Biology

Mycoplasma are a member of the Mollicutes, an order within the Tenericutes phyla that do not contain a peptidoglycan cell wall. This feature is unique to Mollicutes and many of these including Mycoplasma are chemo-organotrophic organisms.

mycoplasma-cell-structure

As such these bacteria are more susceptible to osmotic stress and are resistant to Beta-Lactam derived antibiotics such as Penicillin’s and cephalosporins, although they are sensitive to antibiotics which target other metabolic processes (tetracycline and erythromycin).

Cells of Mollicutes contain only a trilaminar membrane as a barrier to the outside environment, this is fortified with lipoglycans and sterols to compensate for the lack of stability caused by no cell wall. Additionally, this feature means Mollicutes exhibit pleomorphism and can exist in many forms (pear shaped, filamentous) – also allowing them to pass through biological filters.

Mycoplasma colonies tend to be < 1mm in diameter displaying a typical umbonate “fried-egg” apperance – However other colonies may have cauliflower-like or smooth colony surfaces with smooth, irregular or scalloped margins.

Genera within the Mollicutes are parasitic organisms infecting a variety of eukaryotic hosts. This parasitic nature may in part be caused by their small genomes; Mollicutes have the smallest genomes of any bacteria (see diagram 2) which greatly reduces their metabolic functions, meaning they rely on hosts for specific growth requirements. Mollicutes and more specifically Mycoplasmas, can be considered as a ‘bridge’ between virus and bacteria; some viruses known as giant viruses (giruses) have genomes more than double the size of Mycoplasma genitalium (580kb).

nrmicro.2016.197-f1 — Size of giant virus genomes compared with more common viruses

Some clinically important genera within the Mollicutes include:

Mycoplasma
Ureaplasma
Spiroplasma

Mycoplasma and Ureaplasma are parasites of humans, insects and other small animals (e.g. fish). They colonize the mucosal layers of the body including the urogenital tract. Mycoplasma are also common lab contaminants of cell lines.

Mycoplasmas are usually non-motile, however some species possess specialized structures which allow them to exhibit gliding motility, the attachment organelle of Mycoplasma pneumoniae is and example of this. Additionally, Mycoplasma mobile is the fastest Mycoplasma and can travel at 2.0 – 4.5 µm/s – or 3 -7 times its cell length per second.

Spiroplasma also exhibit motility, this time by corkscrew movement through liquid. Spiroplasma are again parasites of many organisms including plants. The motility of these organisms is mediated by contracting elements of their internal filaments – this structure responds via chemotaxis.