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Bacterial gliding motility:
Gliding bacteria have been studied for well over 50 years,
but the molecular mechanism that leads to their gliding behavior still remains
a mystery. I am currently working with
Prof. George Oster
(UC Berkeley) on
developing a theoretical model that utilizes the swelling of a polyelectrolyte
gel (slime) to describe gliding in cyanobacteria and
A-motility in Myxococcus xanthus. As well, we are interested
in understanding the diverse pattern formation that is seen in multicellular
groups of Myxococcus Xanthus. We are working in collaboration with
Prof. Dale Kaiser (Stanford),
Prof. David Zusman (UC Berkeley), and Dr. Egbert Hoiczyk (Rockefeller) on
experiments that can shed more light on the mechanism and guide the theoretical
model. The picture to the right shows the volume fraction (color map) of a gel hydrating out of a nozzle structure similar to what was observed by Hoiczyk and Baumeister in cyanobacteria. The arrows show fluid velocity lines.
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Supercoiling of filamentous bacteria:
Filamentous bacteria are ubiquitous in nature. Many different species of
bacteria form long filaments of end-to-end connected cells
when they fail to seperate upon replication. Some species that are seen
to do this are B. subtilis, E. coli, and tuberculosis. Under some growth
conditions, these filaments will grow to a certain length and then buckle and
coil around themselves like an over-twisted phone cord.
Neil Mendelson (Arizona)
has studied the complex pattern formation that occurs in B. subtilis and
has shown what factors affect the morphology. Utilizing elasticity and
viscous fluid dynamics,
Prof. Thomas Powers (Brown),
Prof. Ray Goldstein (University of Arizona), and I have developed a mathematical that
describes how growth-induced twist can lead to the supercoiling that is seen
in these filaments. Picture to the left shows the numerical solution
of the dynamic PDE's for a growing elastic filament loop with preferred twist.
The 3-point branches that are seen are similar to what is observed in B.
subtilis macrofibers.
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The mathematical model mentioned above, does not explain the molecular advent of
twist generation during growth.
The recent discovery of Mbl and mreB (helical, actin-like cytoskeletal cables in B. subtilis )
suggests a potential mechanism underlying this behavior. George Oster and I recently proposed a
quantitative model that describes how growth coupled to helical cytoskeletal proteins
can lead to supercoiling. In addition, this model may provide insight into maintenance of form
in helical bacteria, such as V. cholerae and C. cresecentus.
Now, in collaboration with Prof. Peter Setlow (UCHC), Barabara Setlow (UCHC), and Sulav Mukherjee (UCHC),
I have begun to experimentally test this model using a cwlF and sigD filamentous mutant
of B. subtilis and fluorescent confocal microscopy. Image to the
right shows a coiled mutant with fluorescently label Mbl. This project is supported
by NSF MCB 0327716.
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Nematode sperm motility:
Utilizing the gel equations that I used to model the slime gun for gliding motility,
I hope to show how polymerization and depolymerization of Major Sperm Protein (MSP)
can lead to the motility
and lamellipod protrusion that is observed in nematode sperm.
Beginning with a model developed
in collaboration with Prof. George Oster (UC Berkeley) and
Prof. Alex
Mogilner (UC Davis), Mark Zajac and I are now working on
developing a 2D computer simulation to further explore the motility of nematode sperm.
In addition, Brian Dacanay,
Prof. Bill Mohler (UCHC),
Prof. Ann Cowan (UCHC),
and I are testing the pH dependence of
motility in C. elegans sperm. pH has already been shown to affect crawling
in sperm from the nematode Ascaris suum and therefore should play a role in
C. elegans . As shown to the left, changes in external pH can
reversibly turn on and off the motility of C. elegans spermatozoa.
Look here for a movie of crawling nematode sperm.
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Spirochete morpholgy and motility
Spirochetes are unique bacteria in that they exist in a helical morphology
and their motion is driven by flagella that are encased within their periplasmic
space. These bacteria are often highly toxic and exceptionally mobile
in gel environments. It has been observed that the cell morphology is
dependent on both the cell wall and the flagella. We hope to show how
morphology and motility is acheived in these bacteria by studying the coupled
elasticity of the cell wall with the flagella. Picture to the right shows
Leptospira interrogans.
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Overall, my research objectives deal with using physics to understand biological
systems. My main interests lie in morphology, propulsion, growth, and fluid dynamics.
Though I have mainly done research on cellular-sized objects, I am interested
in biophysics at all length scales.