Mycoplasma: Smallest Living Cells Without Cell Walls

Mycoplasma are the smallest living cells known to science, so tiny that about five hundred of them could line up across the width of a single human hair. They are bacteria, but very unusual ones: they have no cell wall at all. Without a rigid wall they can change shape freely, which is why they are also called pleomorphic, meaning many-shaped. They were first discovered in cattle suffering from lung disease and were originally named PPLO, short for pleuropneumonia-like organisms.

Mycoplasma matter because they break the normal rules of bacterial life. They can survive without oxygen, they are naturally resistant to penicillin and similar antibiotics that attack cell walls, and several species cause diseases in humans, animals and plants. Scientists also study them as examples of the simplest possible free-living cells.

Key Concepts

• Smallest cells: roughly 0.1 to 0.3 micrometres across, much smaller than typical bacteria.
• No cell wall: the defining feature; the cell is bounded only by a plasma membrane strengthened with sterols.
• Pleomorphic: no fixed shape because there is no rigid wall to hold one.
• Facultative anaerobes: can live with or without oxygen.
• Penicillin resistance: penicillin kills bacteria by blocking cell-wall building, so wall-less mycoplasma are naturally unaffected by it.
• Pathogenic species: cause pleuropneumonia in cattle, witches' broom disease in plants, and a mild lung infection called walking pneumonia in humans.

How It Works

A normal bacterium would burst without its wall, because water rushes in by osmosis. Mycoplasma avoid this by packing their plasma membrane with sterol molecules, the same kind of stiffening molecules found in animal cell membranes, which makes the membrane tougher and less leaky. They reproduce by simple binary fission like other bacteria. When they infect a host, they attach to the surface of host cells using special tip proteins and steal nutrients, damaging tissues such as the lungs in cattle or the growing tips in plants, which causes the abnormal bushy growth called witches' broom.

Worked Example

Question: A human hair is about 100 micrometres wide. A mycoplasma cell is about 0.2 micrometres across, while a typical rod-shaped bacterium is about 2 micrometres long. How many of each fit across the hair, and how many times smaller is mycoplasma?

Step 1: Mycoplasma across hair = 100 / 0.2 = 500 cells.

Step 2: Typical bacteria across hair = 100 / 2 = 50 cells.

Step 3: Size ratio = 2 / 0.2 = 10.

Answer: 500 mycoplasma cells fit across a hair versus 50 ordinary bacteria, so mycoplasma is about 10 times smaller.

Real-World Applications

• Medicine: diagnosing mycoplasma infections needs special tests, since standard penicillin-type antibiotics do not work on them.
• Veterinary care: controlling pleuropneumonia in cattle protects livestock health and farmers' income.
• Plant pathology: identifying witches' broom and similar diseases helps save crops.
• Cell biology research: laboratories guard cell cultures against mycoplasma contamination, and researchers study their tiny genomes to learn the minimum genes needed for life.

Mycoplasma are the smallest known living cells and have no cell wall, which makes them pleomorphic, able to live without oxygen, and resistant to penicillin. They cause diseases in cattle, plants and humans, and their extreme simplicity makes them valuable models for understanding the basics of life.