Microswimmers hit the wall

" - The results of a study published today (7 January) suggest that microbes 'feel' their way along a solid surface, much as a blindfolded person would move near a wall. Using high-speed microscopic imaging, University of Cambridge researchers have found that sperm cells accumulate at surfaces and algae move away from them as a result of between the surface and the cells' flagella or cilia - the hair-like appendages that propel cells through their fluid environment. Interactions between swimming cells and surfaces feature prominently in a wide range of microbiological processes, most importantly in the formation of bacterial films and during the fertilisation of the human egg. Yet, surprisingly little has been known about the physical mechanisms that govern the accumulation of microbes at surfaces. The new experiments, published in Proceedings of the National Academy of Sciences (USA), were performed by Raymond Goldstein's group in the Department of Applied Mathematics and Theoretical Physics, and demonstrate how the interactions of microbes with solid surfaces are considerably more complex than previously thought. Vasily Kantsler, who performed the experiments, said: "Current theories of microbial swimming near surfaces assumed that these processes are governed by hydrodynamic long-range forces - essentially, the movement of fluid around the cells - but our experimental observations show that they are in fact governed by the mechanical properties of the cells themselves." Sperm cells differ from algal cells in how they respond to a solid wall because they are 'pusher swimmers', with a single posterior cilium.