Bacterial - Mycoplasmas (Mollicutes)

The simplest cells?

Mollicutes, informally referred to as the mycoplasmas, are extraordinary organisms! They are the smallest known cells, typically the size of large viruses at only 200 to 300 nm in diameter (1 nanometer, 1 nm = 1 millionth of a millimetre) and they have the smallest genomes of any known bacteria. 

The genome of Mycoplasma genitalium, for example, is only 580,073 bases long and contains a mere 517 genes. Though still complex in real terms, this is a very minimilistic genome and mycoplasmas are of especial interest to scientists working to build simple cells in the lab.

Mycoplasmas are often described as the simplest free-living cells, although most are parasitic or commensals dependent on their host for certain functions, however, they are independent in the sense that they have their own power sources, fermenting fuels to make ATP (unlike the tiny chlamydial elementary bodies which are energy parasites dependent on ATP provided by the host cells they live within). 

Most 'respire' anaerobically by fermentation without the use of electron transport chains (ETCs) although rudimentary ETCs may be present in the cytoplasm (or attached to the cell membrane in Acholeplasma). 

Their simplicity is in large part due to evolutionary degeneration - they are dependant on their hosts for many metabolic functions that they would otherwise have to carry out on their own and so they have lost these functions, including the ETC.

As prokaryotes they lack a nuclear envelope, possessing a DNA/protein nucleoid rather than a nucleus. As bacteria we tend to think of them as single-celled organisms, however, like most bacteria they can form biofilms - loose associations in which the cells remain distinctly separate but are embedded in a common slime matrix.

Despite this, they do not seem to use quorum-sensing to sense the presence of other bacteria and communicate in order to form biofilms. 

Most bacteria use quorum-sensing in biofilm formation, but it is thought that mycoplasmas form these structures without communication between the component cells. 

Biofilms are not true multicellular organisms, since the cells do not communicate via cell-to-cell junctions (pores and channels that join the cytoplasm of neighbouring cells together, as do gap junctions in animal cells, plasmodesmata in plants and microdesmata in some cyanobacteria). 

Indeed, in a typical biofilm the cells are not generally in direct physical contact with other cells.

Mycoplasmas also lack protective cell walls - they have no peptidoglycan like most bacteria and no rigid layer at all in their cell envelopes, which instead consists of single cell membranes coated in carbohydrates (forming the glycocalyx or slime coat) rather like animal cells and protozoa. 

Like mammalian cells they are osmotically sensitive. Having no wall to maintain their shape they will swell and burst in distilled water. 

Again, this may be a feature that their ancestors had which was lost as their host maintains an osmotically stable environment which is also generally free of mechanical trauma. Many do incorporate sterols, manufactured by their hosts, however, which strengthen their membranes by making them more rigid. 

Acholeplasma is able to grow without sterols, but will incorporate them if they are available and may manufacture carotenoids to strengthen its membrane.

Life as parasites

Most mycoplasmas are parasites or commensals, living inside other organisms. 

Thermoplasma acidophilum is unusual in being found in acidic coal refuse piles where internal temperatures reach 55 degrees C. Spiroplasma infects plants and the arthropods which carry it from plant to plant, infecting the haemolymph, gut and salivary glands of insects. 
Some may cause repiratory infections in humans, for example Mycoplasma pneumoniae can cause a type of pneumonia (though it is by no means the only cause of this disease). 
Mycoplasma may also infect the synovial membranes of the joints of vertebrates, causing a form of arthritis.

Mycoplasmas require complex nutritional requirments if grown in the lab, since they depend on their hosts for complex growth requirements, such as fatty acids, vitamins, purines, pyramidines, and also the sterols for their membranes. When grown on agar they form characteristic small circular colonies with a nipple-like or fried egg-like appearance.

Ureaplasma lives in the mouth, respiratory and genital tracts of mammals and humans and has a novel way of obtaining energy. 

Whereas most mollicutes generate ATP by anaerobic fermentation, Ureaplasma exhibits an unusual form of respiration in which urea (a waste product of mammalian metabolism) is hydrolysed by an enzyme called a urease to form ammonium (NH4+) which acts as a source of protons (H+) to power the ATPase by generating a proton gradient across the cell membrane (essentially generating positive electric charge which flows through the ATPase which acts as an electric motor whose rotation energy is sued to make ATP).

Many mycoplasmas have been implicated in causing plant disease, though since these are often poorly characterised they are often referred to tentatively as mycoplasma-like organisms (MLOs).

Cell shape and cell motility - novel mechanisms of locomotion

Mycoplasmas are often described as pleomorphic: having variable shape, especially types like Mycoplasma, although this is more true when growth conditions are sub-optimal (it is dificult to grow parasites out of their hosts) and in optimal conditions their form is more consistent.

The simplest cells?

Mycoplasma grown under optimal conditions tends to have a more regular form. These organisms glide by means of the apical protrusion protruding from their front end.

For a long time it was though that bacteria lacked an internal cytoplasmic skeleton (cytoskeleton) as found in eukaryotic cells. 

This is because bacterial cells are much smaller and have a cell wall to support them (for a while it was thought that plant cells might not have a cytoskeleton because of their cell walls, but they do). 

However, it is now known that bacteria do have a cytoskeleton, although one that is much less developed than in plant and animal cells. 

Indeed, in normal electron microscopy no cytoskeleton may be evident at all in bacteria, except perhaps the occasional tubular structure. 

It is now realised, however, that cytoskeletal structures form during cell division and also during cell growth, at least in some forms, where cytoskeletal filaments direct the deposition of new peptiodoglycan fibres in the cell wall. 

In some cases cytoplasmic filaments are also seen to anchor flagella motors. 

Mycoplasmas, although tiny, lack supporting cell walls and so their cytoskeletons have more work to do and are better developed. 

Animal cells and protozoa, being large and wall-less have especially well developed cytoskeletons. 

The cytoskeletons of mollicutes are also involved in locomotion. These bacteria lack flagella, possibly because they have no rigid cell walls in which to anchor the rotary motors. 

Spiroplasma, being helical in shape moves by flexing, creeping and swimming by rotating in cork-screw fashion, rather like spirochaetes, except that spirochaetes use endoflagella to produce these movements whereas Spiroplasma has no endoflagella. 

Instead Sprioplasma has a unique helical protein fibres (3.6 nm diameter) in the cytoplasm, grouped into bundles or ribbons and a second helical structure made of an actin-like protein MreB. 

Cryo-electron tomography, a new technique for visualising cell structure at a molecular scale (less than 5 nm resolution) whilst preserving the structure of living cells almost intact has revealed two ribbons of thicker filaments with a band of thinner filaments in-between.
 
At least one of these structures is thought to be contractile.

These fibrils are positioned just beneath the cell membrane and are thought to be involved in maintaining helical shape as well as in motility and daughter-chromosome separation during cell division. 

Indeed, these fibrils bear some resemblance to those seen in walled bacteria, for example an MreB-like protein, Mbl (an MRb homologue) in the rod-shaped Bacillus subtilis form helical filaments that direct the deposition of helical fibres of peptidoglycan in the cell wall during cell growth (cell elongation) whilst MreB controls cell width. 

However, the other component of Spiroplasma's helical fibres is apparently unique to Spiroplasma.

Spiroplasma rotates as it swims, corkscrewing its way along, which is an advantage when swimming in highly viscous (sticky) fluids and, perhaps not surprisingly, Spiroplasma is viscotactic, moving towards regions of high fluid viscosity and also exhibits chemotaxis. 

They change helicity, from anticlockwise to clockwise at intervals, and often transitional cells are seen with mixed helicity, caught in the act of changing. They can also change the pitch (steepness of turns) of the helix with waves of change in helicity traveling down the length of the cell as pairs of tiny kinks travelling from anterior to posterior.

The kinks in each pair are on average 0.26 seconds apart, with the second kink appearing as the first nears the posterior end of the cell. 

The kinks move at about 10.5 micrometres per second and the cell swims in the opposite direction at about 3 micrometres per second, faster (up to about 5 micrometres per second) if the medium is thickened (made more viscous) by the addition of 0.5% methylcellulose - like spirochaetes they swim faster in high viscosity fluids. 

These observations suggest that the cells rotate and swim due to flexing of the helical filaments.

Gliding motility in Mycoplasma

Many mollicutes move by gliding across a solid surface. Of these, the fastest is Mycoplasma mobile which can glide at 2.0 to 4.5 micrometres per second on a glass surface. This is astonishingly fast for a gliding organism. 

Like many mollicutes, Mycoplasma mobile moves by means of an anterior nose-like anterior projection which generates traction against the substrate apparently by means of about 400 minute (less than about 50 nm long) 'leg-like' proteins (possibly the protein Gli349) in the cell membrane which are thought to alternately adhere to, detach, advance and reattach to the surface, in a stepping manner, pulling the cell along. 

Alternatively, other electron microscope evidence shows a much smaller number of relatively long spikes (less than about 50 nm long) which may be the leg-like structures or separate adhesion organelles. 

The cells can only glide forwards and can not reverse. ATP hydrolysis provides the energy for this movement. 

Inside the apical protrusion are prominent cytoskeletal structures - a solid nose-cap or hexagonal-lattice of a specific protein in the very tip of the protrusion, forming an oval or hemispherical cap 235 nm wide and 155 nm long attached to dozens of flexible protein 'tentacles' (inside the cytoplasm). 

The tentacles have 20 nm particles (proteins?) attached to them at 30 nm intervals and it is thought that the leg-like proteins in the cell membrane attach to these tentacles, transmitting the force of traction they generate to the rest of the cell.