MicrobiologyBytes

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

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• Bacterial motility

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