Thursday, December 27, 2012

1212.6113 (Samia Dhahri et al.)

In-situ determination of the mechanical properties of crawling or
non-motile bacteria by Atomic Force Microscopy under physiological conditions
without immobilization
   [PDF]

Samia Dhahri, Michel Ramonda, Christian Marlière
We present a new, non-perturbative, easy-to-apply method for AFM imaging of living bacteria in their genuine physiological liquid environment. No immobilization protocol, neither chemical nor mechanical, was needed. For the first time, the native gliding (crawling) movements of gram-negative filamentous Nostoc cyanobacteria, along the surface and at speeds up to 200 nm/s, were studied by AFM. With the AFM working in a fast approach/retract mode, no limitation in neither spatial resolution nor imaging rate was detected. Gram positive and non-motile Rhodococcus wratislaviensis bacteria were studied as well. From the approach curves, Young modulus and turgor pressure were measured for both strains and for different gliding speeds. Young modulus is ranging from 20 to 105MPa and turgor pressure from 40 to 310kPa depending on the bacterium and the gliding speed. For Nostoc, spatially limited zones with higher values of stiffness were observed. The related spatial period is much higher than the mean length of nodules of Nostoc. This was explained by inhomogeneous mechanical activation of nodules in the cyanobacterium. We observed the presence of a soft extra cellular matrix (ECM) around the Nostoc bacterium. Both strains left a track of polymeric slime with variable thicknesses. For Rhodococcus, it is equal to few hundreds of nanometers, likely to promote its adhesion to the sample. While gliding, the Nostoc secretes, behind it, a slime layer the thickness of it is in the nanometer range and increases with the gliding speed. This result reinforces the hypothesis of a propulsion mechanism based, for cyanobacteria, on ejection of slime. These results open a large window on new studies of both dynamical phenomena of practical and fundamental interests such as the formation of conglomerates of bacteria (biofilms) and static properties of bacteria in real physiological conditions.
View original: http://arxiv.org/abs/1212.6113

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