General Microbiology - Schlegel H. 1987

Prokaryotes: A Taxonomic Survey
Mycoplasmas (Group 19)

Mycoplasma group representatives (Class Mollicutes) are the smallest self-replicating prokaryotes. They completely lack Cell walls. Since their protoplast is bounded solely by The Plasma Membrane, these Cells are extremely osmotically fragile. In most Mycoplasmas (such as Mycoplasma and Ureaplasma), The Genome is four times smaller than that of Escherichia coli (a mere 0.5∙10⁹ Da); consequently, they possess the smallest genome among all self-reproducing prokaryotes. The order Mycoplasmatales has been assigned to a distinct bacterial class named Mollicutes ("soft-skinned"), which emphasizes the phylogenetic divergence of mycoplasmas from all other Bacteria.

The first described member of the mycoplasma group was the CAUSATIVE AGENT OF bovine pleuropneumonia. On Agar nutrient media supplemented with serum, it forms small colonies resembling a fried egg. Forms exhibiting this growth pattern were historically designated as PPLO (pleuropneumonia-like organisms). Mycoplasmas have also been identified as pathogens for numerous other diseases; they frequently contaminate tissue cultures, although non-pathogenic commensal species are known as well.

Colonies consist of individual cells and aggregates of varying sizes, which can be described as coccoid cells, filaments, discs, and rosettes. Reproduction occurs through conventional Cell Division, the fragmentation of filaments and rings into coccoid cells, as well as a budding-like process. In liquid media, highly irregular and often branched cells also appear (Fig. 3.18) that are capable of passing through membrane filters, much like Viruses.

Distribution and Species. Representatives of the mycoplasma group (genera Mycoplasma, Acholeplasma, and Spiroplasma) are parasitic bacteria. However, rather than killing their hosts, they typically establish chronic infections, making them highly successful and well-adapted parasites.

Fig. 3.18. Mycoplasmas. Cells of the rat bronchopneumonia agent growing in a liquid nutrient medium; transmission electron micrograph; 11,200×. (Klieneberger-Nobel E., Cuchów F. W., J. Gen. Microbiol., 12 [1955] 95.)

In animals, they commonly occur as relatively harmless parasites on the mucous membranes of the respiratory and reproductive tracts (in mammals and birds). These organisms function as membrane parasites, meaning they firmly attach to the epithelial cells of mucosal surfaces. Although they do not secrete toxins, their intimate contact with the wall-less host cells allows even mildly toxic metabolic byproducts—such as ammonium ions and hydrogen peroxide—to exert adverse effects on the membranes of the infected cells.

Two main genera of animal-parasitic mycoplasmas are distinguished. Members of the genus Mycoplasma can grow in pure culture only on media supplemented with Cholesterol or complex steroid-containing additives (such as Blood serum). Cholesterol-independent species have been classified into the genus Acholeplasma. Mycoplasma infections in some animals remain entirely asymptomatic, whereas in others they provoke inflammation of the respiratory tract, Lungs, or Mammary Glands. Host Specificity is often reflected in specific epithets, such as Mycoplasma canis, M. gallisepticum, M. hominis, and others.

In plants, mycoplasmas are the causative agents of yellows diseases. They predominantly localize within the phloem and, owing to their superficial morphological resemblance to spirilla, have been grouped into the genus Spiroplasma. Spiroplasma citri is the pathogen responsible for citrus stubborn disease. Similar forms have been discovered in other plants (including corn, prickly pear, Bermuda grass, and rice). Spiroplasma has also been isolated from bees and grasshoppers, suggesting that insects serve not merely as vectors, but as true hosts for these species.

Biochemical Characteristics. Mycoplasmas differ from most other bacteria not only by the absence of a Cell wall, but also by a set of distinct biochemical traits. They proliferate exclusively in isotonic or hypertonic media (supplemented with sorbitol or sucrose) and have an absolute requirement for Purines, Pyrimidines, and Lipids, including Steroids. The absence of Quinones and Cytochromes in mycoplasmas strongly indicates that they possess a highly truncated Respiratory Chain.

Relation to L-Forms. In 1934, a strain multiplying in the form of irregularly shaped protoplasts was isolated from a cell population of Streptobacillus moniliformis. These cells were termed L-forms in honor of the Lister Institute. Cells growing as naked protoplasts can also be isolated from cultures of Salmonella, Escherichia, Proteus, and other bacteria when cultivated on serum agar in the presence of penicillin (100 µg/mL). This yields fried-egg-shaped colonies strikingly similar to those of mycoplasmas. Two Types of L-forms have been identified: unstable L-forms, which revert to normal cells upon removal of penicillin, and stable L-forms, which fail to regenerate cell walls even in the absence of the antibiotic. It was initially hypothesized that Mycoplasma species evolved from conventional bacteria via Mutations leading to stable L-form production; however, genomic size data and the GC content of mycoplasmal DNA contradict the eubacterial origin hypothesis, pointing instead to a distinct, separate bacterial class.

¹ The family Ectothiorhodospiraceae is now recognized as a separate taxon encompassing the genus Ectothiorhodospira, which was previously included in the family Chromatiaceae. — Translator's Note.