(With a nod to Aerosmith)
by by Daniel P. Haeusser
Many prokaryotes move actively in liquid (swim) or on moist solid surfaces (swarm and glide) toward or away from a stimulus, such as a nutrient, light, or oxygen. Not surprisingly, prokaryotes have evolved numerous means of locomotion built around distinct molecular mechanisms.
How distinct? A human running, a dog walking, an eagle flying, a fish swimming, a frog hopping, and a starfish crawling are each unique, but all share a root mechanism in the molecular properties of actin and myosin in muscle cells. For prokaryotes however, their movements don’t just look different; they often evolved from disparate molecular systems or organelles unique to a particular genus or even species . Some of the more-studied mechanisms of prokaryotic motility are flagellar-based swimming and swarming, type-IV pilus twitching motility, and the ‘adventurous’ motility of Myxococcus xanthus. This adventurous motility relies on both secreted polysaccharide and a helical motor that produces tread-like protrusions along the cell surface.
Among the handful of other known prokaryotic motility mechanisms are those employed by the Mycoplasma, a genus in the class Mollicutes of the low G+C Gram-positive bacteria (Firmicutes). (Video of Mycoplasma motility) The Mollicutes share several defining characteristics, most obviously the lack of cell wall that gives them their name (mollis is Latin for ‘soft’ or ‘pliable’). Therefore, although they phylogenetically fall in the Firmicutes, the Mollicutes have no cell wall for the Gram stain and are not susceptible to the antibiotics that target bacterial cell wall synthesis. In addition, the Mollicutes are some of the smallest cells known at 0.2 – 0.3 μm in length. Although they can be grown independently, these bacteria are often closely associated with host organisms, allowing the bacteria to attain drastically reduced genome sizes (The ~500,000 kilobases of M. genitalium is the lower limit for forming colonies on agar).
Recently featured in the news and here for its role in the synthetic biology research of the J. Craig Venter Institute, Mycoplasma also has played an important role in pathogenesis in livestock (M. mycoides) and in humans (M. pneumoniae and M. genitalium). They are also a frequent contamination of laboratory eukaryotic cell cultures, a significant risk for inducing data artifacts in cell cultures. Together, these traits make Mycoplasma a fascinating subject for study in a variety of fields. In particular, the mysterious motility observed for species of the M. pneumoniae cluster has inspired enthralling research that begins to illuminate novel mechanisms of prokaryotic motility.
It’s first worth getting a sense of how fast these microbes are moving (with a nod to the wonderful series of “Microbial Olympics” essays to which our own Merry Youle contributed. If you’ve witnessed bacterial motility under a microscope, you’re perhaps most familiar with flagellar-mediated E. coli swimming, which generates speeds of ~10 – 35 mm/sec. This translates to a top burst speed of ~17.5 ‘body’ lengths (bl)/sec. To put this into perspective (based on numbers here), the world’s fastest fish, the sunfish, clocks in at a staggering speed of 90 bl/sec, a goldfish manages a healthy 4.5 bl/sec, and US Olympian Michael Phelps achieves a meager 1 bl/sec. Of the motile Mycoplasma, the swiftest known is the aptly named M. mobile, a presumed pathogenic resident of gills in freshwater fish. Discovered in the late 1970’s, M. mobile has been studied as a potential biomotor with gliding speeds of 2 – 7 mm/sec. Considering their small size (~0.7 mm) this gives a top burst speed of ~10 bl/sec. This M. mobile gliding is about half as fast as E. coli swimming, but still quite respectable. Indeed, it is double the sprinting speed of world-record-holding, Jamaican Olympian Usain Bolt (5.3 bl/sec.)!
So then, M. mobile gets around. How does it do it? Mycoplasma species lack flagella, pili, or homologs to any known motility system, prokaryotic or eukaryotic. It turns out that their motility is closely related to their shape – again, form fits function. Species in the M. pneumoniae cluster (including M. mobile) have a light-bulb shaped morphology consisting of a body and a polar membrane protrusion (head) called the attachment or terminal organelle. The cells glide unidirectionally, leading with this head in an asymmetry that allows attachment to surfaces by the terminal organelle and that has implications in division.
Beneath this bulb-shaped morphology lies a complex cytoskeleton. When exposed to the detergent Triton X-100 to permeabilize the cell membranes, M. motile cells leave behind a 'ghost' consisting of an insoluble cytoskeleton that resembles a 'jellyfish', with a lattice-like 'bell' in the head and 'tentacles' that extend from the base ('neck') of the terminal organelle into the cell body. Interestingly, gliding deficient mutants had discontinuous and significantly reduced tentacles compared to wild type cells. So what is this cytoskeleton made of? While some Mycoplasma encode known prokaryotic cytoskeletal elements such as FtsZ and MreB, these are absent in M. mobile. Instead, researchers identified a set of at least ten unique proteins as making up the M. motile cytoskeleton structure.
To identify the proteins involved in gliding, researchers compared the protein expression profiles of wild-type cells and gliding-deficient mutants. They also made antibodies that specifically inhibited M. mobile gliding and used these to localize the target proteins. Three of the four identified proteins are found in the neck portion of the cell surface, corresponding to the region where the tentacle portion of the internal cytoskeleton resides. Antibody-mediated inhibition of single gliding proteins coupled with structural studies using conventional and atomic force electron microscopy led to a model where the gliding proteins form a leg-like structure extending from inside the cell (associated in some way with the cytoskeleton components) to a ‘foot’ that interacts with surface substrates. In this way, M. mobile is able to ‘walk’ across surfaces and manifest its visible gliding motility.
At this point you may wonder how this walking is powered. Recall that three of the four identified gliding proteins localize to the neck of M. motile. The fourth gliding protein, P42, has not been localized, but it is a putative ATPase that has ATP hydrolysis activity when expressed in recombinant E. coli. Furthermore, ATP is required for M. mobile motility and addition of ATP amazingly restored motility to the Triton X-100 insoluble M. motile 'ghosts' (video here). This, and further data, have led to the 'centipede' model, where cycles of ATP binding, hydrolysis, and release by P42 lead to conformational changes in the three structural gliding proteins and subsequent binding and release from surface substrate. A recent publication further characterized the physics of these movements using cells elongated by low detergent concentrations.
Surprisingly, this molecular system of motility appears specific for M. mobile. The gliding proteins of M. mobile are found in the closely related M. pulmonis, but nowhere else known. Researchers have studied the motility of M. pneumoniae for decades longer than M. motile and, while the overall cell morphology and means of locomotion appear conserved between the two species, the cytoskeletal structure of M. pneumoniae (and that of the related M. gallisepticum) is distinct from that of M. mobile, with a completely different set of gliding proteins that has led to the 'inchworm' model of Mycoplasma motility. This suggests separate evolution of distinct motility systems (‘centipede’ vs. ‘inchworm’ models) in different Mycoplasma species (M. motile vs. M. pneumoniae, respectively).
Considering their close association (and at times dependence) on host species and their reduced genomes did the ancestral Mycoplasma lose their motility only to regain similar systems independently? Were they immotile bacteria that found a need for motility once beholden to host? Do the cytoskeletal and gliding proteins of these species perform other functions that have been coopted? How are these cytoskeletal and motility systems kept organized through the process of cell division? A host of questions remains for these fascinating and deceptively simple organisms sprinting about their hosts.