Suggested Bibliography

Below are some of the key references used in writing this article:

Spiroplasma fibrils - http://www.sciencedirect.com/science/article/pii/S0014482704004525

Makoto Miyata, and Jennifer D. Petersen, 2004. Spike Structure at the Interface between Gliding Mycoplasma mobile Cells and Glass Surfaces Visualized by Rapid-Freeze-and-Fracture Electron Microscopy. J. Bacteriology 186: 4382–4386.

Mayer, F. 2003. Cytoskeletons in prokaryotes. Cell Biology International 27: 429–438.

Makoto Miyata and Hiroshi Ogaki, 2006. Cytoskeleton of Mollicutes. J Mol Microbiol Biotechnol 11: 256–264.

Daisuke Nakane and Makoto Miyata, 2007. Cytoskeletal ‘‘jellyfish’’ structure of Mycoplasma mobile. PNAS 104: 19518–19523.

Models of Mycoplasma pneumoniae show a different arrangement of cytoskeletal structures in the apical protrusion. 

There is still a cap-like structure right inside the tip and attached to this and extending down the central axis of the protrusion is a rod-like structure, attached to the surrounding cell membrane by radial spokes along its length and anchored at the base of the protrusion in a disc-like structure, also connected to the cell membrane by radial filaments. 

Other cytoskeletal structures have also been observed in this species, including a helical mesh just beneath the cell membrane and fibres extending into other regions of the cell. 

A different mode of locomotion has been suggested for Mycoplasma pneumoniae in which the apical protrusion extends, then membrane proteins in the apical protrusion attach to the substrate followed by contraction pulling the rest of the cell forwards before the cycle repeats.

Minimalist cells?

As the simplest living cells so far discovered on earth, the mollicutes have attracted much recent research, not just because they are agents of disease, but because these fascinating organisms may give some insight into how cells first evolved. 

Viruses are simpler, but Mycoplasmas are acellular. 

However, with their 600 genes or so they manage a considerable degree of complexity as can be seen by the (highly simplified) account we have given of their cytoskeleton and motility mechanisms. 

Mollicutes are hard to study, because they are so small, and only recently has real progress been made on understanding their cellular machinery, which is complex.

Much remains uncertain and controversial, which is one reason why we avoided details - it should be most interesting to see the progress in these fascinating areas of research over the coming years.

It would appear that, despite their relative simplicity, the mollicutes are highly evolved: as parasites and commensals with some degree of degeneration of their ancestral systems, in addition to evolution of novel systems. 

Nevertheless they remain the smallest and simplest cells that are known to be able to survive as free-living organisms (albeit under carefully controlled laboratory conditions or in specialist habitats such as coal piles) and understanding how they function is of major importance. 

This still leaves the mystery as to how the first cells evolved. 

There would appear to be no surviving simpler cells, and also none appear to be currently evolving. 

What special conditions enabled cells to evolve but prevent them from doing so today? 

In the current absence of simpler cells this question is difficult to answer. 

However, what mollictes show us is how much can be done with relatively few genes and proteins.

Simpler cells?

There is one line of thought that hypothesises that simpler cells do currently exist on earth, but have so far evaded detection. Indeed, the smallest cellular life-forms, such as mollicutes and Chlamydia are still poorly understood and many more similar organisms no doubt await detection. 

(Such as the MLOs, also called phytoplasms, which cause over 600 known plant diseases). 

In recent years controversial claims that minute bacteria, called nanobacteria, of the order of 100 nm across or less, have been discovered remain to be verified. 

Often found inside rocks and other 'primordial' habitats, these 'organisms' may simply be some form of mineral growth. 

Indeed, as the controversy over the Martian meteorite, ALH84001, and its possible nanofossils has shown - proving the existence of the smallest cells, past or present, will likely be extremely difficult.

Almost certainly, the vast majority of bacteria remain undiscovered. Most do not grow on standard agars and only recently has the vast diversity of bacteria and archaebacteria in the Earth's oceans been realised. 

Bacteria are also being discovered in unlikely places, from Antarctic deserts to rocks deep beneath the Earth's surface. No doubt there are many surprises yet to be revealed by the ancient and complex world of prokaryotes.