MicrobiologyBytes

Cellular movement is key to life and in the case of intracellular parasites, provides a vital mechanism to gain access to the protected niche they require. The parasite Toxoplasma gondii is a model for a group of parasites called apicomplexans, which move by an actin-dependent process referred to as gliding motility. This form of motility is distinct from that used by ciliated or flagellated cells, and from the crawling behavior of amoeba and many mammalian cells.

A new paper demonstrates that the normally highly conserved protein actin is divergent in these parasites and that it displays unusual kinetic properties that result in formation of short unstable filaments, in contrast to the highly stable nature of mammalian actin. The findings reveal that the short dynamic nature of parasite actins is due to a small number of amino acid differences that affect stability of the filament. These properties are essential to normal parasite motility since reversion of these residues to match those seen in mammalian cells was detrimental to gliding movement. The dependence of parasites on rapid turnover of highly unstable actins renders them extremely sensitive to toxins that stabilize actin filaments, thus providing a potential target for development of specific intervention.

Evolutionarily Divergent, Unstable Filamentous Actin Is Essential for Gliding Motility in Apicomplexan Parasites. (2011) PLoS Pathog 7(10): e1002280. doi:10.1371/journal.ppat.1002280
Apicomplexan parasites rely on a novel form of actin-based motility called gliding, which depends on parasite actin polymerization, to migrate through their hosts and invade cells. However, parasite actins are divergent both in sequence and function and only form short, unstable filaments in contrast to the stability of conventional actin filaments. The molecular basis for parasite actin filament instability and its relationship to gliding motility remain unresolved. We demonstrate that recombinant Toxoplasma (TgACTI) and Plasmodium (PfACTI and PfACTII) actins polymerized into very short filaments in vitro but were induced to form long, stable filaments by addition of equimolar levels of phalloidin. Parasite actins contain a conserved phalloidin-binding site as determined by molecular modeling and computational docking, yet vary in several residues that are predicted to impact filament stability. In particular, two residues were identified that form intermolecular contacts between different protomers in conventional actin filaments and these residues showed non-conservative differences in apicomplexan parasites. Substitution of divergent residues found in TgACTI with those from mammalian actin resulted in formation of longer, more stable filaments in vitro. Expression of these stabilized actins in T. gondii increased sensitivity to the actin-stabilizing compound jasplakinolide and disrupted normal gliding motility in the absence of treatment. These results identify the molecular basis for short, dynamic filaments in apicomplexan parasites and demonstrate that inherent instability of parasite actin filaments is a critical adaptation for gliding motility.

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Flagella-driven bacterial motility, referred to as swimming, has been recognized for over 20 years to affect the ability of bacteria to infect and colonize a host. The common theme is that bacteria must be motile to colonize the host but must become non-motile to chronically persist; this has been observed in many pathogenic bacteria including species of Vibrio and Pseudomonas. Therefore it makes sense that the immune system would evolve mechanisms to exploit this virulence determinant of pathogenic bacteria. This paper presents evidence that flagellar motility is recognized by innate immune cells as a phagocytic activation signal. It shows that step-wise loss of flagellar motility confers a proportional ability to evade phagocytic engulfment, independent of the flagellum itself acting as a phagocytic activator. This is not due to motility- co-regulated secretions or compensatory genetic changes by the bacteria, but instead is due to a mechano-sensory response whereby phagocytic cells respond directly to flagellar motility. This represents a novel mechanism by which the innate immune system facilitates clearance of bacterial pathogens, and provides an explanation for how selective pressure may result in bacteria with down-regulated flagellar gene expression and motility as is observed in isolates taken from chronic infections.

Step-Wise Loss of Bacterial Flagellar Torsion Confers Progressive Phagocytic Evasion. (2011) PLoS Pathog 7(9): e1002253. doi:10.1371/journal.ppat.1002253
Phagocytosis of bacteria by innate immune cells is a primary method of bacterial clearance during infection. However, the mechanisms by which the host cell recognizes bacteria and consequentially initiates phagocytosis are largely unclear. Previous studies of the bacterium Pseudomonas aeruginosa have indicated that bacterial flagella and flagellar motility play an important role in colonization of the host and, importantly, that loss of flagellar motility enables phagocytic evasion. Here we use molecular, cellular, and genetic methods to provide the first formal evidence that phagocytic cells recognize bacterial motility rather than flagella and initiate phagocytosis in response to this motility. We demonstrate that deletion of genes coding for the flagellar stator complex, which results in non-swimming bacteria that retain an initial flagellar structure, confers resistance to phagocytic binding and ingestion in several species of the gamma proteobacterial group of Gram-negative bacteria, indicative of a shared strategy for phagocytic evasion. Furthermore, we show for the first time that susceptibility to phagocytosis in swimming bacteria is proportional to mot gene function and, consequently, flagellar rotation since complementary genetically- and biochemically-modulated incremental decreases in flagellar motility result in corresponding and proportional phagocytic evasion. These findings identify that phagocytic cells respond to flagellar movement, which represents a novel mechanism for non-opsonized phagocytic recognition of pathogenic bacteria.

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Bacteria can change course almost instantaneously, zipping towards food or away from toxins. How do such simple organisms do something so complex? It’s all in the flagella, a tail-like structure with rotating helical filaments. The flagella work in unison to propel the cell forward by rotating counterclockwise and thus bundling together. When the flagella reverse their rotation to clockwise, they disrupt the bundle and make the cell tumble in place. When the flagella shift back to counterclockwise again, the bacteria set off on a new course.

This description of bacterial locomotion is well known, but the mechanisms that allow the flagella to shift gears from counterclockwise to clockwise have proven difficult to identify. Now, a new study bring us closer to answering this fundamental question and propose a new model describing how flagella manage this switch. Filaments in the flagella are powered by rotary motors that span the cell membrane. Things of beauty, these motors are tooled so precisely that they are nearly 100% efficient, and their direction is set by a rotor that can turn thousands of revolutions per minute. The rotor shifts from the forward-propelling counterclockwise to the tumble-inducing clockwise when chemical gradients tell bacteria they’ve gone astray, for example, away from food. This activates a cytoplasmic signaling protein that binds proteins in the rotor switch, changing the orientation of another switch protein called FliG and thereby reversing the rotor’s spin to clockwise.

Source: How Bacteria Shift Gears. 2011 PLoS Biol 9(5): e1001061. doi:10.1371/journal.pbio.1001061

Related:

• Bacterial motility

There are many different mechanisms of bacterial motility, by gliding motility has always been something of a mystery. It turns out that in Mycoplasma, gliding motility results from many tiny legs pedalling away like crazy. Cute, wait until the nanotech guys gets hold of this one. Oh wait, they already did.

Unique centipede mechanism of Mycoplasma gliding. Ann Rev Microbiol. (2010) 64: 519-537
Mycoplasma, a genus of pathogenic bacteria, forms a membrane protrusion at a cell pole. It binds to solid surfaces with this protrusion and then glides. The mechanism is not related to known bacterial motility systems, such as flagella or pili, or to conventional motor proteins, including myosin. We have studied the fastest species, Mycoplasma mobile, and have proposed a working model as follows. The gliding machinery is composed of four huge proteins at the base of the membrane protrusion and supported by a cytoskeletal architecture from the cell inside. Many flexible legs approximately 50 nm long are sticking out from the machinery. The movements generated by the ATP hydrolysis cell inside are transmitted to the “leg” protein through a “gear” protein, resulting in repeated binding, pull, and release of the sialylgalactose fixed on the surface by the legs. The gliding of Mycoplasma pneumoniae, a species distantly related to M. mobile, is also discussed.
Related:

• Cytoskeletal structure of Mycoplasma mobile
• Bacterial Gliding Motility: Multiple Mechanisms for Cell Movement over Surfaces. (2007) Ann. Rev. Microbiol. 55: 49-75

Enteric bacteria such as Escherichia coli swim by rotating a set of flagella that forms a bundle when the flagellar motors turn in the counterclockwise (CCW) direction The bundle falls apart when one or more motors turns in the clockwise (CW) direction, and the bacterium tumbles. A new swimming direction is selected upon resuming the CCW rotation of the flagellar motors. By modulating the CCW and CW intervals according to external chemical cues, the cells are able to migrate toward attractants or away from repellents.

This paper report observations of motility patterns of marine bacterium Vibrio alginolyticus. The authors fround found that the bacteria employ a unique cyclic three-step (forward–reverse–flick) swimming pattern for chemotaxis; they regulate both forward and backward swimming times according to a given chemical profile. By employing the three-step chemotactic strategy, cells of V. alginolyticus are able to focus on a point source of attractant rapidly and form a compact swarm around it. This is apparently a significant niche for V. alginolyticus, which live in ocean where nutrients are scarce and rapidly dispersed by currents.

Bacterial flagellum as a propeller and as a rudder for efficient chemotaxis. PNAS USA 4th January 4 2011 doi: 10.1073/pnas.1011953108
We investigate swimming and chemotactic behaviors of the polarly flagellated marine bacteria Vibrio alginolyticus in an aqueous medium. Our observations show that V. alginolyticus execute a cyclic, three-step (forward, reverse, and flick) swimming pattern that is distinctively different from the run–tumble pattern adopted by Escherichia coli. Specifically, the bacterium backtracks its forward swimming path when the motor reverses. However, upon resuming forward swimming, the flagellum flicks and a new swimming direction is selected at random. In a chemically homogeneous medium (no attractant or repellent), the consecutive forward tf and backward tb swimming times are uncorrelated. Interestingly, although tf and tb are not distributed in a Poissonian fashion, their difference Δt = |tf – tb| is. Near a point source of attractant, on the other hand, tf and tb are found to be strongly correlated, and Δt obeys a bimodal distribution. These observations indicate that V. alginolyticus exploit the time-reversal symmetry of forward and backward swimming by using the time difference to regulate their chemotactic behavior. By adopting the three-step cycle, cells of V. alginolyticus are able to quickly respond to a chemical gradient as well as to localize near a point source of attractant.

Related:

• Bacterial Motility