Mycoplasma is commonly known as "minimalist" cells with a diameter ranging from 0.1 to 0.5 micrometers. Their small size allows them to pass through most bacterial filters, earning them the title of "filterable" bacteria.
The lack of a cell wall which is a prominent feature of other bacteria makes them sensitive to osmotic pressure changes. They can also be contorted into various shapes like all mollusks, from round to filamentous. Mycoplasma exhibits resistance to antibiotics targeting cell wall synthesis, such as penicillin. Their cell membrane is rich in sterols to make it more resilient to compensate for the absence of a cell wall.
During the process of evolution, Mycoplasma have undergone extensive genome reduction, resulting in a smaller genome compared to many other bacteria. The genome of Mycoplasma genitalium is only about 580 kb with approximate 480 genes. This reduction in genetic material is considered an adaptation to their parasitic and symbiotic lifestyle, as they rely on host cells to obtain essential nutrients. Their DNA is uniformly dispersed in the cytoplasm without a nucleoid region like bacteria. Mycoplasma reproduces through binary fission or budding.
Mycoplasma is currently known as the smallest microorganism that is capable of growth and reproduction in non-living culture media. They parasitize host cells and penetrate bacterial filtering membranes, so they are a common and challenging contaminant in cell culture. They have higher nutritional requirements compared to bacteria and require the addition of 10% to 20% human or animal serum to provide the necessary sterols. The typical "fried-egg" colonies appear after 2 to 3 days of incubation on agar culture media: round in shape with a diameter of 10 to 16 µm, thick central core, and a thin translucent peripheral region. Additionally, Mycoplasma can also grow in chick embryos or living cells.
The small size of Mycoplasma facilitates their invasion into host cells to evade the immune system's attack. They can cause a range of diseases such as respiratory infections, urinary tract infections, and reproductive system diseases. However, diseases caused by Mycoplasma are generally milder and mostly long-term chronic conditions, sometimes even asymptomatic. The absence of symptoms makes patients unconcerned about it and the delayed treatment eventually leads to permanent impairment of reproductive function.
The transmission routes, symptoms, and treatment of Mycoplasma are similar to Chlamydia but do not include trachoma. For more information, please refer to the article on "Chlamydia". Due to the abundant sterols in their cell membrane, antibiotics that inhibit sterol synthesis, such as amphotericin B, are effective against Mycoplasma.
Mycoplasma can also infect animals and plants. Some examples include pneumonia in animals and diseases like yellows and dwarfism in plants, which are associated with Mycoplasma.