Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

High-angle helix helps bacteria swim

14.08.2013
It’s counterintuitive but true: Some microorganisms that use flagella for locomotion are able to swim faster in gel-like fluids such as mucus.

Research engineers at Brown University have figured out why. It's the angle of the coil that matters. Findings are reported in Physical Review Letters.


A really interesting fluid dynamics problem
Locomotion is a different proposition at the cellular scale. A bacterium — this is a helical leptospira — that is swimming through water “would be like us trying to swim in tar.” Some microorganisms swim with helical flagella, but how?


Powerful swimmers: a comparison
In the graph, the vertical axis is the ratio of speed in a viscoelastic fluid to speed in water. The horizontal axis is the degree of viscoelasticity. A flagellum with a high-angle helix, labeled with triangle, swims faster in a viscoelastic fluid than in water when the viscoelasticity is just right. As the helix angle decreases, the peak enhancement in speed decreases. For low-angle helices (circle), viscoelasticity always makes the swimmer slower than it would be in water. Credit: Powers lab/Brown University

A high-angle helix helps microorganisms like sperm and bacteria swim through mucus and other viscoelastic fluids, according to a new study by researchers from Brown University and the University of Wisconsin. The findings help clear up some seemingly conflicting findings about how microorganisms swim using flagella, helical appendages that provide propulsion as they rotate.

Simple as single-celled creatures may be, understanding how they get around requires some complex science. The physics of helical swimming turns out to be “a really interesting fluid dynamics problem,” said Thomas Powers, a professor of engineering and physics at Brown and one of the new study’s authors.

At the scale of a single cell, fluids become much more viscous than on larger scales. A bacterium swimming through water “would be like us trying to swim in tar,” Powers said. That means swimming at the micron scale is a completely different enterprise than it is for fish or people. Counterintuitive as it may sound, tiny helical swimmers rely exclusively on drag to move forward. The turning flagellum creates an apparent wave that propagates out from behind the creature. The drag force against that wave pushes the creature in the opposite direction.

Powerful swimmers: a comparisonIn the graph, the vertical axis is the ratio of speed in a viscoelastic fluid to speed in water. The horizontal axis is the degree of viscoelasticity. A flagellum with a high-angle helix, labeled with triangle, swims faster in a viscoelastic fluid than in water when the viscoelasticity is just right. As the helix angle decreases, the peak enhancement in speed decreases. For low-angle helices (circle), viscoelasticity always makes the swimmer slower than it would be in water.

In recent years, there has been some theoretical work aimed at fully understanding the physics of this kind of swimming, much of it done by modeling how helical swimmers behave in water. But bacteria and sperm spend a lot of time in fluids like mucus and cervical fluid — fluids that are not only more viscous than water, but also elastic since they are full of springy polymers. Because a rotating helix might be able to push against the polymers, it could be that a viscoelastic fluid makes swimming easier.

“It’s a fairly simple question,” Powers said. “Does viscoelasticity make microorganisms swim faster or slower?” Finding the right answer, however, hasn’t been so simple.

Early theoretical work suggested viscoelastic fluids should slow helical swimmers down. But some experimental work in the Brown School of Engineering by Powers, postdoctoral associate Bin Liu, and Kenneth Breuer, professor of engineering, suggested that viscoelastic fluids should actually help helical swimmers move faster.

This latest study, published in the journal Physical Review Letters, helps to bridge that apparent gap. Powers and Liu worked with Saverio Spagnolie, aprofessor of mathematics at the University of Wisconsin and aformer postdoctoral researcher at Brown. Using what Powers described as “some clever numerical methods and a lot of hard work,” Spagnolie was able to show computationally that the pitch angle of the helix — the degree to which the helix is coiled — matters in how well it performs in viscoelastic fluids. At a low pitch angle (think of a stretched phone cord), helices move more slowly in viscoelastic fluids. When the pitch angle increases, performance improves.

The findings reconcile the experimental and earlier theoretical work. Much of the theoretical work, which suggested more viscosity would cause slower swimming, assumed a small pitch angle for the sake of keeping the computations manageable. The experimental work, which showed viscosity sped swimming, involved higher pitch angles. By showing numerically that a higher pitch angle increases speed, the researchers were able to explain that apparent discrepancy. “This work shows how you can connect that prior work,” Powers said.

While this work was extremely valuable in linking theory and experiment, there’s still much more work to be done on this problem, Powers says. “We don’t really understand the result because it is so hard to visualize the three-dimensional configuration of all the forces involved. It’s actually very frustrating. We’re still trying to get an intuitive picture.”

That, at this point, is still an upstream swim.

Ultimately, the researchers say, a better understanding how tiny swimmers get around could inform studies of bacterial infection and fertility. It could also help scientists develop artificial swimmers that could deliver medicine inside the body.

The work was supported by the National Science Foundation (CBET-0854108).

Editors: Brown University has a fiber link television studio available for domestic and international live and taped interviews, and maintains an ISDN line for radio interviews. For more information, call (401) 863-2476.

Kevin Stacey | EurekAlert!
Further information:
http://www.brown.edu

More articles from Physics and Astronomy:

nachricht Hope to discover sure signs of life on Mars? New research says look for the element vanadium
22.09.2017 | University of Kansas

nachricht Calculating quietness
22.09.2017 | Forschungszentrum MATHEON ECMath

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: The pyrenoid is a carbon-fixing liquid droplet

Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.

A warming planet

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Rainbow colors reveal cell history: Uncovering β-cell heterogeneity

22.09.2017 | Life Sciences

Penn first in world to treat patient with new radiation technology

22.09.2017 | Medical Engineering

Calculating quietness

22.09.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>