My work is driven towards the understanding of biological systems. However, my contributions have always been linked to development of new experimental tools while keeping in mind the initial goal of achieving new biological knowledge. My main research topic is linked to bacterial chemotaxis but optimal flight strategies have piqued my interest lately.
The role of adaptation in bacterial speed races
It is widely believed that the chemotactic pathway is adapted in order to maintain both sensitivity and dynamic range in the signalling pathway. However, DeGennes [1] showed that a bacterium with a purely positive impulse response (non-adapted response) would climb up a linear gradi- ent faster than one with an adapted impulse response. More recently, Celani and Vergassola [2] pointed out the fact that evolutionary constraints might have shaped the chemotactic response to the type of environments seen for the different chemoattractant. In order to assay the role of adaptation in bacterial chemotaxis pathway, I developed a microfluidic setup in which the same bacterial population would climb up a gradient of aspartate (adapted chemotaxis) and a gradient of serine (non-adapted chemotaxis) separately. By monitoring the bacterial density along the gradients, we showed that the bacteria climb the gradient faster in serine (non-adapted response). Moreover, if chemotaxis to aspartate was rendered non-adapted, similar drift could be measured for aspartate and for serine. This work [3] points out that functional and evolutionary aspects of bacterial chemotaxis have yet to be linked.
Bacterial chemotaxis in fluctuating environments
If climbing a simple gradient faster gave a selective advantage (see [3]), why would perfect adaptation be tightly maintained in single cells for chemotaxis to aspartate [4]? Our previous work suggests the selective pressure applied on the signalling pathway by the environment are essentially unknown and that the ecological/physiological conditions are not met in usual laboratory conditions. A first step in this study is to create complex environments where both temporal and spatial fluctuations can be engineered. I have been closely collaborating with the Groisman lab to do so and clear technical solutions have been found. I intend to quantitatively address wether environmental fluctuations shaped the adapted response to aspartate as it is currently known.
Regulation of adaptation in different environments. Links between chemotaxis and physiology
Moreover, it has been found that levels of chemical receptors, Tar and Tsr, might vary with environmental conditions[5] which would suggest that the level of adaptation might also vary. Pushing for understanding the role of adaptation clearly needs a thorough study of the effects of different environment. In collaboration with the Hwa lab, I am currently engineering strains in which level of adaptation can be tuned or monitored. By mapping the environmental conditions in which adaptation is achieved for each particular chemical, we believe that one can extract clear information on the links between chemotaxis and metabolism ultimately reaching an understanding of what function chemotaxis is set to perform.
Soaring in fluctuating environments
Migrating birds and gliders use warm air thermal columns to cover large distances while minimizing the energy cost of propelling themselves using flapping or engines. Strategies that effectively utilize these convective currents are complicated by strong turbulent fluctuations that accompany them. A bird soaring within such noisy environments has to explore to identify and then exploit the large-scale fluctuations by sensing local quantities. We use modern machine learning techniques to find effective soaring strategies for gliders flying within complex, turbulent flows. Results advance understanding of how birds do it and is also applicable in the development of autonomous flying vehicles.
[1] P.-G. de Gennes, “Chemotaxis: the role of internal delays.,” Eur Biophys J, vol. 33, pp. 691–693, Dec 2004. [2] A. Celani and M. Vergassola, “Bacterial strategies for chemotaxis response.,” Proc Natl Acad Sci U S A, Jan 2010. [3] Wong-Ng J, Melbinger A, Celani A and Vergassola M; The Role of Adaptation in Bacterial Speed Races. PLOS Comp Biol 2016, 12(6): e1004974. [4] J.-B. Masson*, G. Voisinne*, J. Wong-Ng*, A. Celani, and M. Vergassola, “Noninvasive inference of the molecular chemotactic response using bacterial trajectories.,” Proc Natl Acad Sci U S A, vol. 109, pp. 1802–1807, Jan 2012 (* equal contributors). [5] H. Salman and A. Libchaber, “A concentration-dependent switch in the bacterial response to temperature.,” Nat Cell Biol, vol. 9, pp. 1098–1100, Sep 2007.