Long-term persistent mollicute infection of mouse embryo cells initiated a multistage cellular process that resulted in irreversible cell transformation, karyotypic alterations, and tumorigenicity in nude mice.²² Their role in the ontogeny of human cancers remains speculative.

Several epidemiologic studies have correlated respiratory infections from Mycoplasma with exacerbation of Crohn’s disease and other chronic inflammatory bowel diseases.²³ Acute-onset gastrointestinal symptoms in patients are accompanied by seroconversion to specific viral or M. pneumoniae antigens.

Various Mycoplasma and Ureaplasma species have been detected in joint tissues of patients with rheumatoid arthritis, sexually transmitted reactive arthritis, and other human arthritides.²⁴ In a review of 358 patients with primary antibody deficiency, mollicute infection was the most common cause of severe erosive arthritis.²⁵