Bacterial motility
Part of a series on |
Microbial and microbot movement |
---|
Microswimmers |
Molecular motors |
Bacterial motility is the ability of
Other types of movement occurring on solid surfaces include twitching, gliding and sliding, which are all independent of flagella.
Bacteria can also exhibit
Currently there is interest in developing biohybrid microswimmers, microscopic swimmers which are part biological and part engineered by humans, such as swimming bacteria modified to carry cargo.
Background
In 1828, the British biologist
Ever since Newton established his equations of motion, the mystery of motion on the microscale has emerged frequently in scientific history, as famously demonstrated by a couple of articles that should be discussed briefly. First, an essential concept, popularized by
Here, ρ represents the density of the fluid; u is a characteristic velocity of the system (for instance, the velocity of a swimming particle); l is a characteristic length scale (e.g., the swimmer size); and μ is the viscosity of the fluid. Taking the suspending fluid to be water, and using experimentally observed values for u, one can determine that inertia is important for macroscopic swimmers like fish (Re = 100), while viscosity dominates the motion of microscale swimmers like bacteria (Re = 10−4).[3]
The overwhelming importance of viscosity for swimming at the micrometer scale has profound implications for swimming strategy. This has been discussed memorably by
Microorganisms have optimized their
where is the velocity of the fluid and is the gradient of the pressure. As Purcell noted, the resulting equation — the Stokes equation — contains no explicit time dependence.[4] This has some important consequences for how a suspended body (e.g., a bacterium) can swim through periodic mechanical motions or deformations (e.g., of a flagellum). First, the rate of motion is practically irrelevant for the motion of the microswimmer and of the surrounding fluid: changing the rate of motion will change the scale of the velocities of the fluid and of the microswimmer, but it will not change the pattern of fluid flow. Secondly, reversing the direction of mechanical motion will simply reverse all velocities in the system. These properties of the Stokes equation severely restrict the range of feasible swimming strategies.[4][3]
As a concrete illustration, consider a mathematical scallop that consists of two rigid pieces connected by a hinge. Can the "scallop" swim by periodically opening and closing the hinge? No: regardless of how the cycle of opening and closing depends on time, the scallop will always return to its starting point at the end of the cycle. Here originated the striking quote: "Fast or slow, it exactly retraces its trajectory and it's back where it started".[4] In light of this scallop theorem, Purcell developed approaches concerning how artificial motion at the micro scale can be generated.[3] This paper continues to inspire ongoing scientific discussion; for example, recent work by the Fischer group from the Max Planck Institute for Intelligent Systems experimentally confirmed that the scallop principle is only valid for Newtonian fluids.[5][3]
Motile systems have developed in the natural world over time and length scales spanning several orders of magnitude, and have evolved
Some of the smallest known motile systems are
Bacteria can be roughly divided into two fundamentally different groups,
Specifically, for microorganisms that live in aqueous environments, locomotion refers to swimming, and hence the world is full of different classes of swimming microorganisms, such as bacteria,
Movement mechanisms
Bacteria have two different primary mechanisms they use for movement. The flagellum is used for swimming and swarming, and the pilus (or fimbria) is used for twitching.
Flagellum
The flagellum (plural, flagella; a group of flagella is called a tuft) is a helical, thin and long appendage attached to the cell surface by one of its ends, performing a rotational motion to push or pull the cell.[14][3] During the rotation of the bacterial flagellar motor, which is located in the membrane, the flagella rotate at speeds between 200 and 2000 rpm, depending on the bacterial species. The hook substructure of the bacterial flagellum acts as a universal joint connecting the motor to the flagellar filament.[13]
Prokaryotes, both bacteria and archaea, primarily use
- Bacterial flagella are helical filaments, each with a rotary motor at its base which can turn clockwise or counterclockwise.[16][17][18] They provide two of several kinds of bacterial motility.[19][20]
- Archaeal flagella are called archaella, and function in much the same way as bacterial flagella. Structurally the archaellum is superficially similar to a bacterial flagellum, but it differs in many details and is considered non-homologous.[21][15]
Some eukaryotic cells also use flagella — and they can be found in some protists and plants as well as animal cells. Eukaryotic flagella are complex cellular projections that lash back and forth, rather than in a circular motion. Prokaryotic flagella use a rotary motor, and the eukaryotic flagella use a complex sliding filament system. Eukaryotic flagella are ATP-driven, while prokaryotic flagella can be ATP-driven (archaea) or proton-driven (bacteria).[22]
Different types of cell flagellation are found depending on the number and arrangement of the flagella on the cell surface, e.g., only at the cell poles or spread over the cell surface.[23] In polar flagellation, the flagella are present at one or both ends of the cell: if a single flagellum is attached at one pole, the cell is called monotrichous; if a tuft of flagella is located at one pole, the cells is lophotrichous; when flagella are present at both ends, the cell is amphitrichous. In peritrichous flagellation, the flagella are distributed in different locations around the cell surface. Nevertheless, variations within this classification can be found, like lateral and subpolar—instead of polar—monotrichous and lophotrichous flagellation.[24][3]
The rotary motor model used by bacteria uses the protons of an electrochemical gradient in order to move their flagella. Torque in the flagella of bacteria is created by particles that conduct protons around the base of the flagellum. The direction of rotation of the flagella in bacteria comes from the occupancy of the proton channels along the perimeter of the flagellar motor.[25]
The bacterial flagellum is a protein-nanomachine that converts electrochemical energy in the form of a gradient of H+ or Na+ ions into mechanical work.
Pilus (fimbria)
A
Other
Gliding motility is a type of translocation that is independent of propulsive structures such as flagella or pili.[42] Gliding allows microorganisms to travel along the surface of low aqueous films. The mechanisms of this motility are only partially known. Gliding motility uses a highly diverse set of different motor complexes, including e.g., the focal adhesion complexes of Myxococcus.[43][44] The speed of gliding varies between organisms, and the reversal of direction is seemingly regulated by some sort of internal clock.[45]
Modes of locomotion
Most
Other types of movement occurring on solid surfaces include twitching, gliding and sliding, which are all independent of flagella. Twitching motility depends on the extension, attachment to a surface, and retraction of
Swimming
External videos | |
---|---|
Why do bacteria move like vibrating chaos snakes? – Journey to the Microcosmos |
Many bacteria swim, propelled by rotation of the flagella outside the cell body. In contrast to
The archetype of bacterial swimming is represented by the well-studied model organism
However, the type of swimming movement (propelled by rotation of flagella outside the cell body) varies significantly with the species and number/distribution of flagella on the cell body. For example, the marine bacterium Vibrio alginolyticus, with its single polar flagellum, swims in a cyclic, three-step (forward, reverse, and flick) pattern. Forward swimming occurs when the flagellum pushes the cell head, while backward swimming is based on the flagellum pulling the head upon motor reversal.[3]
Besides these 180° reversals, the cells can reorient (a "flick") by an angle around 90°, referred to as turning by buckling.[58][54] Rhodobacter sphaeroides with its subpolar monotrichous flagellation, represents yet another motility strategy:[55][24] the flagellum only rotates in one direction, and it stops and coils against the cell body from time to time, leading to cell body reorientations,[56][59][60] In the soil bacterium Pseudomonas putida, a tuft of helical flagella is attached to its posterior pole. P. putida alternates between three swimming modes: pushing, pulling, and wrapping.[57][3]
In the pushing mode, the rotating flagella (assembled in a bundle or as an open tuft of individual filaments) drive the motion from the rear end of the cell body. The trajectories are either straight or, in the vicinity of a solid surface, curved to the right, due to hydrodynamic interaction of the cell with the surface. The direction of curvature indicates that pushers are driven by a left-handed helix turning in CCW direction. In the pulling mode, the rotating flagellar bundle is pointing ahead. In this case the trajectories are either straight or with a tendency to bend to the left, indicating that pullers swim by turning a left-handed helical bundle in CW direction. Finally, P. putida can swim by wrapping the filament bundle around its cell body, with the posterior pole pointing in the direction of motion. In that case, the flagellar bundle takes the form of a left-handed helix that turns in CW direction, and the trajectories are predominantly straight.[57][3]
Swarming
Swarming motility is a rapid (2–10 μm/s) and coordinated translocation of a bacterial population across solid or semi-solid surfaces,[61] and is an example of bacterial multicellularity and swarm behaviour. Swarming motility was first reported in 1972 by Jorgen Henrichsen.[62]
The transition from swimming to swarming mobility is usually associated with an increase in the number of flagella per cell, accompanied by cell elongation.
Cell trajectories and flagellar motion during swarming was thoroughly studied for E. coli, in combination with fluorescently labeled flagella.[66][46] The authors described four different types of tracks during bacterial swarming: forward movement, reversals, lateral movement, and stalls.[46] In forward movement, the long axis of the cell, the flagellar bundle and the direction of movement are aligned, and propulsion is similar to the propulsion of a freely swimming cell. In a reversal, the flagellar bundle loosens, with the filaments in the bundle changing from their "normal form" (left-handed helices) into a "curly" form of right-handed helices with lower pitch and amplitude. Without changing its orientation, the cell body moves backwards through the loosened bundle. The bundle re-forms from curly filaments on the opposite pole of the cell body, and the filaments eventually relax back into their normal form. Lateral motion can be caused by collisions with other cells or by a motor reversal. Finally, stalled cells are paused but the flagella continue spinning and pumping fluid in front of the swarm, usually at the swarm edge.[46][3]
Twitching
A bacterial
Gliding
Non-motile
Non-motile species lack the ability and structures that would allow them to propel themselves, under their own power, through their environment. When non-motile bacteria are cultured in a stab tube, they only grow along the stab line. If the bacteria are mobile, the line will appear diffuse and extend into the medium.[90]
Bacterial taxis: Directed motion
Bacteria are said to exhibit taxis if they move in a manner directed toward or away from some stimulus in their environment. This behaviour allows bacteria to reposition themselves in relation to the stimulus. Different types of taxis can be distinguished according to the nature of the stimulus controlling the directed movement, such as chemotaxis (chemical gradients like glucose), aerotaxis (oxygen), phototaxis (light), thermotaxis (heat), and magnetotaxis (magnetic fields).[3]
Chemotaxis
The overall movement of a bacterium can be the result of alternating tumble and swim phases.[91] As a result, the trajectory of a bacterium swimming in a uniform environment will form a random walk with relatively straight swims interrupted by random tumbles that reorient the bacterium.[92] Bacteria such as E. coli are unable to choose the direction in which they swim, and are unable to swim in a straight line for more than a few seconds due to rotational diffusion; in other words, bacteria "forget" the direction in which they are going. By repeatedly evaluating their course, and adjusting if they are moving in the wrong direction, bacteria can direct their random walk motion toward favorable locations.[93]
In the presence of a chemical gradient bacteria will chemotax, or direct their overall motion based on the gradient. If the bacterium senses that it is moving in the correct direction (toward attractant/away from repellent), it will keep swimming in a straight line for a longer time before tumbling; however, if it is moving in the wrong direction, it will tumble sooner. Bacteria like E. coli use temporal sensing to decide whether their situation is improving or not, and in this way, find the location with the highest concentration of attractant, detecting even small differences in concentration.[94]
This biased random walk is a result of simply choosing between two methods of random movement; namely tumbling and straight swimming.[95] The helical nature of the individual flagellar filament is critical for this movement to occur. The protein structure that makes up the flagellar filament, flagellin, is conserved among all flagellated bacteria. Vertebrates seem to have taken advantage of this fact by possessing an immune receptor (TLR5) designed to recognize this conserved protein.
As in many instances in biology, there are bacteria that do not follow this rule. Many bacteria, such as Vibrio, are monoflagellated and have a single flagellum at one pole of the cell. Their method of chemotaxis is different. Others possess a single flagellum that is kept inside the cell wall. These bacteria move by spinning the whole cell, which is shaped like a corkscrew.[96]
The ability of
The fine-scale interactions between
Phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light.[106] This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite.[107]
Two types of positive phototaxis are observed in
Phototactic responses are observed in a number of bacteria and archae, such as Serratia marcescens. Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are bacteriorhodopsin and bacteriophytochromes in some bacteria. See also: phytochrome and phototropism.
Most prokaryotes (bacteria and archaea) are unable to sense the direction of light, because at such a small scale it is very difficult to make a detector that can distinguish a single light direction. Still, prokaryotes can measure light intensity and move in a light-intensity gradient. Some gliding filamentous prokaryotes can even sense light direction and make directed turns, but their phototactic movement is very slow. Some bacteria and archaea are phototactic.[108][109][110]
In most cases the mechanism of phototaxis is a biased random walk, analogous to bacterial chemotaxis.
Some cyanobacteria (e.g.
Magnetotaxis
Escape response
An
Other taxes
- Aerotaxis is the response of an organism to variation in oxygen concentration, and is mainly found in aerobic bacteria.[106]
- Energy taxis is the orientation of bacteria towards conditions of optimal metabolic activity by sensing the internal energetic conditions of cell. Therefore, in contrast to chemotaxis (taxis towards or away from a specific extracellular compound), energy taxis responds on an intracellular stimulus (e.g.
Mathematical modelling
The mathematical models used to describe the bacterial swimming dynamics can be classified into two categories. The first category is based on a microscopic (i.e. cell-level) view of bacterial swimming through a set of equations where each equation describes the state of a single agent.[138][139][140][141][142] The second category provides a macroscopic (i.e. population-level) view via continuum-based partial differential equations that capture the dynamics of population density over space and time, without considering the intracellular characteristics directly.[143][144][145][146][147][148][149][150][151][137]
Among the present models,
To study the effect of obstacles (another environmental condition) on the motion of bacteria, Chepizhko and his co-workers study the motion of
See also
- Cyanobacterial movement
- Protist locomotion
References
- .
- .
- ^ PMID 33500976. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- ^ doi:10.1119/1.10903.
- PMID 25369018.
- S2CID 3932471.
- PMID 15939237.
- PMID 23166050.
- PMID 11412087.
- PMID 20080560.
- ISBN 9781292235103.
- PMID 25684261.
- ^ PMID 29078764. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- ^ S2CID 4173914.
- ^ PMID 25699024.
- S2CID 10370084.
- S2CID 4242129.
- S2CID 4173914.
- PMID 5318439.
- PMID 14527279.
- ISBN 978-1-904455-48-6.
- PMID 18786541.
- ISBN 9781420007282.
- ^ PMID 21335384.
- PMID 7684268.
- ^ PMID 19741.
- S2CID 85138168.
- PMID 9251788.
- PMID 21673657.
- PMID 1732214.
- PMID 8415608.
- PMID 12500982.
- PMID 20926516.
- PMID 24973293.
- PMID 25613993.
- PMID 20076642. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- ^ "pilus" at Dorland's Medical Dictionary
- ^ PMID 11381130.
- PMID 12142488.
- S2CID 4425775.
- PMID 13230101.
- ^ PMID 28222296.
- ^ PMID 26520023.
- PMID 27028358.
- ^ PMID 24556443.
- ^ PMID 28656975.
- ^ Henrichsen, J. (1972) "Bacterial surface translocation: a survey and a classification". Bacteriol. Rev., 36: 478–503.
- PMID 12142488.
- PMID 27028358.
- PMID 28370801.
- S2CID 3297704.
- PMID 21087385.
- ^ ISBN 9780387216386.
- ^ .
- ^ PMID 3492489.
- ^ PMID 10438751.
- ^ PMID 29196650.
- PMID 21205908.
- PMID 19571004.
- PMID 24872500.
- PMID 14527279.
- PMID 4631369.
- ^ PMID 20694026.
- PMID 28873304.
- ^ PMID 24145500.
- PMID 10781548.
- ^ PMID 31601892. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- PMID 12142488.
- S2CID 4425775.
- PMID 4631369.
- PMID 26968016.
- PMID 26497940.
- PMID 21768344.
- PMID 28393835.
- PMID 10537208.
- PMID 27851948.
- .
- PMID 11381130.
- PMID 12446837.
- PMID 18416602.
- PMID 34017216.
- .
- PMID 29074778.
- PMID 26041805.
- PMID 29891864.
- S2CID 17555804.
- PMID 20580218.
- PMID 15487930.
- PMID 21910630.
- ^ "BIOL 230 Lab Manual: Nonmotile Bacteria in Motility Medium". faculty.ccbcmd.edu. Archived from the original on 15 April 2017. Retrieved 8 June 2021. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- S2CID 1909173.
- PMID 22169400.
- ISBN 978-0-691-00064-0.
- PMID 22169400.
- PMID 4560688.
- ISBN 978-0-387-00888-2.[page needed]
- ^ hdl:11343/274205. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- S2CID 83762501.
- PMID 22028628.
- S2CID 36627341.
- ISBN 9780691190310.
- ^ S2CID 7919921.
- PMID 22540625.
- PMID 22203971.
- PMID 31015402.
- ^ ISBN 978-0-333-34867-3
- ISBN 978-3-540-08837-0
- S2CID 20435383.
- S2CID 9325066.
- ^ PMID 19720645. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- PMID 11452084.
- PMID 17005405.
- PMID 2344465.
- S2CID 4425659.
- S2CID 2811584.
- PMID 7882970.
- PMID 3654583.
- S2CID 12242837.
- S2CID 25364218.
- S2CID 4424642.
- ^ Zhulin, I.B. (2000) "A novel phototaxis receptor hidden in the cyanobacterial genome". Journal of molecular microbiology and biotechnology, 2(4): 491–494.
- S2CID 9549058.
- S2CID 27566851.
- PMID 11134414.
- PMID 11053396.
- doi:10.3389/fmicb.2013.00344. Modified text was copied from this source, which is available under a Creative Commons Attribution 3.0 International License.
- PMID 29581530.
- ^ ISBN 978-0-674-03116-6.
- ^ PMID 33487111. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- .
- PMID 32343961.
- PMID 8655473.
- PMID 28808010.
- ^ Schweinitzer T, Josenhans C. Bacterial energy taxis: a global strategy? Arch Microbiol. 2010 Jul;192(7):507-20.
- ^ BNSim University of Texas. Accessed 10 June 2021.
- S2CID 1888353.
- ^ PMID 28804259. Modified text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- PMID 21494629.
- S2CID 123428558.
- PMID 15951372.
- PMID 18426776.
- PMID 24098352.
- PMID 27297496.
- S2CID 1174081.
- ISBN 978-3-642-35497-7.
- S2CID 18153478.
- PMID 19784399.
- S2CID 249201.
- S2CID 17774620.
- PMID 16479498.
- S2CID 17797787.
- PMID 9960890.
- S2CID 412945.
- ^ PMID 21806920.
- S2CID 11143298.
- S2CID 3122097.
- PMID 26403719.
External links
- Review of the hydrodynamics of bacterial swimming: Lauga, Eric (2016). "Bacterial Hydrodynamics". Annual Review of Fluid Mechanics. 48 (1): 105–130. S2CID 13849152.
- On-line text book on bacteriology (2015)