Mycoplasmas or mollicutes are wall-less bacteria.They are a group of bacteria that do not have a cell wall. Wall-less bacteria are pleomorphic, and they exist in varying morphological shapes. Generally, there are six (6) recognized genera of wall-less bacteria, and they include:
- Anaeroplasma, and
Only Mycoplasmas and Ureaplasma are important pathogens of humans while the others are mainly pathogens of plants and other animals. These other categories of wall-less bacteria rarely cause disease in human population. Ureaplasma is another group of wall-less bacteria that have lost the ability to form cell wall. U. urealyticum is a member of the genus Ureaplasma that inhabits the GIT and causes non-gonococcal urethritis in humans. Mycoplasmas also lack peptidoglycan, and are only enclosed in a plasma membrane. They occur in the urogenital and respiratory tract of humans and animals. Mycoplasmas are pleomorphic, non-motile and anaerobic or facultatively anaerobic bacteria, and some species are fastidious in nature.
Typical examples of the Mycoplasmas are Mycoplasma pneumoniae – that causes pharyngitis (i.e., inflammation of the pharynx) and atypical pneumoniae in children and young people. Mycoplasmas are the smallest free-living microorganisms, and they are poorly stained by Gram staining technique because they are wall-less bacteria and do not possess peptidoglycan. However, the cell membrane of Mycoplasmas is rich in sterols (e.g., cholesterol) which are not found in other prokaryotic cells.
Other Mycoplasma species include M. orale, M. hominis, M. genitalium and M. fermentans.
Mycoplasmas are a typical example of “loss of function” by a bacterial cell; and they are capable of independent existence. Why does Mycoplasma have stronger cytoplasmic membrane than other bacteria? Mycoplasmas are wall-less bacteria i.e., they do not have cell walls like the traditional Gram-positive and Gram-negative bacteria with cell walls. Thus, their cytoplasmic membrane (which usually underlies the cell walls in bacteria that have cell walls) is the only protection against external aggression and it also help to maintain the internal osmotic pressure of the cell as well as control the entry and exit of materials in and out of the cell. Therefore the cytoplasmic membrane of mycoplasmas is much stronger than that of other bacteria with cell walls.
L-form bacteria are group of bacteria with defective cell walls. In L-form bacteria, the cell wall is either partially or completely absent. They are highly resistant to beta-lactam antibiotics – which targets the murein component of the cell wall. Antibiotics block murein synthesis in such organisms, and L-form bacteria particularly have defects in their murein layer. Cell wall or murein-defective bacteria can be produced by treating bacterial cells with cell wall inhibiting chemicals or drugs. However, some L-form bacteria especially the unstable ones can revert back to the normal bacteria while the stable L-form bacteria will not revert back to the normal or parental bacterial cell after being treated with substances that inhibit cell wall formation. Mutation in L-form bacteria, as well as chemical treatment which affects their cell wall results in the formation of two types of cells known as protoplasts andspheroplasts.
Spheroplasts originate from Gram-negative bacteria and may possess some outer-membrane component of their parent bacterial cell but protoplasts originate from Gram-positive bacteria, and they lack cell wall components. L-form bacteria are generally mutated or transformed bacterial cells which can be formed in the laboratory under certain stimuli conditions such as treatment of bacterial cells with chemicals or antibiotic. Heat shock and osmotic shock are other examples of stimuli that can be used to transform a bacterial cell into an L-form bacterium. Spheroplasts are unstable L-form bacteria because they can revert back to their parent bacterial cell when the stimulus is removed but the protoplasts are stable and cannot revert back to their parent cells even when the stimulus is removed or inhibited. Note: stable L-form bacteria continue to reproduce as L-form bacteria since they cannot relapse to the parent bacterial cell from which they developed. Bacteria genera that can easily form L-form bacteria include: Streptococcus, Proteus and Bacillus.
Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.
Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.
Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.
Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.
Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.