Which of the following options indicates the size of PPLO (Pleuro Pneumonia Like Organisms)?

Correct — C. PPLO, the organisms now classed as Mycoplasma, are the smallest known cells, of the order of 0.1 µm — that is 100 nanometres.

Two familiar facts follow from that size. They pass through filters that hold back ordinary bacteria, which is why they were first taken for viruses, and they carry no cell wall, which is why penicillin does not touch them.

Set the scale against the alternatives: a typical bacterium is 1-2 µm long and an animal cell 10-30 µm, so a figure in the tens of micrometres describes a cell hundreds of times too big. Option (c) is the one at the right scale.

Why the others are wrong

• (a)
  About 10 µm — 10 µm is the scale of a human cell — a red blood cell is about 7-8 µm across. PPLO are roughly a hundred times smaller than that.
• (b)
  About 20 µm — 20 µm describes a large eukaryotic cell, not the smallest organism known. At this scale a mycoplasma would be a speck on the same drawing.
• (d)
  About 30 µm — 30 µm is larger still, the range of a plant cell or a big protozoan. Nothing in the mycoplasma group comes anywhere near it.

PPLO stands for Pleuro Pneumonia Like Organisms, named because the first of them was isolated from cattle suffering contagious bovine pleuropneumonia. They are now placed in the genus Mycoplasma.

They are prokaryotes defined by an absence: no cell wall. That leaves them pleomorphic, with no fixed shape, and untouched by antibiotics such as penicillin that work by blocking wall synthesis.

Their smallness is what exams use. In size they sit at the boundary between cell and virus, though unlike a virus a mycoplasma is a complete cell that reproduces on its own.

Key facts

• PPLO are the smallest known cells, about 0.1 µm across.
• Mycoplasmas have no cell wall, which is why penicillin is ineffective against them.
• The name comes from contagious bovine pleuropneumonia, the cattle disease from which the first PPLO was isolated.
• 0.1 µm is 100 nanometres, smaller than many bacteria and comparable to the size of large viruses.

Common traps

• Treating PPLO as viruses because they pass through bacterial filters, when they are complete cells with cytoplasm and ribosomes.
• Slipping between micrometres and nanometres: 0.1 µm is 100 nm, not 0.1 nm.

No directly related past PYQ was found.